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Transistorized "cascode" RF amp?
Back from good old tube days, cascoded triode RF preamps were good for high
gain, stability and low noise. Anything out there in transistorised version, or there is there better stuff available? Looking mainly for preamps on HF to be used with low gain antennas, like small loops or beverages. Yuri, K3BU.us |
Jim Kortge, K8IQY, uses a cascode amp in his famous 2N2/40. He actually
uses it in the transmitter ... even with a passive mixer you don't need a lot of gain on 40! See http://www.qsl.net/k8iqy ... "Yuri Blanarovich" wrote in message ... Back from good old tube days, cascoded triode RF preamps were good for high gain, stability and low noise. Anything out there in transistorised version, or there is there better stuff available? Looking mainly for preamps on HF to be used with low gain antennas, like small loops or beverages. Yuri, K3BU.us |
Jim Kortge, K8IQY, uses a cascode amp in his famous 2N2/40. He actually
uses it in the transmitter ... even with a passive mixer you don't need a lot of gain on 40! See http://www.qsl.net/k8iqy ... "Yuri Blanarovich" wrote in message ... Back from good old tube days, cascoded triode RF preamps were good for high gain, stability and low noise. Anything out there in transistorised version, or there is there better stuff available? Looking mainly for preamps on HF to be used with low gain antennas, like small loops or beverages. Yuri, K3BU.us |
"Yuri Blanarovich" wrote in message ... Back from good old tube days, cascoded triode RF preamps were good for high gain, stability and low noise. Anything out there in transistorised version, or there is there better stuff available? Looking mainly for preamps on HF to be used with low gain antennas, like small loops or beverages. Yuri, K3BU.us Back in the 'good old tube days' it was difficult to get below a NF of 3 db on 2 meters. Now it is easy to get way below that with a good FET. For HF almost any RF device now will take you very low in NF with good gain. There are versions of the cascode that use FETs for HF and above. |
"Yuri Blanarovich" wrote in message ... Back from good old tube days, cascoded triode RF preamps were good for high gain, stability and low noise. Anything out there in transistorised version, or there is there better stuff available? Looking mainly for preamps on HF to be used with low gain antennas, like small loops or beverages. Yuri, K3BU.us Back in the 'good old tube days' it was difficult to get below a NF of 3 db on 2 meters. Now it is easy to get way below that with a good FET. For HF almost any RF device now will take you very low in NF with good gain. There are versions of the cascode that use FETs for HF and above. |
Yuri Blanarovich ) writes:
Back from good old tube days, cascoded triode RF preamps were good for high gain, stability and low noise. Anything out there in transistorised version, or there is there better stuff available? Looking mainly for preamps on HF to be used with low gain antennas, like small loops or beverages. Yuri, K3BU.us You definitely saw them in the early days. The one that comes immediately to mind was a cheap preamp described in Ham Radio in the early seventies, and was available as a cheap kit from Hamtronics. I think there were some shown with bipolars, but I can't dredge up any specific memories at the moment. It was obviously a transitional thing, since you're right, one doesn't see solid state cascodes very often. I have no idea if they were used decades ago because it worked around limitations of early solid state devices, or if it just seemed to be the thing to do since they had been common in tube circuits. The author of that Ham Radio article (actually, there were two, and he used cascode FET amplifiers in various projects described later), I think his name was Jerry Voigt, had grumlbed about the then relatively new dual-gate MOSFETs, but in a followup letter he admitted than a dual-gate MOSFET was basically a cascode device. If there was an advantage to be using cascode circuits today, then you'd be seeing them. A lot has changed. Bipolars came in, and they couldn't handle strong signals well. The JFET came along, and it was seen as the device to use for best performance. The MOSFET came along, and again there was a switch. Sort of simultaneously with MOSFETs, but not really adopted till later, bipolars started being capable of low noise and strong signal handling. MOSFETs are now rare for receiving applications. Gasfets came along and they seem to be be the thing for low noise RF amplification. Michael VE2BVW |
Yuri Blanarovich ) writes:
Back from good old tube days, cascoded triode RF preamps were good for high gain, stability and low noise. Anything out there in transistorised version, or there is there better stuff available? Looking mainly for preamps on HF to be used with low gain antennas, like small loops or beverages. Yuri, K3BU.us You definitely saw them in the early days. The one that comes immediately to mind was a cheap preamp described in Ham Radio in the early seventies, and was available as a cheap kit from Hamtronics. I think there were some shown with bipolars, but I can't dredge up any specific memories at the moment. It was obviously a transitional thing, since you're right, one doesn't see solid state cascodes very often. I have no idea if they were used decades ago because it worked around limitations of early solid state devices, or if it just seemed to be the thing to do since they had been common in tube circuits. The author of that Ham Radio article (actually, there were two, and he used cascode FET amplifiers in various projects described later), I think his name was Jerry Voigt, had grumlbed about the then relatively new dual-gate MOSFETs, but in a followup letter he admitted than a dual-gate MOSFET was basically a cascode device. If there was an advantage to be using cascode circuits today, then you'd be seeing them. A lot has changed. Bipolars came in, and they couldn't handle strong signals well. The JFET came along, and it was seen as the device to use for best performance. The MOSFET came along, and again there was a switch. Sort of simultaneously with MOSFETs, but not really adopted till later, bipolars started being capable of low noise and strong signal handling. MOSFETs are now rare for receiving applications. Gasfets came along and they seem to be be the thing for low noise RF amplification. Michael VE2BVW |
