| Page 1 of 2 | 1 | 2 |
|
|
Very Low Power Preamp
I am working on a project for receiving a very narrow bandwidth signal
at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? -- Rick |
Very Low Power Preamp
rickman wrote:
I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? Specifying the output, impedance, and degree of linearlity required might get a meaningful response. The Maxim MAX480 opamp will run from a single supply down to 1.6V with a supply current of less tha 20 uA. Most of the micropower opamps I've seen run in the range of 30 to 50 uW quiescent which doesn't leave a lot for output. -- Jim Pennino |
Very Low Power Preamp
On 11/4/2014 6:29 PM, rickman wrote:
I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
Very Low Power Preamp
|
Very Low Power Preamp
On 11/4/2014 9:42 PM, Jerry Stuckle wrote:
On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! -- Rick |
Very Low Power Preamp
On 11/5/2014 1:57 PM, wrote:
rickman wrote: On 11/4/2014 8:29 PM, wrote: rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? Specifying the output, impedance, and degree of linearlity required might get a meaningful response. The Maxim MAX480 opamp will run from a single supply down to 1.6V with a supply current of less tha 20 uA. Most of the micropower opamps I've seen run in the range of 30 to 50 uW quiescent which doesn't leave a lot for output. Thanks for the reply. I'm not sure what you are asking regarding the output and impedance. The input would be high impedance and the output low driving another high impedance input. I can design the circuit around the amp, I just can't find anything very low power. The main problem seems to be getting enough gain-bandwidth product to actually amplify the signal. The output and impedance determine E^2/R, which subtracts from the 100 uW leaving you with approximetly the quiescent power budget. The degree of linearity has a big effect on efficiency which has a big effect on the power budget. Also the desired gain would be a nice thing to know as in would a single transistor be enough. 20 dB would be a good start. The part you suggest has a number of problems. One is that it is not recommended for new designs. The other is that it only has a gain bandwidth product of 20 kHz. It was just a micropower opamp off the top of my head. I assume you have looked into designs using various FET's? Such as? -- Rick |
Very Low Power Preamp
rickman wrote:
On 11/5/2014 1:57 PM, wrote: rickman wrote: On 11/4/2014 8:29 PM, wrote: rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? Specifying the output, impedance, and degree of linearlity required might get a meaningful response. The Maxim MAX480 opamp will run from a single supply down to 1.6V with a supply current of less tha 20 uA. Most of the micropower opamps I've seen run in the range of 30 to 50 uW quiescent which doesn't leave a lot for output. Thanks for the reply. I'm not sure what you are asking regarding the output and impedance. The input would be high impedance and the output low driving another high impedance input. I can design the circuit around the amp, I just can't find anything very low power. The main problem seems to be getting enough gain-bandwidth product to actually amplify the signal. The output and impedance determine E^2/R, which subtracts from the 100 uW leaving you with approximetly the quiescent power budget. The degree of linearity has a big effect on efficiency which has a big effect on the power budget. Also the desired gain would be a nice thing to know as in would a single transistor be enough. 20 dB would be a good start. Off the top of my head that seems to be a bit high for a single transistor. The part you suggest has a number of problems. One is that it is not recommended for new designs. The other is that it only has a gain bandwidth product of 20 kHz. It was just a micropower opamp off the top of my head. I assume you have looked into designs using various FET's? Such as? Well, you could start with an amplifier design that uses a theoretical transistor which meets your goals then search maker spec sheets for a real transistor that has the needed qualities. I thought you said you had Spice? If I were doing it, I would start with some common transistor then tweak the transistor specs until it did what I wanted or became obvious it can't be done with a single transistor. If there were no real transistor with the required specs or it is not possible to do with a single transistor, I would go to two transistors and repeat. -- Jim Pennino |
Very Low Power Preamp
On 11/5/2014 2:56 PM, wrote:
rickman wrote: On 11/5/2014 1:57 PM, wrote: rickman wrote: On 11/4/2014 8:29 PM, wrote: rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? Specifying the output, impedance, and degree of linearlity required might get a meaningful response. The Maxim MAX480 opamp will run from a single supply down to 1.6V with a supply current of less tha 20 uA. Most of the micropower opamps I've seen run in the range of 30 to 50 uW quiescent which doesn't leave a lot for output. Thanks for the reply. I'm not sure what you are asking regarding the output and impedance. The input would be high impedance and the output low driving another high impedance input. I can design the circuit around the amp, I just can't find anything very low power. The main problem seems to be getting enough gain-bandwidth product to actually amplify the signal. The output and impedance determine E^2/R, which subtracts from the 100 uW leaving you with approximetly the quiescent power budget. The degree of linearity has a big effect on efficiency which has a big effect on the power budget. Also the desired gain would be a nice thing to know as in would a single transistor be enough. 20 dB would be a good start. Off the top of my head that seems to be a bit high for a single transistor. If the current is low enough two could be used each with a gain of 10 or more. The part you suggest has a number of problems. One is that it is not recommended for new designs. The other is that it only has a gain bandwidth product of 20 kHz. It was just a micropower opamp off the top of my head. I assume you have looked into designs using various FET's? Such as? Well, you could start with an amplifier design that uses a theoretical transistor which meets your goals then search maker spec sheets for a real transistor that has the needed qualities. I thought you said you had Spice? I don't think spice is needed to design a single transistor amp. I was actually looking for an IC that would do the job. If using a transistor someone with experience in transistors would have an idea of the limits. I have been told that bipolar devices have very low gain at low collector currents which becomes a limitation as the bias current is reduced. I don't know about FETs. It is not at all clear to me that transistor models are accurate enough to give a good representation of this in spice. If I were doing it, I would start with some common transistor then tweak the transistor specs until it did what I wanted or became obvious it can't be done with a single transistor. If there were no real transistor with the required specs or it is not possible to do with a single transistor, I would go to two transistors and repeat. If I can't find someone with more experience I may do that. The actual goal is to do this without a preamp, but I'm starting to have some doubts about being able to pull it off. I need to characterize my digital inputs. Then I will have a better idea of the requirements, but that is more easily said than done. -- Rick |
Very Low Power Preamp
On 11/5/2014 1:29 PM, rickman wrote:
On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
Very Low Power Preamp
On 11/6/2014 10:04 AM, Jerry Stuckle wrote:
On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. -- Rick |
Very Low Power Preamp
In article , rickman wrote:
I don't follow on this. How does a low output impedance drive the current drain? Well, the current to pull up the output against the low impedance has to come from somewhere! It has to accounted for as part of the budget. Consider the current used only by the amp, not the load. You might want to experiment with using a one- or two-stage common-drain amplifier using "electrometer grade" JFETs such as the 2N4117-2N4119 family (obsolete, but still available if you hunt around a bit). They have extremely low gate leakage (i.e. very high input impedance). Even with 0 volts on the gate (relative to source) they have a low Idss, and of course you can add a resistor between source and ground and reduce the standing current as low as you want. |
Very Low Power Preamp
On 11/6/2014 2:02 PM, David Platt wrote:
In article , rickman wrote: I don't follow on this. How does a low output impedance drive the current drain? Well, the current to pull up the output against the low impedance has to come from somewhere! It has to accounted for as part of the budget. I never said the load is low impedance. I said the output should be low impedance. Consider the current used only by the amp, not the load. You might want to experiment with using a one- or two-stage common-drain amplifier using "electrometer grade" JFETs such as the 2N4117-2N4119 family (obsolete, but still available if you hunt around a bit). They have extremely low gate leakage (i.e. very high input impedance). Even with 0 volts on the gate (relative to source) they have a low Idss, and of course you can add a resistor between source and ground and reduce the standing current as low as you want. I'll take a look. The input impedance does not need to be *huge*. My concern is the drain current. This needs to be operated in an analog range rather than digital and provide gain. -- Rick |
Very Low Power Preamp
On 11/6/2014 11:45 AM, rickman wrote:
On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ .. You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
Very Low Power Preamp
On 11/7/2014 10:49 AM, Jerry Stuckle wrote:
On 11/6/2014 11:45 AM, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ . You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. I have done this before and found nothing. But I did it again at both Mouser and Digikey and found several. One listed by Mouser looked especially good only to find rather than 0.75 uA of supply current, it had 0.75 mA of supply current. lol But then the next part, same thing... another one... and another... one part I'm not sure what to make of it. The selection table shows supply current of 0.034 mA and the data sheet shows 25 A! Yes, that's right, the data sheet shows between 25 and 300 Amps for typical supply current!!! I would contact TI about this obvious typo, but this part is not suitable because of the GBW which is also incorrect in the selection table. Same thing at Digikey, everything in the selection table that meets these two requirements is a mistake. -- Rick |
Very Low Power Preamp
On 11/7/2014 1:02 PM, rickman wrote:
On 11/7/2014 10:49 AM, Jerry Stuckle wrote: On 11/6/2014 11:45 AM, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ . You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. I have done this before and found nothing. But I did it again at both Mouser and Digikey and found several. One listed by Mouser looked especially good only to find rather than 0.75 uA of supply current, it had 0.75 mA of supply current. lol But then the next part, same thing... another one... and another... one part I'm not sure what to make of it. The selection table shows supply current of 0.034 mA and the data sheet shows 25 A! Yes, that's right, the data sheet shows between 25 and 300 Amps for typical supply current!!! I would contact TI about this obvious typo, but this part is not suitable because of the GBW which is also incorrect in the selection table. Same thing at Digikey, everything in the selection table that meets these two requirements is a mistake. A couple of things. First of all, I've found minor errors in the listings at Mouser (I don't use Digikey much), but never real glaring errors. And this is th first time I've seen a TI datasheet that far off. Looks like someone dropped a decimal point :). However, I've found Mouser is interested in correcting errors; they are input by humans, after all, at some point in time, and errors do creep in. Secondly, the current shown is going to be max current, which will depend on the output impedance (and the amount that has to be sourced/sunk). It's not going to pull this all the time; I would expect your actual current draw to be much less since you're 1) going into a high impedance load and 2) not going from rail to rail. Also, if you use a bipolar supply, then current drain should be less because you'll be operating near ground, instead of the midpoint of a single supply voltage (where the output would be at 1/2 Vcc). Some of these are quite low voltage, and I would think a couple of the larger lithium coin batteries should last quite a while. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
Very Low Power Preamp
On 11/7/2014 1:17 PM, Jerry Stuckle wrote:
On 11/7/2014 1:02 PM, rickman wrote: On 11/7/2014 10:49 AM, Jerry Stuckle wrote: On 11/6/2014 11:45 AM, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ . You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. I have done this before and found nothing. But I did it again at both Mouser and Digikey and found several. One listed by Mouser looked especially good only to find rather than 0.75 uA of supply current, it had 0.75 mA of supply current. lol But then the next part, same thing... another one... and another... one part I'm not sure what to make of it. The selection table shows supply current of 0.034 mA and the data sheet shows 25 A! Yes, that's right, the data sheet shows between 25 and 300 Amps for typical supply current!!! I would contact TI about this obvious typo, but this part is not suitable because of the GBW which is also incorrect in the selection table. Same thing at Digikey, everything in the selection table that meets these two requirements is a mistake. A couple of things. First of all, I've found minor errors in the listings at Mouser (I don't use Digikey much), but never real glaring errors. And this is th first time I've seen a TI datasheet that far off. Looks like someone dropped a decimal point :). However, I've found Mouser is interested in correcting errors; they are input by humans, after all, at some point in time, and errors do creep in. Yes, when you list millions of parts there will be errors. I have written digikey many times about listing errors and they always thank me. I'm sure Mouser is no different. Secondly, the current shown is going to be max current, which will depend on the output impedance (and the amount that has to be sourced/sunk). It's not going to pull this all the time; I would expect your actual current draw to be much less since you're 1) going into a high impedance load and 2) not going from rail to rail. I find the opposite. The current listed is under specified conditions which usually *do not* include output drive. In fact, it usually listed as a quiescent current. Also, if you use a bipolar supply, then current drain should be less because you'll be operating near ground, instead of the midpoint of a single supply voltage (where the output would be at 1/2 Vcc). Some of these are quite low voltage, and I would think a couple of the larger lithium coin batteries should last quite a while. Not sure how the ground level would affect the bias currents. When the supply voltage is lowered the GBW lowers as well. -- Rick |
Very Low Power Preamp
On 11/7/2014 1:26 PM, rickman wrote:
On 11/7/2014 1:17 PM, Jerry Stuckle wrote: On 11/7/2014 1:02 PM, rickman wrote: On 11/7/2014 10:49 AM, Jerry Stuckle wrote: On 11/6/2014 11:45 AM, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ . You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. I have done this before and found nothing. But I did it again at both Mouser and Digikey and found several. One listed by Mouser looked especially good only to find rather than 0.75 uA of supply current, it had 0.75 mA of supply current. lol But then the next part, same thing... another one... and another... one part I'm not sure what to make of it. The selection table shows supply current of 0.034 mA and the data sheet shows 25 A! Yes, that's right, the data sheet shows between 25 and 300 Amps for typical supply current!!! I would contact TI about this obvious typo, but this part is not suitable because of the GBW which is also incorrect in the selection table. Same thing at Digikey, everything in the selection table that meets these two requirements is a mistake. A couple of things. First of all, I've found minor errors in the listings at Mouser (I don't use Digikey much), but never real glaring errors. And this is th first time I've seen a TI datasheet that far off. Looks like someone dropped a decimal point :). However, I've found Mouser is interested in correcting errors; they are input by humans, after all, at some point in time, and errors do creep in. Yes, when you list millions of parts there will be errors. I have written digikey many times about listing errors and they always thank me. I'm sure Mouser is no different. Secondly, the current shown is going to be max current, which will depend on the output impedance (and the amount that has to be sourced/sunk). It's not going to pull this all the time; I would expect your actual current draw to be much less since you're 1) going into a high impedance load and 2) not going from rail to rail. I find the opposite. The current listed is under specified conditions which usually *do not* include output drive. In fact, it usually listed as a quiescent current. Well, yes and no. Op amps typically sink more than they source, and the sink current does not come from the chip. Source current at the output is supplied by the chip, of course. And I've found a wide difference between how op amp specs are listed; some show quiescent current, some show average current under typical operating conditions. Some even show maximum current which can be drawn. So I'll retract that statement above. Wasn't thinking clearly. Also, if you use a bipolar supply, then current drain should be less because you'll be operating near ground, instead of the midpoint of a single supply voltage (where the output would be at 1/2 Vcc). Some of these are quite low voltage, and I would think a couple of the larger lithium coin batteries should last quite a while. Not sure how the ground level would affect the bias currents. When the supply voltage is lowered the GBW lowers as well. If the output is at ground level, no current will be pulled from either rail (at the output). Shifting above or below that will draw a little current, reference zero. However, if you're running a single ended supply, your output will be at 1/2 Vcc, and will always be pulling some current to maintain that level. The signal will change that slightly, increasing or decreasing. But unless you have a square wave with a 50% duty cycle, you'll end up needing more current from the single ended supply. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
