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#1
November 16th 14, 03:17 AM
posted to rec.radio.amateur.equipment,sci.electronics.design
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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 |
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#2
November 16th 14, 08:18 AM
posted to rec.radio.amateur.equipment,sci.electronics.design
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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 ? |
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#3
November 16th 14, 08:47 AM
posted to rec.radio.amateur.equipment,sci.electronics.design
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Very Low Power Preamp
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#4
November 16th 14, 10:08 AM
posted to rec.radio.amateur.equipment,sci.electronics.design
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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. |
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#6
November 16th 14, 04:14 PM
posted to rec.radio.amateur.equipment,sci.electronics.design
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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/ |
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#7
November 16th 14, 06:54 PM
posted to rec.radio.amateur.equipment,sci.electronics.design
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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 |
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#8
November 16th 14, 08:41 PM
posted to rec.radio.amateur.equipment,sci.electronics.design
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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 |
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#9
November 17th 14, 03:58 PM
posted to rec.radio.amateur.equipment,sci.electronics.design
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Very Low Power Preamp
On 11/16/2014 01:54 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. My data are pretty limited--a few examples of the SKY65050 had flatband 1-Hz noise of about 0.4 nV and a 1/f corner of about 50 MHz. (!) The Avago ATF38143s I tested were quieter, 0.35 nV and 10 MHz, which is still pretty high--JFETs usually come in at about 1 kHz and good bipolars, 10 Hz. Besides the 1/f noise, the main down side of pHEMTs is that they have pathetically low drain impedances, down in the hundreds of ohms. That makes them useless for followers, for instance, and seriously limits the voltage gain you can get out of them. Cascoding them helps a lot. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net |
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#10
November 16th 14, 09:18 PM
posted to rec.radio.amateur.equipment,sci.electronics.design
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Very Low Power Preamp
On 11/16/2014 11:14 AM, 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. Can you explain this? The info I have on the BF862 shows 10 pF range gate to source. If I replace the FET gate with a 10 pF load on the first stage the output looks normal. With the FET in place the frequency response of the first stage output looks very bad with a complex curve rather than just a simple capacitor loading. 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. I know nothing about dual gate FETs. The BJTs I have looked at won't get the high input impedance or the low biasing currents I want. The JFET is operating with about 5 uA of drain current. -- Rick |
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