Very Low Power Preamp
On 11/7/2014 1:53 PM, Jerry Stuckle wrote:
On 11/7/2014 1:26 PM, rickman wrote: On 11/7/2014 1:17 PM, Jerry Stuckle wrote: On 11/7/2014 1:02 PM, rickman wrote: On 11/7/2014 10:49 AM, Jerry Stuckle wrote: On 11/6/2014 11:45 AM, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ . You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. I have done this before and found nothing. But I did it again at both Mouser and Digikey and found several. One listed by Mouser looked especially good only to find rather than 0.75 uA of supply current, it had 0.75 mA of supply current. lol But then the next part, same thing... another one... and another... one part I'm not sure what to make of it. The selection table shows supply current of 0.034 mA and the data sheet shows 25 A! Yes, that's right, the data sheet shows between 25 and 300 Amps for typical supply current!!! I would contact TI about this obvious typo, but this part is not suitable because of the GBW which is also incorrect in the selection table. Same thing at Digikey, everything in the selection table that meets these two requirements is a mistake. A couple of things. First of all, I've found minor errors in the listings at Mouser (I don't use Digikey much), but never real glaring errors. And this is th first time I've seen a TI datasheet that far off. Looks like someone dropped a decimal point :). However, I've found Mouser is interested in correcting errors; they are input by humans, after all, at some point in time, and errors do creep in. Yes, when you list millions of parts there will be errors. I have written digikey many times about listing errors and they always thank me. I'm sure Mouser is no different. Secondly, the current shown is going to be max current, which will depend on the output impedance (and the amount that has to be sourced/sunk). It's not going to pull this all the time; I would expect your actual current draw to be much less since you're 1) going into a high impedance load and 2) not going from rail to rail. I find the opposite. The current listed is under specified conditions which usually *do not* include output drive. In fact, it usually listed as a quiescent current. Well, yes and no. Op amps typically sink more than they source, and the sink current does not come from the chip. Source current at the output is supplied by the chip, of course. And I've found a wide difference between how op amp specs are listed; some show quiescent current, some show average current under typical operating conditions. Some even show maximum current which can be drawn. So I'll retract that statement above. Wasn't thinking clearly. Also, if you use a bipolar supply, then current drain should be less because you'll be operating near ground, instead of the midpoint of a single supply voltage (where the output would be at 1/2 Vcc). Some of these are quite low voltage, and I would think a couple of the larger lithium coin batteries should last quite a while. Not sure how the ground level would affect the bias currents. When the supply voltage is lowered the GBW lowers as well. If the output is at ground level, no current will be pulled from either rail (at the output). Shifting above or below that will draw a little current, reference zero. However, if you're running a single ended supply, your output will be at 1/2 Vcc, and will always be pulling some current to maintain that level. The signal will change that slightly, increasing or decreasing. But unless you have a square wave with a 50% duty cycle, you'll end up needing more current from the single ended supply. What you are saying is only true if your load is ground connected. The load for this circuit will be a voltage source through a high impedance. The input is differential and to make it as sensitive as possible a bias will be applied to one input sufficient to offset the input bias voltage. So in reality the load will be biased to approx 1/2 Vcc. The real point is that this is *LOAD* current, not amplifier current and is independent of the amplifier and so considered separately since selection of the amplifier has no impact on it. -- Rick |
Very Low Power Preamp
On Thu, 6 Nov 2014, rickman wrote:
On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? If you use a large resister in the collector, you'll get high impedance output. But load it down with a low impedance, and there won't be a proper transfer of the signal. So you use a low value collector resistor, current goes up because it pushes more current through the device, but you get your lower impedance. I thought generally people wanted more current into preamps, because that helped them in handling strong signals. So take a look at WWVB preamps/receivers from the seventies. Certainly they'd be using bipolar transistors, but one might think they might be reasonably low current. On the other hand, I can't remember why you need low current for this, and something like a WWVB receiver usually doesn't need to fuss about being extra low current. So I suspect those projects never tried to be ultra-low current. My Casio Waveceptor watch does, but I have no idea what kind of circuitry is in there, and even if I opened the watch, I bet it would be difficult to trace. On the other hand, I have a Radio Shack "atomic clock" that runs for years on one AAA or AA battery, so someone figured out how to receive WWVB with low current and low voltage. But then, the WWVB front end is likely a module, which is another way to solve the problem, just buy a module, or strip one out of an existing clock. But again, I can't remember why you are needing this, so I suspect there's some reason why these options aren't being used. Is a preamp really going to be low current compared to the later circuitry's needs? Once you add the rest, maybe it's not worth pursuing ultra-low current for the preamp. Michael |
Very Low Power Preamp
On 11/7/2014 3:07 PM, rickman wrote:
On 11/7/2014 1:53 PM, Jerry Stuckle wrote: On 11/7/2014 1:26 PM, rickman wrote: On 11/7/2014 1:17 PM, Jerry Stuckle wrote: On 11/7/2014 1:02 PM, rickman wrote: On 11/7/2014 10:49 AM, Jerry Stuckle wrote: On 11/6/2014 11:45 AM, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ . You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. I have done this before and found nothing. But I did it again at both Mouser and Digikey and found several. One listed by Mouser looked especially good only to find rather than 0.75 uA of supply current, it had 0.75 mA of supply current. lol But then the next part, same thing... another one... and another... one part I'm not sure what to make of it. The selection table shows supply current of 0.034 mA and the data sheet shows 25 A! Yes, that's right, the data sheet shows between 25 and 300 Amps for typical supply current!!! I would contact TI about this obvious typo, but this part is not suitable because of the GBW which is also incorrect in the selection table. Same thing at Digikey, everything in the selection table that meets these two requirements is a mistake. A couple of things. First of all, I've found minor errors in the listings at Mouser (I don't use Digikey much), but never real glaring errors. And this is th first time I've seen a TI datasheet that far off. Looks like someone dropped a decimal point :). However, I've found Mouser is interested in correcting errors; they are input by humans, after all, at some point in time, and errors do creep in. Yes, when you list millions of parts there will be errors. I have written digikey many times about listing errors and they always thank me. I'm sure Mouser is no different. Secondly, the current shown is going to be max current, which will depend on the output impedance (and the amount that has to be sourced/sunk). It's not going to pull this all the time; I would expect your actual current draw to be much less since you're 1) going into a high impedance load and 2) not going from rail to rail. I find the opposite. The current listed is under specified conditions which usually *do not* include output drive. In fact, it usually listed as a quiescent current. Well, yes and no. Op amps typically sink more than they source, and the sink current does not come from the chip. Source current at the output is supplied by the chip, of course. And I've found a wide difference between how op amp specs are listed; some show quiescent current, some show average current under typical operating conditions. Some even show maximum current which can be drawn. So I'll retract that statement above. Wasn't thinking clearly. Also, if you use a bipolar supply, then current drain should be less because you'll be operating near ground, instead of the midpoint of a single supply voltage (where the output would be at 1/2 Vcc). Some of these are quite low voltage, and I would think a couple of the larger lithium coin batteries should last quite a while. Not sure how the ground level would affect the bias currents. When the supply voltage is lowered the GBW lowers as well. If the output is at ground level, no current will be pulled from either rail (at the output). Shifting above or below that will draw a little current, reference zero. However, if you're running a single ended supply, your output will be at 1/2 Vcc, and will always be pulling some current to maintain that level. The signal will change that slightly, increasing or decreasing. But unless you have a square wave with a 50% duty cycle, you'll end up needing more current from the single ended supply. What you are saying is only true if your load is ground connected. The load for this circuit will be a voltage source through a high impedance. The input is differential and to make it as sensitive as possible a bias will be applied to one input sufficient to offset the input bias voltage. So in reality the load will be biased to approx 1/2 Vcc. True, but with a bipolar supply, the input is referenced to ground and no current flows with no input. The output is also referenced to ground, so no current flows their, either. And with both input and output at ground potential, there is less current flowing internally. The real point is that this is *LOAD* current, not amplifier current and is independent of the amplifier and so considered separately since selection of the amplifier has no impact on it. That is true - to an extent. When the op amp is sinking current, the current comes from the load, not the op amp. However, internally there must still be current flowing to provide that sink. But when the op amp is sourcing current, the amplifier has to provide the source current plus the drive current. Now you can bias the output so that the op amp is always sinking, but then you have a steady drain from the standby current. When you're using a bipolar supply and input is at ground, the output will also be at ground, and very little current will be flowing. You can create an artificial ground at 1/2 Vcc, but even creating that artificial ground draws current. The difference in current drain is not important in the vast majority of cases. But they become important when you're talking the low drain you wish. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
Very Low Power Preamp
On 11/7/2014 4:20 PM, Michael Black wrote:
On Thu, 6 Nov 2014, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? If you use a large resister in the collector, you'll get high impedance output. But load it down with a low impedance, and there won't be a proper transfer of the signal. So you use a low value collector resistor, current goes up because it pushes more current through the device, but you get your lower impedance. I thought generally people wanted more current into preamps, because that helped them in handling strong signals. So take a look at WWVB preamps/receivers from the seventies. Certainly they'd be using bipolar transistors, but one might think they might be reasonably low current. On the other hand, I can't remember why you need low current for this, and something like a WWVB receiver usually doesn't need to fuss about being extra low current. So I suspect those projects never tried to be ultra-low current. My Casio Waveceptor watch does, but I have no idea what kind of circuitry is in there, and even if I opened the watch, I bet it would be difficult to trace. On the other hand, I have a Radio Shack "atomic clock" that runs for years on one AAA or AA battery, so someone figured out how to receive WWVB with low current and low voltage. But then, the WWVB front end is likely a module, which is another way to solve the problem, just buy a module, or strip one out of an existing clock. But again, I can't remember why you are needing this, so I suspect there's some reason why these options aren't being used. Is a preamp really going to be low current compared to the later circuitry's needs? Once you add the rest, maybe it's not worth pursuing ultra-low current for the preamp. Michael Michael, Yes, battery operated analog clocks run for a long time on an AA battery. But the real question here is - is your clock receiving WWVB all the time, or does it only sync up when you insert a battery and once or twice a day after that? The latter would save a lot of current draw, and it probably doesn't need to sync more often than that (well, maybe 4 times a day if it's way off :) ). I have two analog battery-operated analog clocks here, and only set them twice a year. Between, they keep perfect time (within a few seconds, anyway). -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
Very Low Power Preamp
On 11/7/2014 4:23 PM, Jerry Stuckle wrote:
On 11/7/2014 3:07 PM, rickman wrote: On 11/7/2014 1:53 PM, Jerry Stuckle wrote: On 11/7/2014 1:26 PM, rickman wrote: On 11/7/2014 1:17 PM, Jerry Stuckle wrote: On 11/7/2014 1:02 PM, rickman wrote: On 11/7/2014 10:49 AM, Jerry Stuckle wrote: On 11/6/2014 11:45 AM, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ . You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. I have done this before and found nothing. But I did it again at both Mouser and Digikey and found several. One listed by Mouser looked especially good only to find rather than 0.75 uA of supply current, it had 0.75 mA of supply current. lol But then the next part, same thing... another one... and another... one part I'm not sure what to make of it. The selection table shows supply current of 0.034 mA and the data sheet shows 25 A! Yes, that's right, the data sheet shows between 25 and 300 Amps for typical supply current!!! I would contact TI about this obvious typo, but this part is not suitable because of the GBW which is also incorrect in the selection table. Same thing at Digikey, everything in the selection table that meets these two requirements is a mistake. A couple of things. First of all, I've found minor errors in the listings at Mouser (I don't use Digikey much), but never real glaring errors. And this is th first time I've seen a TI datasheet that far off. Looks like someone dropped a decimal point :). However, I've found Mouser is interested in correcting errors; they are input by humans, after all, at some point in time, and errors do creep in. Yes, when you list millions of parts there will be errors. I have written digikey many times about listing errors and they always thank me. I'm sure Mouser is no different. Secondly, the current shown is going to be max current, which will depend on the output impedance (and the amount that has to be sourced/sunk). It's not going to pull this all the time; I would expect your actual current draw to be much less since you're 1) going into a high impedance load and 2) not going from rail to rail. I find the opposite. The current listed is under specified conditions which usually *do not* include output drive. In fact, it usually listed as a quiescent current. Well, yes and no. Op amps typically sink more than they source, and the sink current does not come from the chip. Source current at the output is supplied by the chip, of course. And I've found a wide difference between how op amp specs are listed; some show quiescent current, some show average current under typical operating conditions. Some even show maximum current which can be drawn. So I'll retract that statement above. Wasn't thinking clearly. Also, if you use a bipolar supply, then current drain should be less because you'll be operating near ground, instead of the midpoint of a single supply voltage (where the output would be at 1/2 Vcc). Some of these are quite low voltage, and I would think a couple of the larger lithium coin batteries should last quite a while. Not sure how the ground level would affect the bias currents. When the supply voltage is lowered the GBW lowers as well. If the output is at ground level, no current will be pulled from either rail (at the output). Shifting above or below that will draw a little current, reference zero. However, if you're running a single ended supply, your output will be at 1/2 Vcc, and will always be pulling some current to maintain that level. The signal will change that slightly, increasing or decreasing. But unless you have a square wave with a 50% duty cycle, you'll end up needing more current from the single ended supply. What you are saying is only true if your load is ground connected. The load for this circuit will be a voltage source through a high impedance. The input is differential and to make it as sensitive as possible a bias will be applied to one input sufficient to offset the input bias voltage. So in reality the load will be biased to approx 1/2 Vcc. True, but with a bipolar supply, the input is referenced to ground and no current flows with no input. The output is also referenced to ground, so no current flows their, either. And with both input and output at ground potential, there is less current flowing internally. I just explained a scenario where the load will draw current from the amp regardless of power supply arrangement. You are making an assumption that the input and output are ground referenced. That is independent of the supply arrangement. The real point is that this is *LOAD* current, not amplifier current and is independent of the amplifier and so considered separately since selection of the amplifier has no impact on it. That is true - to an extent. When the op amp is sinking current, the current comes from the load, not the op amp. However, internally there must still be current flowing to provide that sink. You aren't grasping the issue. It is not about which direction the current is flowing, it is about what is responsible for setting the amount of current. The load determines the current that flows in or out of the load and is independent of the opamp characteristics or the power supply arrangement. What I can control by picking the opamp is the current that flows through the opamp that is independent of the load or input. But when the op amp is sourcing current, the amplifier has to provide the source current plus the drive current. Now you can bias the output so that the op amp is always sinking, but then you have a steady drain from the standby current. When you're using a bipolar supply and input is at ground, the output will also be at ground, and very little current will be flowing. You can create an artificial ground at 1/2 Vcc, but even creating that artificial ground draws current. The difference in current drain is not important in the vast majority of cases. But they become important when you're talking the low drain you wish. None of this is relevant to the issue of picking an amplifier. -- Rick |
Very Low Power Preamp
On 11/7/2014 4:20 PM, Michael Black wrote:
On Thu, 6 Nov 2014, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? If you use a large resister in the collector, you'll get high impedance output. But load it down with a low impedance, and there won't be a proper transfer of the signal. So you use a low value collector resistor, current goes up because it pushes more current through the device, but you get your lower impedance. I thought generally people wanted more current into preamps, because that helped them in handling strong signals. So take a look at WWVB preamps/receivers from the seventies. Certainly they'd be using bipolar transistors, but one might think they might be reasonably low current. On the other hand, I can't remember why you need low current for this, and something like a WWVB receiver usually doesn't need to fuss about being extra low current. So I suspect those projects never tried to be ultra-low current. No, none of the designs I have seen are remotely low current. They usually have collector resistors in the range of 1k or less. My Casio Waveceptor watch does, but I have no idea what kind of circuitry is in there, and even if I opened the watch, I bet it would be difficult to trace. Single chip ASIC. On the other hand, I have a Radio Shack "atomic clock" that runs for years on one AAA or AA battery, so someone figured out how to receive WWVB with low current and low voltage. But then, the WWVB front end is likely a module, which is another way to solve the problem, just buy a module, or strip one out of an existing clock. But again, I can't remember why you are needing this, so I suspect there's some reason why these options aren't being used. Yes, you can get a chip that does the front end reception. The block diagram includes an amplifier which is absolutely required with a ferrite antenna. I am using a large air core loop which should give me a larger voltage than the ferrite antennas which have small loops. Is a preamp really going to be low current compared to the later circuitry's needs? Once you add the rest, maybe it's not worth pursuing ultra-low current for the preamp. That's the whole point. The rest of this receiver is very low power and I don't want to double the power draw with an amplifier. -- Rick |
Very Low Power Preamp
On 11/7/2014 4:40 PM, rickman wrote:
On 11/7/2014 4:23 PM, Jerry Stuckle wrote: On 11/7/2014 3:07 PM, rickman wrote: On 11/7/2014 1:53 PM, Jerry Stuckle wrote: On 11/7/2014 1:26 PM, rickman wrote: On 11/7/2014 1:17 PM, Jerry Stuckle wrote: On 11/7/2014 1:02 PM, rickman wrote: On 11/7/2014 10:49 AM, Jerry Stuckle wrote: On 11/6/2014 11:45 AM, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ . You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. I have done this before and found nothing. But I did it again at both Mouser and Digikey and found several. One listed by Mouser looked especially good only to find rather than 0.75 uA of supply current, it had 0.75 mA of supply current. lol But then the next part, same thing... another one... and another... one part I'm not sure what to make of it. The selection table shows supply current of 0.034 mA and the data sheet shows 25 A! Yes, that's right, the data sheet shows between 25 and 300 Amps for typical supply current!!! I would contact TI about this obvious typo, but this part is not suitable because of the GBW which is also incorrect in the selection table. Same thing at Digikey, everything in the selection table that meets these two requirements is a mistake. A couple of things. First of all, I've found minor errors in the listings at Mouser (I don't use Digikey much), but never real glaring errors. And this is th first time I've seen a TI datasheet that far off. Looks like someone dropped a decimal point :). However, I've found Mouser is interested in correcting errors; they are input by humans, after all, at some point in time, and errors do creep in. Yes, when you list millions of parts there will be errors. I have written digikey many times about listing errors and they always thank me. I'm sure Mouser is no different. Secondly, the current shown is going to be max current, which will depend on the output impedance (and the amount that has to be sourced/sunk). It's not going to pull this all the time; I would expect your actual current draw to be much less since you're 1) going into a high impedance load and 2) not going from rail to rail. I find the opposite. The current listed is under specified conditions which usually *do not* include output drive. In fact, it usually listed as a quiescent current. Well, yes and no. Op amps typically sink more than they source, and the sink current does not come from the chip. Source current at the output is supplied by the chip, of course. And I've found a wide difference between how op amp specs are listed; some show quiescent current, some show average current under typical operating conditions. Some even show maximum current which can be drawn. So I'll retract that statement above. Wasn't thinking clearly. Also, if you use a bipolar supply, then current drain should be less because you'll be operating near ground, instead of the midpoint of a single supply voltage (where the output would be at 1/2 Vcc). Some of these are quite low voltage, and I would think a couple of the larger lithium coin batteries should last quite a while. Not sure how the ground level would affect the bias currents. When the supply voltage is lowered the GBW lowers as well. If the output is at ground level, no current will be pulled from either rail (at the output). Shifting above or below that will draw a little current, reference zero. However, if you're running a single ended supply, your output will be at 1/2 Vcc, and will always be pulling some current to maintain that level. The signal will change that slightly, increasing or decreasing. But unless you have a square wave with a 50% duty cycle, you'll end up needing more current from the single ended supply. What you are saying is only true if your load is ground connected. The load for this circuit will be a voltage source through a high impedance. The input is differential and to make it as sensitive as possible a bias will be applied to one input sufficient to offset the input bias voltage. So in reality the load will be biased to approx 1/2 Vcc. True, but with a bipolar supply, the input is referenced to ground and no current flows with no input. The output is also referenced to ground, so no current flows their, either. And with both input and output at ground potential, there is less current flowing internally. I just explained a scenario where the load will draw current from the amp regardless of power supply arrangement. You are making an assumption that the input and output are ground referenced. That is independent of the supply arrangement. But if they aren't ground referenced, then they must be referenced to an artificial ground, i.e. 1/2 Vcc. And creating that artificial ground will require a certain amount of current. For instance - it's common to bias the input of an op amp running from a single ended supply at 1/2 Vcc. This is generally done with a couple of resistors, in various configurations. But you will always have a small current through those resistors. The lower the impedance of the input, the lower the resistors must be. Output in this case will also be referenced at 1/2 Vcc, which means the op amp output is conducting some current all of the time. Even if the output is capacitive coupled to the load, internally the op amp must draw some current to maintain that 1/2 Vcc. Again, the amount of current is dependent on the output impedance, but it is still there. With a bipolar supply, the op amp doesn't draw input current with no input signal, and doesn't have to source or sink any current when you have 0V output. I hope this is a bit clearer. The real point is that this is *LOAD* current, not amplifier current and is independent of the amplifier and so considered separately since selection of the amplifier has no impact on it. That is true - to an extent. When the op amp is sinking current, the current comes from the load, not the op amp. However, internally there must still be current flowing to provide that sink. You aren't grasping the issue. It is not about which direction the current is flowing, it is about what is responsible for setting the amount of current. The load determines the current that flows in or out of the load and is independent of the opamp characteristics or the power supply arrangement. What I can control by picking the opamp is the current that flows through the opamp that is independent of the load or input. But the direction is important, also. You wanted to know how much the op amp itself will draw; when sourcing the load, you will find more current on Vcc then when the op amp is sinking the load. But when the op amp is sourcing current, the amplifier has to provide the source current plus the drive current. Now you can bias the output so that the op amp is always sinking, but then you have a steady drain from the standby current. When you're using a bipolar supply and input is at ground, the output will also be at ground, and very little current will be flowing. You can create an artificial ground at 1/2 Vcc, but even creating that artificial ground draws current. The difference in current drain is not important in the vast majority of cases. But they become important when you're talking the low drain you wish. None of this is relevant to the issue of picking an amplifier. It is if you're trying to minimize power usage to the maximum extent. I admit my op amp theory is around 40 years old, but I don't think the laws of physics have changed in that time :) As I said before - in the vast majority of cases, the difference in current draw I cited are unimportant. A milliamp or two doesn't generally make much difference. 100 microamps makes a difference even less of the time. But you have a case where it is important. -- ================== Remove the "x" from my email address Jerry Stuckle ================== |
Very Low Power Preamp
On 11/7/2014 5:07 PM, Jerry Stuckle wrote:
On 11/7/2014 4:40 PM, rickman wrote: On 11/7/2014 4:23 PM, Jerry Stuckle wrote: On 11/7/2014 3:07 PM, rickman wrote: On 11/7/2014 1:53 PM, Jerry Stuckle wrote: On 11/7/2014 1:26 PM, rickman wrote: On 11/7/2014 1:17 PM, Jerry Stuckle wrote: On 11/7/2014 1:02 PM, rickman wrote: On 11/7/2014 10:49 AM, Jerry Stuckle wrote: On 11/6/2014 11:45 AM, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ . You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. I have done this before and found nothing. But I did it again at both Mouser and Digikey and found several. One listed by Mouser looked especially good only to find rather than 0.75 uA of supply current, it had 0.75 mA of supply current. lol But then the next part, same thing... another one... and another... one part I'm not sure what to make of it. The selection table shows supply current of 0.034 mA and the data sheet shows 25 A! Yes, that's right, the data sheet shows between 25 and 300 Amps for typical supply current!!! I would contact TI about this obvious typo, but this part is not suitable because of the GBW which is also incorrect in the selection table. Same thing at Digikey, everything in the selection table that meets these two requirements is a mistake. A couple of things. First of all, I've found minor errors in the listings at Mouser (I don't use Digikey much), but never real glaring errors. And this is th first time I've seen a TI datasheet that far off. Looks like someone dropped a decimal point :). However, I've found Mouser is interested in correcting errors; they are input by humans, after all, at some point in time, and errors do creep in. Yes, when you list millions of parts there will be errors. I have written digikey many times about listing errors and they always thank me. I'm sure Mouser is no different. Secondly, the current shown is going to be max current, which will depend on the output impedance (and the amount that has to be sourced/sunk). It's not going to pull this all the time; I would expect your actual current draw to be much less since you're 1) going into a high impedance load and 2) not going from rail to rail. I find the opposite. The current listed is under specified conditions which usually *do not* include output drive. In fact, it usually listed as a quiescent current. Well, yes and no. Op amps typically sink more than they source, and the sink current does not come from the chip. Source current at the output is supplied by the chip, of course. And I've found a wide difference between how op amp specs are listed; some show quiescent current, some show average current under typical operating conditions. Some even show maximum current which can be drawn. So I'll retract that statement above. Wasn't thinking clearly. Also, if you use a bipolar supply, then current drain should be less because you'll be operating near ground, instead of the midpoint of a single supply voltage (where the output would be at 1/2 Vcc). Some of these are quite low voltage, and I would think a couple of the larger lithium coin batteries should last quite a while. Not sure how the ground level would affect the bias currents. When the supply voltage is lowered the GBW lowers as well. If the output is at ground level, no current will be pulled from either rail (at the output). Shifting above or below that will draw a little current, reference zero. However, if you're running a single ended supply, your output will be at 1/2 Vcc, and will always be pulling some current to maintain that level. The signal will change that slightly, increasing or decreasing. But unless you have a square wave with a 50% duty cycle, you'll end up needing more current from the single ended supply. What you are saying is only true if your load is ground connected. The load for this circuit will be a voltage source through a high impedance. The input is differential and to make it as sensitive as possible a bias will be applied to one input sufficient to offset the input bias voltage. So in reality the load will be biased to approx 1/2 Vcc. True, but with a bipolar supply, the input is referenced to ground and no current flows with no input. The output is also referenced to ground, so no current flows their, either. And with both input and output at ground potential, there is less current flowing internally. I just explained a scenario where the load will draw current from the amp regardless of power supply arrangement. You are making an assumption that the input and output are ground referenced. That is independent of the supply arrangement. But if they aren't ground referenced, then they must be referenced to an artificial ground, i.e. 1/2 Vcc. And creating that artificial ground will require a certain amount of current. For instance - it's common to bias the input of an op amp running from a single ended supply at 1/2 Vcc. This is generally done with a couple of resistors, in various configurations. But you will always have a small current through those resistors. The lower the impedance of the input, the lower the resistors must be. Output in this case will also be referenced at 1/2 Vcc, which means the op amp output is conducting some current all of the time. Even if the output is capacitive coupled to the load, internally the op amp must draw some current to maintain that 1/2 Vcc. Again, the amount of current is dependent on the output impedance, but it is still there. With a bipolar supply, the op amp doesn't draw input current with no input signal, and doesn't have to source or sink any current when you have 0V output. I hope this is a bit clearer. It is not a question of clear. It isn't relevant to the power consumption of the opamp. No matter what the reference, somebody, somewhere even if it is in the power supply, is using power sometime unless there are no voltages on any of the resistors in the design. But none of that is relevant to the power consumed by the opamp when in the quiescent state. I'm trying to pick an opamp. I have no need to evaluate the rest of the design when the problem is trying to find an amp with sufficient GBW and low quiescent current. The real point is that this is *LOAD* current, not amplifier current and is independent of the amplifier and so considered separately since selection of the amplifier has no impact on it. That is true - to an extent. When the op amp is sinking current, the current comes from the load, not the op amp. However, internally there must still be current flowing to provide that sink. You aren't grasping the issue. It is not about which direction the current is flowing, it is about what is responsible for setting the amount of current. The load determines the current that flows in or out of the load and is independent of the opamp characteristics or the power supply arrangement. What I can control by picking the opamp is the current that flows through the opamp that is independent of the load or input. But the direction is important, also. You wanted to know how much the op amp itself will draw; when sourcing the load, you will find more current on Vcc then when the op amp is sinking the load.4 No, I am not asking what the opamp will "draw". I'm asking about low power amplifiers. I'd be happy knowing how it is done in the chips in the radio controlled clocks since I'm pretty sure I'm not going to find a standard opamp that will do this. But when the op amp is sourcing current, the amplifier has to provide the source current plus the drive current. Now you can bias the output so that the op amp is always sinking, but then you have a steady drain from the standby current. When you're using a bipolar supply and input is at ground, the output will also be at ground, and very little current will be flowing. You can create an artificial ground at 1/2 Vcc, but even creating that artificial ground draws current. The difference in current drain is not important in the vast majority of cases. But they become important when you're talking the low drain you wish. None of this is relevant to the issue of picking an amplifier. It is if you're trying to minimize power usage to the maximum extent. I admit my op amp theory is around 40 years old, but I don't think the laws of physics have changed in that time :) No, and the laws of relevancy haven't changed either. The power consumed by the load doesn't impact my selection of amplifier. -- Rick |
Very Low Power Preamp
On 11/7/2014 7:18 PM, rickman wrote:
On 11/7/2014 5:07 PM, Jerry Stuckle wrote: On 11/7/2014 4:40 PM, rickman wrote: On 11/7/2014 4:23 PM, Jerry Stuckle wrote: On 11/7/2014 3:07 PM, rickman wrote: On 11/7/2014 1:53 PM, Jerry Stuckle wrote: On 11/7/2014 1:26 PM, rickman wrote: On 11/7/2014 1:17 PM, Jerry Stuckle wrote: On 11/7/2014 1:02 PM, rickman wrote: On 11/7/2014 10:49 AM, Jerry Stuckle wrote: On 11/6/2014 11:45 AM, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ . You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. I have done this before and found nothing. But I did it again at both Mouser and Digikey and found several. One listed by Mouser looked especially good only to find rather than 0.75 uA of supply current, it had 0.75 mA of supply current. lol But then the next part, same thing... another one... and another... one part I'm not sure what to make of it. The selection table shows supply current of 0.034 mA and the data sheet shows 25 A! Yes, that's right, the data sheet shows between 25 and 300 Amps for typical supply current!!! I would contact TI about this obvious typo, but this part is not suitable because of the GBW which is also incorrect in the selection table. Same thing at Digikey, everything in the selection table that meets these two requirements is a mistake. A couple of things. First of all, I've found minor errors in the listings at Mouser (I don't use Digikey much), but never real glaring errors. And this is th first time I've seen a TI datasheet that far off. Looks like someone dropped a decimal point :). However, I've found Mouser is interested in correcting errors; they are input by humans, after all, at some point in time, and errors do creep in. Yes, when you list millions of parts there will be errors. I have written digikey many times about listing errors and they always thank me. I'm sure Mouser is no different. Secondly, the current shown is going to be max current, which will depend on the output impedance (and the amount that has to be sourced/sunk). It's not going to pull this all the time; I would expect your actual current draw to be much less since you're 1) going into a high impedance load and 2) not going from rail to rail. I find the opposite. The current listed is under specified conditions which usually *do not* include output drive. In fact, it usually listed as a quiescent current. Well, yes and no. Op amps typically sink more than they source, and the sink current does not come from the chip. Source current at the output is supplied by the chip, of course. And I've found a wide difference between how op amp specs are listed; some show quiescent current, some show average current under typical operating conditions. Some even show maximum current which can be drawn. So I'll retract that statement above. Wasn't thinking clearly. Also, if you use a bipolar supply, then current drain should be less because you'll be operating near ground, instead of the midpoint of a single supply voltage (where the output would be at 1/2 Vcc). Some of these are quite low voltage, and I would think a couple of the larger lithium coin batteries should last quite a while. Not sure how the ground level would affect the bias currents. When the supply voltage is lowered the GBW lowers as well. If the output is at ground level, no current will be pulled from either rail (at the output). Shifting above or below that will draw a little current, reference zero. However, if you're running a single ended supply, your output will be at 1/2 Vcc, and will always be pulling some current to maintain that level. The signal will change that slightly, increasing or decreasing. But unless you have a square wave with a 50% duty cycle, you'll end up needing more current from the single ended supply. What you are saying is only true if your load is ground connected. The load for this circuit will be a voltage source through a high impedance. The input is differential and to make it as sensitive as possible a bias will be applied to one input sufficient to offset the input bias voltage. So in reality the load will be biased to approx 1/2 Vcc. True, but with a bipolar supply, the input is referenced to ground and no current flows with no input. The output is also referenced to ground, so no current flows their, either. And with both input and output at ground potential, there is less current flowing internally. I just explained a scenario where the load will draw current from the amp regardless of power supply arrangement. You are making an assumption that the input and output are ground referenced. That is independent of the supply arrangement. But if they aren't ground referenced, then they must be referenced to an artificial ground, i.e. 1/2 Vcc. And creating that artificial ground will require a certain amount of current. For instance - it's common to bias the input of an op amp running from a single ended supply at 1/2 Vcc. This is generally done with a couple of resistors, in various configurations. But you will always have a small current through those resistors. The lower the impedance of the input, the lower the resistors must be. Output in this case will also be referenced at 1/2 Vcc, which means the op amp output is conducting some current all of the time. Even if the output is capacitive coupled to the load, internally the op amp must draw some current to maintain that 1/2 Vcc. Again, the amount of current is dependent on the output impedance, but it is still there. With a bipolar supply, the op amp doesn't draw input current with no input signal, and doesn't have to source or sink any current when you have 0V output. I hope this is a bit clearer. It is not a question of clear. It isn't relevant to the power consumption of the opamp. No matter what the reference, somebody, somewhere even if it is in the power supply, is using power sometime unless there are no voltages on any of the resistors in the design. But none of that is relevant to the power consumed by the opamp when in the quiescent state. Yes and no. Op amps are by design bipolar devices; they go plus and minus from some value. It can be zero volts (ground), or it can be some value between Vcc and ground. In the latter case, an artificial ground must be established; by definition this takes current to establish a voltage between Vcc and ground. I'm trying to pick an opamp. I have no need to evaluate the rest of the design when the problem is trying to find an amp with sufficient GBW and low quiescent current. But when you're talking very low power like you are, low quiescent current is very dependent on the power supply, as well as input and output impedance and the chip used. The real point is that this is *LOAD* current, not amplifier current and is independent of the amplifier and so considered separately since selection of the amplifier has no impact on it. That is true - to an extent. When the op amp is sinking current, the current comes from the load, not the op amp. However, internally there must still be current flowing to provide that sink. You aren't grasping the issue. It is not about which direction the current is flowing, it is about what is responsible for setting the amount of current. The load determines the current that flows in or out of the load and is independent of the opamp characteristics or the power supply arrangement. What I can control by picking the opamp is the current that flows through the opamp that is independent of the load or input. But the direction is important, also. You wanted to know how much the op amp itself will draw; when sourcing the load, you will find more current on Vcc then when the op amp is sinking the load.4 No, I am not asking what the opamp will "draw". I'm asking about low power amplifiers. I'd be happy knowing how it is done in the chips in the radio controlled clocks since I'm pretty sure I'm not going to find a standard opamp that will do this. You can; there are a number with that match your requirements. But you need to design the entire circuit around your requirements, not just the op amp. But when the op amp is sourcing current, the amplifier has to provide the source current plus the drive current. Now you can bias the output so that the op amp is always sinking, but then you have a steady drain from the standby current. When you're using a bipolar supply and input is at ground, the output will also be at ground, and very little current will be flowing. You can create an artificial ground at 1/2 Vcc, but even creating that artificial ground draws current. The difference in current drain is not important in the vast majority of cases. But they become important when you're talking the low drain you wish. None of this is relevant to the issue of picking an amplifier. It is if you're trying to minimize power usage to the maximum extent. I admit my op amp theory is around 40 years old, but I don't think the laws of physics have changed in that time :) No, and the laws of relevancy haven't changed either. The power consumed by the load doesn't impact my selection of amplifier. In many cases, it really doesn't matter much. But it does, when you're down in the microamp range. That's what you don't understand. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
Very Low Power Preamp
On 11/7/2014 7:57 PM, Jerry Stuckle wrote:
On 11/7/2014 7:18 PM, rickman wrote: On 11/7/2014 5:07 PM, Jerry Stuckle wrote: On 11/7/2014 4:40 PM, rickman wrote: On 11/7/2014 4:23 PM, Jerry Stuckle wrote: On 11/7/2014 3:07 PM, rickman wrote: On 11/7/2014 1:53 PM, Jerry Stuckle wrote: On 11/7/2014 1:26 PM, rickman wrote: On 11/7/2014 1:17 PM, Jerry Stuckle wrote: On 11/7/2014 1:02 PM, rickman wrote: On 11/7/2014 10:49 AM, Jerry Stuckle wrote: On 11/6/2014 11:45 AM, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ . You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. I have done this before and found nothing. But I did it again at both Mouser and Digikey and found several. One listed by Mouser looked especially good only to find rather than 0.75 uA of supply current, it had 0.75 mA of supply current. lol But then the next part, same thing... another one... and another... one part I'm not sure what to make of it. The selection table shows supply current of 0.034 mA and the data sheet shows 25 A! Yes, that's right, the data sheet shows between 25 and 300 Amps for typical supply current!!! I would contact TI about this obvious typo, but this part is not suitable because of the GBW which is also incorrect in the selection table. Same thing at Digikey, everything in the selection table that meets these two requirements is a mistake. A couple of things. First of all, I've found minor errors in the listings at Mouser (I don't use Digikey much), but never real glaring errors. And this is th first time I've seen a TI datasheet that far off. Looks like someone dropped a decimal point :). However, I've found Mouser is interested in correcting errors; they are input by humans, after all, at some point in time, and errors do creep in. Yes, when you list millions of parts there will be errors. I have written digikey many times about listing errors and they always thank me. I'm sure Mouser is no different. Secondly, the current shown is going to be max current, which will depend on the output impedance (and the amount that has to be sourced/sunk). It's not going to pull this all the time; I would expect your actual current draw to be much less since you're 1) going into a high impedance load and 2) not going from rail to rail. I find the opposite. The current listed is under specified conditions which usually *do not* include output drive. In fact, it usually listed as a quiescent current. Well, yes and no. Op amps typically sink more than they source, and the sink current does not come from the chip. Source current at the output is supplied by the chip, of course. And I've found a wide difference between how op amp specs are listed; some show quiescent current, some show average current under typical operating conditions. Some even show maximum current which can be drawn. So I'll retract that statement above. Wasn't thinking clearly. Also, if you use a bipolar supply, then current drain should be less because you'll be operating near ground, instead of the midpoint of a single supply voltage (where the output would be at 1/2 Vcc). Some of these are quite low voltage, and I would think a couple of the larger lithium coin batteries should last quite a while. Not sure how the ground level would affect the bias currents. When the supply voltage is lowered the GBW lowers as well. If the output is at ground level, no current will be pulled from either rail (at the output). Shifting above or below that will draw a little current, reference zero. However, if you're running a single ended supply, your output will be at 1/2 Vcc, and will always be pulling some current to maintain that level. The signal will change that slightly, increasing or decreasing. But unless you have a square wave with a 50% duty cycle, you'll end up needing more current from the single ended supply. What you are saying is only true if your load is ground connected. The load for this circuit will be a voltage source through a high impedance. The input is differential and to make it as sensitive as possible a bias will be applied to one input sufficient to offset the input bias voltage. So in reality the load will be biased to approx 1/2 Vcc. True, but with a bipolar supply, the input is referenced to ground and no current flows with no input. The output is also referenced to ground, so no current flows their, either. And with both input and output at ground potential, there is less current flowing internally. I just explained a scenario where the load will draw current from the amp regardless of power supply arrangement. You are making an assumption that the input and output are ground referenced. That is independent of the supply arrangement. But if they aren't ground referenced, then they must be referenced to an artificial ground, i.e. 1/2 Vcc. And creating that artificial ground will require a certain amount of current. For instance - it's common to bias the input of an op amp running from a single ended supply at 1/2 Vcc. This is generally done with a couple of resistors, in various configurations. But you will always have a small current through those resistors. The lower the impedance of the input, the lower the resistors must be. Output in this case will also be referenced at 1/2 Vcc, which means the op amp output is conducting some current all of the time. Even if the output is capacitive coupled to the load, internally the op amp must draw some current to maintain that 1/2 Vcc. Again, the amount of current is dependent on the output impedance, but it is still there. With a bipolar supply, the op amp doesn't draw input current with no input signal, and doesn't have to source or sink any current when you have 0V output. I hope this is a bit clearer. It is not a question of clear. It isn't relevant to the power consumption of the opamp. No matter what the reference, somebody, somewhere even if it is in the power supply, is using power sometime unless there are no voltages on any of the resistors in the design. But none of that is relevant to the power consumed by the opamp when in the quiescent state. Yes and no. Op amps are by design bipolar devices; they go plus and minus from some value. It can be zero volts (ground), or it can be some value between Vcc and ground. In the latter case, an artificial ground must be established; by definition this takes current to establish a voltage between Vcc and ground. Actually your characterization of opamps is not accurate. A very few are designed to use dual supplies but most can work with unipolar supplies. Basically they don't know where the ground is and they don't care. The quiescent power the opamp dissipates is not related to where ground is. As I said before, "But none of that is relevant to the power consumed by the opamp when in the quiescent state." I'm trying to pick an opamp. I have no need to evaluate the rest of the design when the problem is trying to find an amp with sufficient GBW and low quiescent current. But when you're talking very low power like you are, low quiescent current is very dependent on the power supply, as well as input and output impedance and the chip used. It is certainly not dependent on the power supply configuration. I can design the rest of the circuit. I'm just looking for a low power device. The real point is that this is *LOAD* current, not amplifier current and is independent of the amplifier and so considered separately since selection of the amplifier has no impact on it. That is true - to an extent. When the op amp is sinking current, the current comes from the load, not the op amp. However, internally there must still be current flowing to provide that sink. You aren't grasping the issue. It is not about which direction the current is flowing, it is about what is responsible for setting the amount of current. The load determines the current that flows in or out of the load and is independent of the opamp characteristics or the power supply arrangement. What I can control by picking the opamp is the current that flows through the opamp that is independent of the load or input. But the direction is important, also. You wanted to know how much the op amp itself will draw; when sourcing the load, you will find more current on Vcc then when the op amp is sinking the load.4 No, I am not asking what the opamp will "draw". I'm asking about low power amplifiers. I'd be happy knowing how it is done in the chips in the radio controlled clocks since I'm pretty sure I'm not going to find a standard opamp that will do this. You can; there are a number with that match your requirements. But you need to design the entire circuit around your requirements, not just the op amp. I only found one that was even close. As I said, the specs listed in the selection guilds were all in error, usually by three orders of magnitude. But when the op amp is sourcing current, the amplifier has to provide the source current plus the drive current. Now you can bias the output so that the op amp is always sinking, but then you have a steady drain from the standby current. When you're using a bipolar supply and input is at ground, the output will also be at ground, and very little current will be flowing. You can create an artificial ground at 1/2 Vcc, but even creating that artificial ground draws current. The difference in current drain is not important in the vast majority of cases. But they become important when you're talking the low drain you wish. None of this is relevant to the issue of picking an amplifier. It is if you're trying to minimize power usage to the maximum extent. I admit my op amp theory is around 40 years old, but I don't think the laws of physics have changed in that time :) No, and the laws of relevancy haven't changed either. The power consumed by the load doesn't impact my selection of amplifier. In many cases, it really doesn't matter much. But it does, when you're down in the microamp range. That's what you don't understand. Ok, thanks for your comments. -- Rick |
Very Low Power Preamp
On 11/7/2014 8:40 PM, rickman wrote:
On 11/7/2014 7:57 PM, Jerry Stuckle wrote: On 11/7/2014 7:18 PM, rickman wrote: On 11/7/2014 5:07 PM, Jerry Stuckle wrote: On 11/7/2014 4:40 PM, rickman wrote: On 11/7/2014 4:23 PM, Jerry Stuckle wrote: On 11/7/2014 3:07 PM, rickman wrote: On 11/7/2014 1:53 PM, Jerry Stuckle wrote: On 11/7/2014 1:26 PM, rickman wrote: On 11/7/2014 1:17 PM, Jerry Stuckle wrote: On 11/7/2014 1:02 PM, rickman wrote: On 11/7/2014 10:49 AM, Jerry Stuckle wrote: On 11/6/2014 11:45 AM, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ . You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. I have done this before and found nothing. But I did it again at both Mouser and Digikey and found several. One listed by Mouser looked especially good only to find rather than 0.75 uA of supply current, it had 0.75 mA of supply current. lol But then the next part, same thing... another one... and another... one part I'm not sure what to make of it. The selection table shows supply current of 0.034 mA and the data sheet shows 25 A! Yes, that's right, the data sheet shows between 25 and 300 Amps for typical supply current!!! I would contact TI about this obvious typo, but this part is not suitable because of the GBW which is also incorrect in the selection table. Same thing at Digikey, everything in the selection table that meets these two requirements is a mistake. A couple of things. First of all, I've found minor errors in the listings at Mouser (I don't use Digikey much), but never real glaring errors. And this is th first time I've seen a TI datasheet that far off. Looks like someone dropped a decimal point :). However, I've found Mouser is interested in correcting errors; they are input by humans, after all, at some point in time, and errors do creep in. Yes, when you list millions of parts there will be errors. I have written digikey many times about listing errors and they always thank me. I'm sure Mouser is no different. Secondly, the current shown is going to be max current, which will depend on the output impedance (and the amount that has to be sourced/sunk). It's not going to pull this all the time; I would expect your actual current draw to be much less since you're 1) going into a high impedance load and 2) not going from rail to rail. I find the opposite. The current listed is under specified conditions which usually *do not* include output drive. In fact, it usually listed as a quiescent current. Well, yes and no. Op amps typically sink more than they source, and the sink current does not come from the chip. Source current at the output is supplied by the chip, of course. And I've found a wide difference between how op amp specs are listed; some show quiescent current, some show average current under typical operating conditions. Some even show maximum current which can be drawn. So I'll retract that statement above. Wasn't thinking clearly. Also, if you use a bipolar supply, then current drain should be less because you'll be operating near ground, instead of the midpoint of a single supply voltage (where the output would be at 1/2 Vcc). Some of these are quite low voltage, and I would think a couple of the larger lithium coin batteries should last quite a while. Not sure how the ground level would affect the bias currents. When the supply voltage is lowered the GBW lowers as well. If the output is at ground level, no current will be pulled from either rail (at the output). Shifting above or below that will draw a little current, reference zero. However, if you're running a single ended supply, your output will be at 1/2 Vcc, and will always be pulling some current to maintain that level. The signal will change that slightly, increasing or decreasing. But unless you have a square wave with a 50% duty cycle, you'll end up needing more current from the single ended supply. What you are saying is only true if your load is ground connected. The load for this circuit will be a voltage source through a high impedance. The input is differential and to make it as sensitive as possible a bias will be applied to one input sufficient to offset the input bias voltage. So in reality the load will be biased to approx 1/2 Vcc. True, but with a bipolar supply, the input is referenced to ground and no current flows with no input. The output is also referenced to ground, so no current flows their, either. And with both input and output at ground potential, there is less current flowing internally. I just explained a scenario where the load will draw current from the amp regardless of power supply arrangement. You are making an assumption that the input and output are ground referenced. That is independent of the supply arrangement. But if they aren't ground referenced, then they must be referenced to an artificial ground, i.e. 1/2 Vcc. And creating that artificial ground will require a certain amount of current. For instance - it's common to bias the input of an op amp running from a single ended supply at 1/2 Vcc. This is generally done with a couple of resistors, in various configurations. But you will always have a small current through those resistors. The lower the impedance of the input, the lower the resistors must be. Output in this case will also be referenced at 1/2 Vcc, which means the op amp output is conducting some current all of the time. Even if the output is capacitive coupled to the load, internally the op amp must draw some current to maintain that 1/2 Vcc. Again, the amount of current is dependent on the output impedance, but it is still there. With a bipolar supply, the op amp doesn't draw input current with no input signal, and doesn't have to source or sink any current when you have 0V output. I hope this is a bit clearer. It is not a question of clear. It isn't relevant to the power consumption of the opamp. No matter what the reference, somebody, somewhere even if it is in the power supply, is using power sometime unless there are no voltages on any of the resistors in the design. But none of that is relevant to the power consumed by the opamp when in the quiescent state. Yes and no. Op amps are by design bipolar devices; they go plus and minus from some value. It can be zero volts (ground), or it can be some value between Vcc and ground. In the latter case, an artificial ground must be established; by definition this takes current to establish a voltage between Vcc and ground. Actually your characterization of opamps is not accurate. A very few are designed to use dual supplies but most can work with unipolar supplies. Basically they don't know where the ground is and they don't care. The quiescent power the opamp dissipates is not related to where ground is. I suggest you build an op amp out of discreet components to understand how it works. It was one of the projects in an EE class. It was very educational to see just how they work. As I said before, "But none of that is relevant to the power consumed by the opamp when in the quiescent state." Actually, it does. See above. I'm trying to pick an opamp. I have no need to evaluate the rest of the design when the problem is trying to find an amp with sufficient GBW and low quiescent current. But when you're talking very low power like you are, low quiescent current is very dependent on the power supply, as well as input and output impedance and the chip used. It is certainly not dependent on the power supply configuration. I can design the rest of the circuit. I'm just looking for a low power device. Once again, it does. But I see I won't convince you. The real point is that this is *LOAD* current, not amplifier current and is independent of the amplifier and so considered separately since selection of the amplifier has no impact on it. That is true - to an extent. When the op amp is sinking current, the current comes from the load, not the op amp. However, internally there must still be current flowing to provide that sink. You aren't grasping the issue. It is not about which direction the current is flowing, it is about what is responsible for setting the amount of current. The load determines the current that flows in or out of the load and is independent of the opamp characteristics or the power supply arrangement. What I can control by picking the opamp is the current that flows through the opamp that is independent of the load or input. But the direction is important, also. You wanted to know how much the op amp itself will draw; when sourcing the load, you will find more current on Vcc then when the op amp is sinking the load.4 No, I am not asking what the opamp will "draw". I'm asking about low power amplifiers. I'd be happy knowing how it is done in the chips in the radio controlled clocks since I'm pretty sure I'm not going to find a standard opamp that will do this. You can; there are a number with that match your requirements. But you need to design the entire circuit around your requirements, not just the op amp. I only found one that was even close. As I said, the specs listed in the selection guilds were all in error, usually by three orders of magnitude. Interesting - I don't know which ones you checked, but I've never found that many errors in Mouser's data sheets. Not to say there aren't errors - I have found several over the years. But not the high percentage you've found. But when the op amp is sourcing current, the amplifier has to provide the source current plus the drive current. Now you can bias the output so that the op amp is always sinking, but then you have a steady drain from the standby current. When you're using a bipolar supply and input is at ground, the output will also be at ground, and very little current will be flowing. You can create an artificial ground at 1/2 Vcc, but even creating that artificial ground draws current. The difference in current drain is not important in the vast majority of cases. But they become important when you're talking the low drain you wish. None of this is relevant to the issue of picking an amplifier. It is if you're trying to minimize power usage to the maximum extent. I admit my op amp theory is around 40 years old, but I don't think the laws of physics have changed in that time :) No, and the laws of relevancy haven't changed either. The power consumed by the load doesn't impact my selection of amplifier. In many cases, it really doesn't matter much. But it does, when you're down in the microamp range. That's what you don't understand. Ok, thanks for your comments. -- ================== Remove the "x" from my email address Jerry Stuckle ================== |
Very Low Power Preamp
On 11/8/2014 3:27 PM, Jerry Stuckle wrote:
On 11/7/2014 8:40 PM, rickman wrote: On 11/7/2014 7:57 PM, Jerry Stuckle wrote: On 11/7/2014 7:18 PM, rickman wrote: On 11/7/2014 5:07 PM, Jerry Stuckle wrote: On 11/7/2014 4:40 PM, rickman wrote: On 11/7/2014 4:23 PM, Jerry Stuckle wrote: On 11/7/2014 3:07 PM, rickman wrote: On 11/7/2014 1:53 PM, Jerry Stuckle wrote: On 11/7/2014 1:26 PM, rickman wrote: On 11/7/2014 1:17 PM, Jerry Stuckle wrote: On 11/7/2014 1:02 PM, rickman wrote: On 11/7/2014 10:49 AM, Jerry Stuckle wrote: On 11/6/2014 11:45 AM, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ . You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. I have done this before and found nothing. But I did it again at both Mouser and Digikey and found several. One listed by Mouser looked especially good only to find rather than 0.75 uA of supply current, it had 0.75 mA of supply current. lol But then the next part, same thing... another one... and another... one part I'm not sure what to make of it. The selection table shows supply current of 0.034 mA and the data sheet shows 25 A! Yes, that's right, the data sheet shows between 25 and 300 Amps for typical supply current!!! I would contact TI about this obvious typo, but this part is not suitable because of the GBW which is also incorrect in the selection table. Same thing at Digikey, everything in the selection table that meets these two requirements is a mistake. A couple of things. First of all, I've found minor errors in the listings at Mouser (I don't use Digikey much), but never real glaring errors. And this is th first time I've seen a TI datasheet that far off. Looks like someone dropped a decimal point :). However, I've found Mouser is interested in correcting errors; they are input by humans, after all, at some point in time, and errors do creep in. Yes, when you list millions of parts there will be errors. I have written digikey many times about listing errors and they always thank me. I'm sure Mouser is no different. Secondly, the current shown is going to be max current, which will depend on the output impedance (and the amount that has to be sourced/sunk). It's not going to pull this all the time; I would expect your actual current draw to be much less since you're 1) going into a high impedance load and 2) not going from rail to rail. I find the opposite. The current listed is under specified conditions which usually *do not* include output drive. In fact, it usually listed as a quiescent current. Well, yes and no. Op amps typically sink more than they source, and the sink current does not come from the chip. Source current at the output is supplied by the chip, of course. And I've found a wide difference between how op amp specs are listed; some show quiescent current, some show average current under typical operating conditions. Some even show maximum current which can be drawn. So I'll retract that statement above. Wasn't thinking clearly. Also, if you use a bipolar supply, then current drain should be less because you'll be operating near ground, instead of the midpoint of a single supply voltage (where the output would be at 1/2 Vcc). Some of these are quite low voltage, and I would think a couple of the larger lithium coin batteries should last quite a while. Not sure how the ground level would affect the bias currents. When the supply voltage is lowered the GBW lowers as well. If the output is at ground level, no current will be pulled from either rail (at the output). Shifting above or below that will draw a little current, reference zero. However, if you're running a single ended supply, your output will be at 1/2 Vcc, and will always be pulling some current to maintain that level. The signal will change that slightly, increasing or decreasing. But unless you have a square wave with a 50% duty cycle, you'll end up needing more current from the single ended supply. What you are saying is only true if your load is ground connected. The load for this circuit will be a voltage source through a high impedance. The input is differential and to make it as sensitive as possible a bias will be applied to one input sufficient to offset the input bias voltage. So in reality the load will be biased to approx 1/2 Vcc. True, but with a bipolar supply, the input is referenced to ground and no current flows with no input. The output is also referenced to ground, so no current flows their, either. And with both input and output at ground potential, there is less current flowing internally. I just explained a scenario where the load will draw current from the amp regardless of power supply arrangement. You are making an assumption that the input and output are ground referenced. That is independent of the supply arrangement. But if they aren't ground referenced, then they must be referenced to an artificial ground, i.e. 1/2 Vcc. And creating that artificial ground will require a certain amount of current. For instance - it's common to bias the input of an op amp running from a single ended supply at 1/2 Vcc. This is generally done with a couple of resistors, in various configurations. But you will always have a small current through those resistors. The lower the impedance of the input, the lower the resistors must be. Output in this case will also be referenced at 1/2 Vcc, which means the op amp output is conducting some current all of the time. Even if the output is capacitive coupled to the load, internally the op amp must draw some current to maintain that 1/2 Vcc. Again, the amount of current is dependent on the output impedance, but it is still there. With a bipolar supply, the op amp doesn't draw input current with no input signal, and doesn't have to source or sink any current when you have 0V output. I hope this is a bit clearer. It is not a question of clear. It isn't relevant to the power consumption of the opamp. No matter what the reference, somebody, somewhere even if it is in the power supply, is using power sometime unless there are no voltages on any of the resistors in the design. But none of that is relevant to the power consumed by the opamp when in the quiescent state. Yes and no. Op amps are by design bipolar devices; they go plus and minus from some value. It can be zero volts (ground), or it can be some value between Vcc and ground. In the latter case, an artificial ground must be established; by definition this takes current to establish a voltage between Vcc and ground. Actually your characterization of opamps is not accurate. A very few are designed to use dual supplies but most can work with unipolar supplies. Basically they don't know where the ground is and they don't care. The quiescent power the opamp dissipates is not related to where ground is. I suggest you build an op amp out of discreet components to understand how it works. It was one of the projects in an EE class. It was very educational to see just how they work. As I said before, "But none of that is relevant to the power consumed by the opamp when in the quiescent state." Actually, it does. See above. I'm trying to pick an opamp. I have no need to evaluate the rest of the design when the problem is trying to find an amp with sufficient GBW and low quiescent current. But when you're talking very low power like you are, low quiescent current is very dependent on the power supply, as well as input and output impedance and the chip used. It is certainly not dependent on the power supply configuration. I can design the rest of the circuit. I'm just looking for a low power device. Once again, it does. But I see I won't convince you. No, you won't. If you are talking about the minute differences in internal biasing, etc, then I can't rule out all effects absolutely, but if you had a valid point you would be able to explain it other than just stating the fact repeatedly. The real point is that this is *LOAD* current, not amplifier current and is independent of the amplifier and so considered separately since selection of the amplifier has no impact on it. That is true - to an extent. When the op amp is sinking current, the current comes from the load, not the op amp. However, internally there must still be current flowing to provide that sink. You aren't grasping the issue. It is not about which direction the current is flowing, it is about what is responsible for setting the amount of current. The load determines the current that flows in or out of the load and is independent of the opamp characteristics or the power supply arrangement. What I can control by picking the opamp is the current that flows through the opamp that is independent of the load or input. But the direction is important, also. You wanted to know how much the op amp itself will draw; when sourcing the load, you will find more current on Vcc then when the op amp is sinking the load.4 No, I am not asking what the opamp will "draw". I'm asking about low power amplifiers. I'd be happy knowing how it is done in the chips in the radio controlled clocks since I'm pretty sure I'm not going to find a standard opamp that will do this. You can; there are a number with that match your requirements. But you need to design the entire circuit around your requirements, not just the op amp. I only found one that was even close. As I said, the specs listed in the selection guilds were all in error, usually by three orders of magnitude. Interesting - I don't know which ones you checked, but I've never found that many errors in Mouser's data sheets. Not to say there aren't errors - I have found several over the years. But not the high percentage you've found. It is not a high percentage. There are over 10,000 amps in Mouser's list. I found around half a dozen or so that were listed incorrectly. Search for amps that are under 40 uA quiescent current and GBW of 6 or more. But when the op amp is sourcing current, the amplifier has to provide the source current plus the drive current. Now you can bias the output so that the op amp is always sinking, but then you have a steady drain from the standby current. When you're using a bipolar supply and input is at ground, the output will also be at ground, and very little current will be flowing. You can create an artificial ground at 1/2 Vcc, but even creating that artificial ground draws current. The difference in current drain is not important in the vast majority of cases. But they become important when you're talking the low drain you wish. None of this is relevant to the issue of picking an amplifier. It is if you're trying to minimize power usage to the maximum extent. I admit my op amp theory is around 40 years old, but I don't think the laws of physics have changed in that time :) No, and the laws of relevancy haven't changed either. The power consumed by the load doesn't impact my selection of amplifier. In many cases, it really doesn't matter much. But it does, when you're down in the microamp range. That's what you don't understand. Ok, thanks for your comments. -- Rick |
Very Low Power Preamp
On 11/8/2014 4:34 PM, rickman wrote:
On 11/8/2014 3:27 PM, Jerry Stuckle wrote: On 11/7/2014 8:40 PM, rickman wrote: On 11/7/2014 7:57 PM, Jerry Stuckle wrote: On 11/7/2014 7:18 PM, rickman wrote: On 11/7/2014 5:07 PM, Jerry Stuckle wrote: On 11/7/2014 4:40 PM, rickman wrote: On 11/7/2014 4:23 PM, Jerry Stuckle wrote: On 11/7/2014 3:07 PM, rickman wrote: On 11/7/2014 1:53 PM, Jerry Stuckle wrote: On 11/7/2014 1:26 PM, rickman wrote: On 11/7/2014 1:17 PM, Jerry Stuckle wrote: On 11/7/2014 1:02 PM, rickman wrote: On 11/7/2014 10:49 AM, Jerry Stuckle wrote: On 11/6/2014 11:45 AM, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ . You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. I have done this before and found nothing. But I did it again at both Mouser and Digikey and found several. One listed by Mouser looked especially good only to find rather than 0.75 uA of supply current, it had 0.75 mA of supply current. lol But then the next part, same thing... another one... and another... one part I'm not sure what to make of it. The selection table shows supply current of 0.034 mA and the data sheet shows 25 A! Yes, that's right, the data sheet shows between 25 and 300 Amps for typical supply current!!! I would contact TI about this obvious typo, but this part is not suitable because of the GBW which is also incorrect in the selection table. Same thing at Digikey, everything in the selection table that meets these two requirements is a mistake. A couple of things. First of all, I've found minor errors in the listings at Mouser (I don't use Digikey much), but never real glaring errors. And this is th first time I've seen a TI datasheet that far off. Looks like someone dropped a decimal point :). However, I've found Mouser is interested in correcting errors; they are input by humans, after all, at some point in time, and errors do creep in. Yes, when you list millions of parts there will be errors. I have written digikey many times about listing errors and they always thank me. I'm sure Mouser is no different. Secondly, the current shown is going to be max current, which will depend on the output impedance (and the amount that has to be sourced/sunk). It's not going to pull this all the time; I would expect your actual current draw to be much less since you're 1) going into a high impedance load and 2) not going from rail to rail. I find the opposite. The current listed is under specified conditions which usually *do not* include output drive. In fact, it usually listed as a quiescent current. Well, yes and no. Op amps typically sink more than they source, and the sink current does not come from the chip. Source current at the output is supplied by the chip, of course. And I've found a wide difference between how op amp specs are listed; some show quiescent current, some show average current under typical operating conditions. Some even show maximum current which can be drawn. So I'll retract that statement above. Wasn't thinking clearly. Also, if you use a bipolar supply, then current drain should be less because you'll be operating near ground, instead of the midpoint of a single supply voltage (where the output would be at 1/2 Vcc). Some of these are quite low voltage, and I would think a couple of the larger lithium coin batteries should last quite a while. Not sure how the ground level would affect the bias currents. When the supply voltage is lowered the GBW lowers as well. If the output is at ground level, no current will be pulled from either rail (at the output). Shifting above or below that will draw a little current, reference zero. However, if you're running a single ended supply, your output will be at 1/2 Vcc, and will always be pulling some current to maintain that level. The signal will change that slightly, increasing or decreasing. But unless you have a square wave with a 50% duty cycle, you'll end up needing more current from the single ended supply. What you are saying is only true if your load is ground connected. The load for this circuit will be a voltage source through a high impedance. The input is differential and to make it as sensitive as possible a bias will be applied to one input sufficient to offset the input bias voltage. So in reality the load will be biased to approx 1/2 Vcc. True, but with a bipolar supply, the input is referenced to ground and no current flows with no input. The output is also referenced to ground, so no current flows their, either. And with both input and output at ground potential, there is less current flowing internally. I just explained a scenario where the load will draw current from the amp regardless of power supply arrangement. You are making an assumption that the input and output are ground referenced. That is independent of the supply arrangement. But if they aren't ground referenced, then they must be referenced to an artificial ground, i.e. 1/2 Vcc. And creating that artificial ground will require a certain amount of current. For instance - it's common to bias the input of an op amp running from a single ended supply at 1/2 Vcc. This is generally done with a couple of resistors, in various configurations. But you will always have a small current through those resistors. The lower the impedance of the input, the lower the resistors must be. Output in this case will also be referenced at 1/2 Vcc, which means the op amp output is conducting some current all of the time. Even if the output is capacitive coupled to the load, internally the op amp must draw some current to maintain that 1/2 Vcc. Again, the amount of current is dependent on the output impedance, but it is still there. With a bipolar supply, the op amp doesn't draw input current with no input signal, and doesn't have to source or sink any current when you have 0V output. I hope this is a bit clearer. It is not a question of clear. It isn't relevant to the power consumption of the opamp. No matter what the reference, somebody, somewhere even if it is in the power supply, is using power sometime unless there are no voltages on any of the resistors in the design. But none of that is relevant to the power consumed by the opamp when in the quiescent state. Yes and no. Op amps are by design bipolar devices; they go plus and minus from some value. It can be zero volts (ground), or it can be some value between Vcc and ground. In the latter case, an artificial ground must be established; by definition this takes current to establish a voltage between Vcc and ground. Actually your characterization of opamps is not accurate. A very few are designed to use dual supplies but most can work with unipolar supplies. Basically they don't know where the ground is and they don't care. The quiescent power the opamp dissipates is not related to where ground is. I suggest you build an op amp out of discreet components to understand how it works. It was one of the projects in an EE class. It was very educational to see just how they work. As I said before, "But none of that is relevant to the power consumed by the opamp when in the quiescent state." Actually, it does. See above. I'm trying to pick an opamp. I have no need to evaluate the rest of the design when the problem is trying to find an amp with sufficient GBW and low quiescent current. But when you're talking very low power like you are, low quiescent current is very dependent on the power supply, as well as input and output impedance and the chip used. It is certainly not dependent on the power supply configuration. I can design the rest of the circuit. I'm just looking for a low power device. Once again, it does. But I see I won't convince you. No, you won't. If you are talking about the minute differences in internal biasing, etc, then I can't rule out all effects absolutely, but if you had a valid point you would be able to explain it other than just stating the fact repeatedly. I have tried explaining it several times. However, you haven't accepted the explanations. I can't help that. The real point is that this is *LOAD* current, not amplifier current and is independent of the amplifier and so considered separately since selection of the amplifier has no impact on it. That is true - to an extent. When the op amp is sinking current, the current comes from the load, not the op amp. However, internally there must still be current flowing to provide that sink. You aren't grasping the issue. It is not about which direction the current is flowing, it is about what is responsible for setting the amount of current. The load determines the current that flows in or out of the load and is independent of the opamp characteristics or the power supply arrangement. What I can control by picking the opamp is the current that flows through the opamp that is independent of the load or input. But the direction is important, also. You wanted to know how much the op amp itself will draw; when sourcing the load, you will find more current on Vcc then when the op amp is sinking the load.4 No, I am not asking what the opamp will "draw". I'm asking about low power amplifiers. I'd be happy knowing how it is done in the chips in the radio controlled clocks since I'm pretty sure I'm not going to find a standard opamp that will do this. You can; there are a number with that match your requirements. But you need to design the entire circuit around your requirements, not just the op amp. I only found one that was even close. As I said, the specs listed in the selection guilds were all in error, usually by three orders of magnitude. Interesting - I don't know which ones you checked, but I've never found that many errors in Mouser's data sheets. Not to say there aren't errors - I have found several over the years. But not the high percentage you've found. It is not a high percentage. There are over 10,000 amps in Mouser's list. I found around half a dozen or so that were listed incorrectly. Search for amps that are under 40 uA quiescent current and GBW of 6 or more. It's a high percentage of those with the specs you need, which was my point. But when the op amp is sourcing current, the amplifier has to provide the source current plus the drive current. Now you can bias the output so that the op amp is always sinking, but then you have a steady drain from the standby current. When you're using a bipolar supply and input is at ground, the output will also be at ground, and very little current will be flowing. You can create an artificial ground at 1/2 Vcc, but even creating that artificial ground draws current. The difference in current drain is not important in the vast majority of cases. But they become important when you're talking the low drain you wish. None of this is relevant to the issue of picking an amplifier. It is if you're trying to minimize power usage to the maximum extent. I admit my op amp theory is around 40 years old, but I don't think the laws of physics have changed in that time :) No, and the laws of relevancy haven't changed either. The power consumed by the load doesn't impact my selection of amplifier. In many cases, it really doesn't matter much. But it does, when you're down in the microamp range. That's what you don't understand. Ok, thanks for your comments. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
Very Low Power Preamp
On 11/8/2014 4:44 PM, Jerry Stuckle wrote:
On 11/8/2014 4:34 PM, rickman wrote: On 11/8/2014 3:27 PM, Jerry Stuckle wrote: On 11/7/2014 8:40 PM, rickman wrote: On 11/7/2014 7:57 PM, Jerry Stuckle wrote: On 11/7/2014 7:18 PM, rickman wrote: On 11/7/2014 5:07 PM, Jerry Stuckle wrote: On 11/7/2014 4:40 PM, rickman wrote: On 11/7/2014 4:23 PM, Jerry Stuckle wrote: On 11/7/2014 3:07 PM, rickman wrote: On 11/7/2014 1:53 PM, Jerry Stuckle wrote: On 11/7/2014 1:26 PM, rickman wrote: On 11/7/2014 1:17 PM, Jerry Stuckle wrote: On 11/7/2014 1:02 PM, rickman wrote: On 11/7/2014 10:49 AM, Jerry Stuckle wrote: On 11/6/2014 11:45 AM, rickman wrote: On 11/6/2014 10:04 AM, Jerry Stuckle wrote: On 11/5/2014 1:29 PM, rickman wrote: On 11/4/2014 9:42 PM, Jerry Stuckle wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I agree with Jim. We need many more specifics to provide a meaningful answer. There are a lot of micropower opamps out there now, but the devil is in the details. I've only found one detail that is giving me the devil. That is the bandwidth. The signal is 60 kHz. I can't think of any other issues I would have with any amp capable of amplifying this signal with a low power level. What more info do you feel is needed? Can you ask questions? Better yet, just point me to any amp that will meet my two stated requirements! The other posts you made had the info - things like impedance and gain are important, as is frequency of operation (but we already know that). A couple of things to consider, however. The higher the impedance, the more susceptible it will be to ambient noise pickup. You're starting with a very small signal and may need to add shielding to limit external noise. The other problem is you're asking for low impedance output. Low impedance limits noise pickup, but increases current drain. So how low of an impedance do you want? I don't follow on this. How does a low output impedance drive the current drain? There are op amps with very high (in the gigaohm range) input impedance and pretty low quiescent current drain. How much it draws during use will be greatly dependent on the output current required, which obviously depends on output voltage and impedance. Consider the current used only by the amp, not the load. I don't have time right now, but later today I'll look through some of my data sheets on op amps to see what I can find. Thanks. Total current is not just dependent on output current; it also is affected by the design of the chip. Op amps are not just single transistor devices; a lower output impedance also means more current to drive the output stage, which affects other components. So even if you have a high impedance load, the lower the output impedance of the op amp (i.e. the more current it can source/sink at a specific supply voltage), the more overall current the op amp will draw. With that said, I did some looking around (sorry for not getting back to you quicker - yesterday was pretty busy). Depending on your needs, there are hundreds you can choose from. I might recommend you check out http://www.mouser.com/Semiconductors...mps/_/N-6j73m/ . You can pick and choose the parameters you want. Another one I've used is http://www.newark.com/operational-amplifiers. Between the two I found several hundred possibilities, but you know the details of what you want better than I do, so rather than guess at what you might want, I think this would be better. It should give you a start. I have done this before and found nothing. But I did it again at both Mouser and Digikey and found several. One listed by Mouser looked especially good only to find rather than 0.75 uA of supply current, it had 0.75 mA of supply current. lol But then the next part, same thing... another one... and another... one part I'm not sure what to make of it. The selection table shows supply current of 0.034 mA and the data sheet shows 25 A! Yes, that's right, the data sheet shows between 25 and 300 Amps for typical supply current!!! I would contact TI about this obvious typo, but this part is not suitable because of the GBW which is also incorrect in the selection table. Same thing at Digikey, everything in the selection table that meets these two requirements is a mistake. A couple of things. First of all, I've found minor errors in the listings at Mouser (I don't use Digikey much), but never real glaring errors. And this is th first time I've seen a TI datasheet that far off. Looks like someone dropped a decimal point :). However, I've found Mouser is interested in correcting errors; they are input by humans, after all, at some point in time, and errors do creep in. Yes, when you list millions of parts there will be errors. I have written digikey many times about listing errors and they always thank me. I'm sure Mouser is no different. Secondly, the current shown is going to be max current, which will depend on the output impedance (and the amount that has to be sourced/sunk). It's not going to pull this all the time; I would expect your actual current draw to be much less since you're 1) going into a high impedance load and 2) not going from rail to rail. I find the opposite. The current listed is under specified conditions which usually *do not* include output drive. In fact, it usually listed as a quiescent current. Well, yes and no. Op amps typically sink more than they source, and the sink current does not come from the chip. Source current at the output is supplied by the chip, of course. And I've found a wide difference between how op amp specs are listed; some show quiescent current, some show average current under typical operating conditions. Some even show maximum current which can be drawn. So I'll retract that statement above. Wasn't thinking clearly. Also, if you use a bipolar supply, then current drain should be less because you'll be operating near ground, instead of the midpoint of a single supply voltage (where the output would be at 1/2 Vcc). Some of these are quite low voltage, and I would think a couple of the larger lithium coin batteries should last quite a while. Not sure how the ground level would affect the bias currents. When the supply voltage is lowered the GBW lowers as well. If the output is at ground level, no current will be pulled from either rail (at the output). Shifting above or below that will draw a little current, reference zero. However, if you're running a single ended supply, your output will be at 1/2 Vcc, and will always be pulling some current to maintain that level. The signal will change that slightly, increasing or decreasing. But unless you have a square wave with a 50% duty cycle, you'll end up needing more current from the single ended supply. What you are saying is only true if your load is ground connected. The load for this circuit will be a voltage source through a high impedance. The input is differential and to make it as sensitive as possible a bias will be applied to one input sufficient to offset the input bias voltage. So in reality the load will be biased to approx 1/2 Vcc. True, but with a bipolar supply, the input is referenced to ground and no current flows with no input. The output is also referenced to ground, so no current flows their, either. And with both input and output at ground potential, there is less current flowing internally. I just explained a scenario where the load will draw current from the amp regardless of power supply arrangement. You are making an assumption that the input and output are ground referenced. That is independent of the supply arrangement. But if they aren't ground referenced, then they must be referenced to an artificial ground, i.e. 1/2 Vcc. And creating that artificial ground will require a certain amount of current. For instance - it's common to bias the input of an op amp running from a single ended supply at 1/2 Vcc. This is generally done with a couple of resistors, in various configurations. But you will always have a small current through those resistors. The lower the impedance of the input, the lower the resistors must be. Output in this case will also be referenced at 1/2 Vcc, which means the op amp output is conducting some current all of the time. Even if the output is capacitive coupled to the load, internally the op amp must draw some current to maintain that 1/2 Vcc. Again, the amount of current is dependent on the output impedance, but it is still there. With a bipolar supply, the op amp doesn't draw input current with no input signal, and doesn't have to source or sink any current when you have 0V output. I hope this is a bit clearer. It is not a question of clear. It isn't relevant to the power consumption of the opamp. No matter what the reference, somebody, somewhere even if it is in the power supply, is using power sometime unless there are no voltages on any of the resistors in the design. But none of that is relevant to the power consumed by the opamp when in the quiescent state. Yes and no. Op amps are by design bipolar devices; they go plus and minus from some value. It can be zero volts (ground), or it can be some value between Vcc and ground. In the latter case, an artificial ground must be established; by definition this takes current to establish a voltage between Vcc and ground. Actually your characterization of opamps is not accurate. A very few are designed to use dual supplies but most can work with unipolar supplies. Basically they don't know where the ground is and they don't care. The quiescent power the opamp dissipates is not related to where ground is. I suggest you build an op amp out of discreet components to understand how it works. It was one of the projects in an EE class. It was very educational to see just how they work. As I said before, "But none of that is relevant to the power consumed by the opamp when in the quiescent state." Actually, it does. See above. I'm trying to pick an opamp. I have no need to evaluate the rest of the design when the problem is trying to find an amp with sufficient GBW and low quiescent current. But when you're talking very low power like you are, low quiescent current is very dependent on the power supply, as well as input and output impedance and the chip used. It is certainly not dependent on the power supply configuration. I can design the rest of the circuit. I'm just looking for a low power device. Once again, it does. But I see I won't convince you. No, you won't. If you are talking about the minute differences in internal biasing, etc, then I can't rule out all effects absolutely, but if you had a valid point you would be able to explain it other than just stating the fact repeatedly. I have tried explaining it several times. However, you haven't accepted the explanations. I can't help that. Yes, and in each case I have explained why your description of what is happening is not valid. The issue I am discussing is the power supply current that is determined by the selection of the amplifier. You seem to want to replace that with all power that flows into the power supply pins of the device even when that power is determined by other parts of the circuit or even power consumed in totally separate parts of the circuit. In other words, you want to address other problems than the one I am addressing. The real point is that this is *LOAD* current, not amplifier current and is independent of the amplifier and so considered separately since selection of the amplifier has no impact on it. That is true - to an extent. When the op amp is sinking current, the current comes from the load, not the op amp. However, internally there must still be current flowing to provide that sink. You aren't grasping the issue. It is not about which direction the current is flowing, it is about what is responsible for setting the amount of current. The load determines the current that flows in or out of the load and is independent of the opamp characteristics or the power supply arrangement. What I can control by picking the opamp is the current that flows through the opamp that is independent of the load or input. But the direction is important, also. You wanted to know how much the op amp itself will draw; when sourcing the load, you will find more current on Vcc then when the op amp is sinking the load.4 No, I am not asking what the opamp will "draw". I'm asking about low power amplifiers. I'd be happy knowing how it is done in the chips in the radio controlled clocks since I'm pretty sure I'm not going to find a standard opamp that will do this. You can; there are a number with that match your requirements. But you need to design the entire circuit around your requirements, not just the op amp. I only found one that was even close. As I said, the specs listed in the selection guilds were all in error, usually by three orders of magnitude. Interesting - I don't know which ones you checked, but I've never found that many errors in Mouser's data sheets. Not to say there aren't errors - I have found several over the years. But not the high percentage you've found. It is not a high percentage. There are over 10,000 amps in Mouser's list. I found around half a dozen or so that were listed incorrectly. Search for amps that are under 40 uA quiescent current and GBW of 6 or more. It's a high percentage of those with the specs you need, which was my point. I'm not sure what you mean. There are *no* parts available with the specs I need. So I guess you could look at it as 100% which is indeed a high percentage. But when the op amp is sourcing current, the amplifier has to provide the source current plus the drive current. Now you can bias the output so that the op amp is always sinking, but then you have a steady drain from the standby current. When you're using a bipolar supply and input is at ground, the output will also be at ground, and very little current will be flowing. You can create an artificial ground at 1/2 Vcc, but even creating that artificial ground draws current. The difference in current drain is not important in the vast majority of cases. But they become important when you're talking the low drain you wish. None of this is relevant to the issue of picking an amplifier. It is if you're trying to minimize power usage to the maximum extent. I admit my op amp theory is around 40 years old, but I don't think the laws of physics have changed in that time :) No, and the laws of relevancy haven't changed either. The power consumed by the load doesn't impact my selection of amplifier. In many cases, it really doesn't matter much. But it does, when you're down in the microamp range. That's what you don't understand. Ok, thanks for your comments. -- Rick |
Very Low Power Preamp
On 11/4/2014 6:29 PM, rickman wrote:
I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I had found one op amp that might get me in the ballpark of power consumption and I did some spice simulation on it. The current ends up being in the 50 uA range which is more than I would like and the gain is only around 100 before the bandwidth limits are felt which is less than I would like. At 50 uA there is not the power to add a second stage. Instead I was looking at some JFETs and found one I like, BF862 made by NXP. I can construct a stage that gives a gain of 40 dB at only a handful of uA. But when I try to cascade a second stage I have trouble. The input capacitance is stated in the data sheet to be in the range of 10 pF. If I add a 10 pF cap to the output of the first stage I get close to 40 dB of gain at the frequency of interest, 60 kHz. But when a second stage is added with capacitive coupling the gain of the first stage drops to 19 dB at 60 kHz while maintaining 40 dB at 1 kHz. As a simple test, I put a capacitor in series with the gate and drove it from a voltage source. I found the gate was at about half the voltage of the voltage source when the capacitor was 300 pF. That says to me the JFET model has 300 pF of capacitance. That just doesn't sound right. I have seen other oddities from trying to drive the input of this part. I have it biased correctly so the gate is not conducting. Any suggestions? I am including the LTspice files below. I found one thread on an audio web site where someone "improved" the model file. Model file - spice_BF862.prm - put in "Simulations" directory below schematic location ******************* * BF862 SPICE MODEL MARCH 2007 NXP SEMICONDUCTORS * ENVELOPE SOT23 * JBF862: 1, Drain, 2,Gate, 3,Source Ld 1 4 L= 1.1nH Ls 3 6 L= 1.25nH Lg 2 5 L= 0.78nH Rg 5 7 R= 0.535 Ohm Cds 1 3 C= 0.0001pF Cgs 2 3 C= 1.05pF Cgd 1 2 C= 0.201pF Co 4 6 C= 0.35092pF JBF862 model parameters: ..model JBF862 NJF(Beta=47.800E-3 Betatce=-.5 Rd=.8 Rs=7.5000 Lambda=37.300E-3 Vto=-.57093 + Vtotc=-2.0000E-3 Is=424.60E-12 Isr=2.995p N=1 Nr=2 Xti=3 Alpha=-1.0000E-3 + Vk=59.97 Cgd=7.4002E-12 M=.6015 Pb=.5 Fc=.5 Cgs=8.2890E-12 Kf=87.5E-18 + Af=1) ENDS BF862 Schematic file - LowPowerPreAmp_JFET.asc ******************* Version 4 SHEET 1 1340 680 WIRE 32 -128 -16 -128 WIRE 128 -128 32 -128 WIRE 368 -128 368 -160 WIRE 1008 -128 1008 -160 WIRE 128 -112 128 -128 WIRE -16 -96 -16 -128 WIRE -16 0 -16 -16 WIRE 128 0 128 -48 WIRE 368 0 368 -48 WIRE 416 0 368 0 WIRE 448 0 416 0 WIRE 608 0 512 0 WIRE 768 0 608 0 WIRE 1008 0 1008 -48 WIRE 1152 0 1008 0 WIRE 1264 0 1152 0 WIRE 368 32 368 0 WIRE 1008 32 1008 0 WIRE 1264 48 1264 0 WIRE 240 96 -16 96 WIRE 320 96 240 96 WIRE 768 96 768 0 WIRE 832 96 768 96 WIRE 960 96 832 96 WIRE 240 144 240 96 WIRE 368 144 368 128 WIRE 448 144 368 144 WIRE 496 144 448 144 WIRE 1008 144 1008 128 WIRE 1088 144 1008 144 WIRE 1136 144 1088 144 WIRE -16 160 -16 96 WIRE 768 160 768 96 WIRE 368 176 368 144 WIRE 1008 176 1008 144 WIRE 496 192 496 144 WIRE 1136 192 1136 144 WIRE 1264 224 1264 112 WIRE 240 256 240 224 WIRE -16 288 -16 240 WIRE 368 288 368 256 WIRE 496 288 496 256 WIRE 496 288 368 288 WIRE 1008 288 1008 256 WIRE 1136 288 1136 256 WIRE 1136 288 1008 288 WIRE 368 336 368 288 WIRE 768 336 768 240 WIRE 1008 336 1008 288 FLAG 368 336 0 FLAG -16 0 0 FLAG 32 -128 V2.2 FLAG -16 96 Vin FLAG 240 256 0 FLAG -16 288 0 FLAG 128 0 0 FLAG 368 -160 V2.2 FLAG 448 144 Vs FLAG 1008 336 0 FLAG 1152 0 Vout FLAG 1008 -160 V2.2 FLAG 1088 144 Vs2 FLAG 768 336 0 FLAG 416 0 G1 FLAG 608 0 Vin2 FLAG 832 96 Vin3 FLAG 1264 224 0 SYMBOL voltage -16 -112 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 124 Left 2 SYMATTR InstName V1 SYMATTR Value 2.2v SYMATTR SpiceLine Rser=1 SYMBOL voltage -16 144 R0 WINDOW 123 24 152 Left 2 WINDOW 39 24 124 Left 2 SYMATTR InstName V2 SYMATTR Value SINE(0 50uV 60K) SYMATTR Value2 AC 1 SYMATTR SpiceLine Rser=10 SYMBOL res 224 128 R0 SYMATTR InstName R1 SYMATTR Value 10Meg SYMBOL cap 112 -112 R0 SYMATTR InstName C5 SYMATTR Value 100µF SYMBOL res 352 -144 R0 SYMATTR InstName R3 SYMATTR Value 100k SYMBOL njf 320 32 R0 SYMATTR InstName T1 SYMATTR Value JBF862 SYMBOL res 352 160 R0 SYMATTR InstName R2 SYMATTR Value 100k SYMBOL cap 480 192 R0 SYMATTR InstName C1 SYMATTR Value 10µF SYMBOL res 992 -144 R0 SYMATTR InstName R6 SYMATTR Value 100k SYMBOL njf 960 32 R0 SYMATTR InstName T2 SYMATTR Value JBF862 SYMBOL res 992 160 R0 SYMATTR InstName R5 SYMATTR Value 100k SYMBOL cap 1120 192 R0 SYMATTR InstName C3 SYMATTR Value 1000nf SYMBOL cap 448 16 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName C2 SYMATTR Value 10µF SYMBOL res 752 144 R0 SYMATTR InstName R4 SYMATTR Value 10Meg SYMBOL cap 1248 48 R0 SYMATTR InstName C4 SYMATTR Value 10pF TEXT 502 -200 Left 2 !.ac dec 10 0.1 10Meg TEXT -24 400 Left 2 !.lib Simulations\\spice_BF862.prm -- Rick |
Very Low Power Preamp
On Sat, 15 Nov 2014 22:17:38 -0500, rickman wrote:
On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I haven't seen the original post, but are you building some type of clock receiver ? Those work for a year with a single battery. What kind of antenna are you using ? Do you really need a preamp ? Do you have room for a tank circuit (L/C) on the collector/drain ? |
Very Low Power Preamp
|
Very Low Power Preamp
On Sun, 16 Nov 2014 03:47:44 -0500, rickman wrote:
On 11/16/2014 3:18 AM, wrote: On Sat, 15 Nov 2014 22:17:38 -0500, rickman wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I haven't seen the original post, but are you building some type of clock receiver ? Those work for a year with a single battery. Yes, it is a radio controlled clock. What kind of antenna are you using ? Do you really need a preamp ? I was planning on a loop antenna made from RG6 cable, but if I have to add an amplifier I may use a ferrite loop. Are you going to use a big (several meters) loop with the RG-6 center conductor as a loop and cutting the shield at the top and using the rest of the cable shield as a grounded static shield and using a small coupling loop into the receiver ? With the main loop resonated by a capacitor to 60 kHz, you should get quite decent signal without preamplifier. For anything smaller, a 5 cm ferrite bar is quite adequate due to the high band noise, even if the ferrite antenna gain might be -40 dBi or even -60 dBi. Do you have room for a tank circuit (L/C) on the collector/drain ? Room should not be a problem. But what is the point of a tank? 1. if you do not have a frequency selective antenna, this tank circuit will provide the selectivity. Since this stage has a low gain at unwanted frequencies, this reduces the risk of IP3 distortion, which becomes critical at low collector/drain currents. 2. you get at least twice the voltage swing compared to the battery voltage. Tapping the inductor or capacitor chain will provide nice impedance matching avoiding the need for a cascaded stage. |
Very Low Power Preamp
rickman wrote:
On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I had found one op amp that might get me in the ballpark of power consumption and I did some spice simulation on it. The current ends up being in the 50 uA range which is more than I would like and the gain is only around 100 before the bandwidth limits are felt which is less than I would like. At 50 uA there is not the power to add a second stage. Instead I was looking at some JFETs and found one I like, BF862 made by NXP. I can construct a stage that gives a gain of 40 dB at only a handful of uA. But when I try to cascade a second stage I have trouble. The input capacitance is stated in the data sheet to be in the range of 10 pF. If I add a 10 pF cap to the output of the first stage I get close to 40 dB of gain at the frequency of interest, 60 kHz. But when a second stage is added with capacitive coupling the gain of the first stage drops to 19 dB at 60 kHz while maintaining 40 dB at 1 kHz. You need a FET with an input capacitance an order of magnitude lower. Got to run now and can't find it so quickly but ask John Larkin. He suggested a FET a while ago that is IIRC under 1pF. Dual gate FETs are another option. An example, although this one still has 2pF at gate 1: http://www.nxp.com/documents/data_sheet/BF998.pdf Have you tried BJTs? Only sad thing is, many of the very low power Japanese ones have been discontinued. [...] -- Regards, Joerg http://www.analogconsultants.com/ |
Very Low Power Preamp
On Sun, 16 Nov 2014 08:14:11 -0800, Joerg
wrote: rickman wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I had found one op amp that might get me in the ballpark of power consumption and I did some spice simulation on it. The current ends up being in the 50 uA range which is more than I would like and the gain is only around 100 before the bandwidth limits are felt which is less than I would like. At 50 uA there is not the power to add a second stage. Instead I was looking at some JFETs and found one I like, BF862 made by NXP. I can construct a stage that gives a gain of 40 dB at only a handful of uA. But when I try to cascade a second stage I have trouble. The input capacitance is stated in the data sheet to be in the range of 10 pF. If I add a 10 pF cap to the output of the first stage I get close to 40 dB of gain at the frequency of interest, 60 kHz. But when a second stage is added with capacitive coupling the gain of the first stage drops to 19 dB at 60 kHz while maintaining 40 dB at 1 kHz. You need a FET with an input capacitance an order of magnitude lower. Got to run now and can't find it so quickly but ask John Larkin. He suggested a FET a while ago that is IIRC under 1pF. NE3509 maybe... a bit under 1 pF. Phemts have high 1/f noise corners, so I don't know how well they might work at 60 KHz and low current. Phil probably has lf noise data on a Skyworks part. The key to low-noise, low-power gain in narrowband amps is proper input network tuning. A tuned circuit makes voltage gain for zero power consumption. Ditto interstage coupling. This problem may not actually need a super-low-capacitance part. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com |
Very Low Power Preamp
On 11/16/2014 5:08 AM, wrote:
On Sun, 16 Nov 2014 03:47:44 -0500, rickman wrote: On 11/16/2014 3:18 AM, wrote: On Sat, 15 Nov 2014 22:17:38 -0500, rickman wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I haven't seen the original post, but are you building some type of clock receiver ? Those work for a year with a single battery. Yes, it is a radio controlled clock. What kind of antenna are you using ? Do you really need a preamp ? I was planning on a loop antenna made from RG6 cable, but if I have to add an amplifier I may use a ferrite loop. Are you going to use a big (several meters) loop with the RG-6 center conductor as a loop and cutting the shield at the top and using the rest of the cable shield as a grounded static shield and using a small coupling loop into the receiver ? With the main loop resonated by a capacitor to 60 kHz, you should get quite decent signal without preamplifier. That's the general idea but in an 8 turn 2 foot loop. I may add another 50 foot of RG6 (helps with the split) to boost the signal further. "Decent" must be defined. This signal is not so strong, 100 uV/m and this loop will only give 26 uV counting a Q of 90 which might not fully materialize by the time it is plugged into the receiver. As I think about this (I do more thinking than designing sometimes) I am becoming less and less convinced I can do this without a preamp. For anything smaller, a 5 cm ferrite bar is quite adequate due to the high band noise, even if the ferrite antenna gain might be -40 dBi or even -60 dBi. Not sure why you would compare the ferrite antenna to an isotropic antenna, but when compared to the 2 foot loop the equations show the 2 foot loop provides a stronger signal. Now that I am considering a preamp I may return to the idea of the ferrite loop antenna. Lol, if I do that I can explore the joys of Litz wire. Do you have room for a tank circuit (L/C) on the collector/drain ? Room should not be a problem. But what is the point of a tank? 1. if you do not have a frequency selective antenna, this tank circuit will provide the selectivity. Since this stage has a low gain at unwanted frequencies, this reduces the risk of IP3 distortion, which becomes critical at low collector/drain currents. The antenna is already highly tuned to the frequency of interest. Unlike voice broadcasts the bandwidth of this signal is just double digit Hz so a Q as high as feasible is useful. That is why RG6 was picked, with a center conductor that pushes the skin effect the Q will be as good as practical (without using Litz wire, lol). 2. you get at least twice the voltage swing compared to the battery voltage. Tapping the inductor or capacitor chain will provide nice impedance matching avoiding the need for a cascaded stage. Impedance matching to what? My "receiver" is an FPGA with a rather high impedance input, measured in Megaohms in parallel with single digit pF. I don't need to worry about Vcc (or Vdd) limiting voltage swing even with the amp the voltage is low. If the tuned circuit will boost the voltage otherwise I would consider it. Would a tank circuit be put in series with a resistance? Otherwise how is the DC point established? I am using a source resistor with bypass cap to bias the gate-source voltage (reminds me of tube circuits) but a resistor is also needed in the drain connection, no? -- Rick |
Very Low Power Preamp
On 11/16/2014 1:54 PM, John Larkin wrote:
On Sun, 16 Nov 2014 08:14:11 -0800, Joerg wrote: rickman wrote: On 11/4/2014 6:29 PM, rickman wrote: I am working on a project for receiving a very narrow bandwidth signal at 60 kHz. One of the design goals is to keep the power consumption to an absolute minimum. I'm trying to figure out how to run a pre-amplifier on less than 100 uW. So far I have found nothing. Any suggestions? I had found one op amp that might get me in the ballpark of power consumption and I did some spice simulation on it. The current ends up being in the 50 uA range which is more than I would like and the gain is only around 100 before the bandwidth limits are felt which is less than I would like. At 50 uA there is not the power to add a second stage. Instead I was looking at some JFETs and found one I like, BF862 made by NXP. I can construct a stage that gives a gain of 40 dB at only a handful of uA. But when I try to cascade a second stage I have trouble. The input capacitance is stated in the data sheet to be in the range of 10 pF. If I add a 10 pF cap to the output of the first stage I get close to 40 dB of gain at the frequency of interest, 60 kHz. But when a second stage is added with capacitive coupling the gain of the first stage drops to 19 dB at 60 kHz while maintaining 40 dB at 1 kHz. You need a FET with an input capacitance an order of magnitude lower. Got to run now and can't find it so quickly but ask John Larkin. He suggested a FET a while ago that is IIRC under 1pF. NE3509 maybe... a bit under 1 pF. Phemts have high 1/f noise corners, so I don't know how well they might work at 60 KHz and low current. Phil probably has lf noise data on a Skyworks part. The key to low-noise, low-power gain in narrowband amps is proper input network tuning. A tuned circuit makes voltage gain for zero power consumption. Ditto interstage coupling. This problem may not actually need a super-low-capacitance part. Thanks for the suggestion. Noise shouldn't be a problem in this app. The noise is typically dominated by terrestrial sources of interference. The antenna has a Q of 90 but the signal is still very low level. The thing I don't get is that the BF862 data sheet says the gate source capacitance is in the 10 pF ballpark. But in the simulation it seems to be more like 300 pF. The frequency response curves don't look anything like capacitive loading either. Is this some strange non-linear thing because I am using the part with a very low drain current ~5 uA? Someone here pointed out to me once that at low collector currents the gain falls off. That didn't make a lot of sense until just now I was looking at the ID vs VG1 of the BBF998 and I realized how it is like a leaky faucet. You can easily change the flow rate from 1 gal/min to 1.1 gal/min. But trying to change it from 1 drop per minute to 1.1 drop per minute is not so easy. The curve is asymptotic to the X axis making it very hard to get much change in current as it approaches 0. -- Rick |
| All times are GMT +1. The time now is 05:21 AM. |
|
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com