xpyttl wrote:
Jim Kortge, K8IQY, uses a cascode amp in his famous 2N2/40. He actually uses it in the transmitter ... even with a passive mixer you don't need a lot of gain on 40! See http://www.qsl.net/k8iqy .. There are two ways of looking at cascade, the devices are in series for dc and ac, or only ac. The above link shows the latter. I had an idea for a hybrid cascade amp of the first kind. The input would be a jfet which sorta becomes the emitter resistor for a bipolar transistor as the second stage of the cascade amp. The bipolar is run in grounded base (for ac, normal bias network and base bypassed to ground with a cap). Would give the advantages of high input impedance of the fet and high gain from the bipolar. |
xpyttl wrote:
Jim Kortge, K8IQY, uses a cascode amp in his famous 2N2/40. He actually uses it in the transmitter ... even with a passive mixer you don't need a lot of gain on 40! See http://www.qsl.net/k8iqy .. There are two ways of looking at cascade, the devices are in series for dc and ac, or only ac. The above link shows the latter. I had an idea for a hybrid cascade amp of the first kind. The input would be a jfet which sorta becomes the emitter resistor for a bipolar transistor as the second stage of the cascade amp. The bipolar is run in grounded base (for ac, normal bias network and base bypassed to ground with a cap). Would give the advantages of high input impedance of the fet and high gain from the bipolar. |
Ken Scharf ) writes:
xpyttl wrote: Jim Kortge, K8IQY, uses a cascode amp in his famous 2N2/40. He actually uses it in the transmitter ... even with a passive mixer you don't need a lot of gain on 40! See http://www.qsl.net/k8iqy .. There are two ways of looking at cascade, the devices are in series for dc and ac, or only ac. The above link shows the latter. I had an idea for a hybrid cascade amp of the first kind. The input would be a jfet which sorta becomes the emitter resistor for a bipolar transistor as the second stage of the cascade amp. The bipolar is run in grounded base (for ac, normal bias network and base bypassed to ground with a cap). Would give the advantages of high input impedance of the fet and high gain from the bipolar. It's been done. There was an article in Ham Radio magazine in 1970 or 1971 about a six meter converter. Just as you described, a common source JFET with a bipolar transistor as the upper element. Used a Vackar oscillator to make it tuneable. It might be this one: Six-meter converter, improved K1BQT 50 Aug 70 The date is about when I remember it, though I don't remember K1BQT being the author. Michael VE2BVW |
Ken Scharf ) writes:
xpyttl wrote: Jim Kortge, K8IQY, uses a cascode amp in his famous 2N2/40. He actually uses it in the transmitter ... even with a passive mixer you don't need a lot of gain on 40! See http://www.qsl.net/k8iqy .. There are two ways of looking at cascade, the devices are in series for dc and ac, or only ac. The above link shows the latter. I had an idea for a hybrid cascade amp of the first kind. The input would be a jfet which sorta becomes the emitter resistor for a bipolar transistor as the second stage of the cascade amp. The bipolar is run in grounded base (for ac, normal bias network and base bypassed to ground with a cap). Would give the advantages of high input impedance of the fet and high gain from the bipolar. It's been done. There was an article in Ham Radio magazine in 1970 or 1971 about a six meter converter. Just as you described, a common source JFET with a bipolar transistor as the upper element. Used a Vackar oscillator to make it tuneable. It might be this one: Six-meter converter, improved K1BQT 50 Aug 70 The date is about when I remember it, though I don't remember K1BQT being the author. Michael VE2BVW |
Michael Black wrote:
Ken Scharf ) writes: xpyttl wrote: Jim Kortge, K8IQY, uses a cascode amp in his famous 2N2/40. He actually uses it in the transmitter ... even with a passive mixer you don't need a lot of gain on 40! See http://www.qsl.net/k8iqy .. There are two ways of looking at cascade, the devices are in series for dc and ac, or only ac. The above link shows the latter. I had an idea for a hybrid cascade amp of the first kind. The input would be a jfet which sorta becomes the emitter resistor for a bipolar transistor as the second stage of the cascade amp. The bipolar is run in grounded base (for ac, normal bias network and base bypassed to ground with a cap). Would give the advantages of high input impedance of the fet and high gain from the bipolar. It's been done. There was an article in Ham Radio magazine in 1970 or 1971 about a six meter converter. Just as you described, a common source JFET with a bipolar transistor as the upper element. Used a Vackar oscillator to make it tuneable. It might be this one: Six-meter converter, improved K1BQT 50 Aug 70 The date is about when I remember it, though I don't remember K1BQT being the author. Michael VE2BVW Nothing new under the sun! Guess I need to get the HamRadio collection on CRrom. |
Michael Black wrote:
Ken Scharf ) writes: xpyttl wrote: Jim Kortge, K8IQY, uses a cascode amp in his famous 2N2/40. He actually uses it in the transmitter ... even with a passive mixer you don't need a lot of gain on 40! See http://www.qsl.net/k8iqy .. There are two ways of looking at cascade, the devices are in series for dc and ac, or only ac. The above link shows the latter. I had an idea for a hybrid cascade amp of the first kind. The input would be a jfet which sorta becomes the emitter resistor for a bipolar transistor as the second stage of the cascade amp. The bipolar is run in grounded base (for ac, normal bias network and base bypassed to ground with a cap). Would give the advantages of high input impedance of the fet and high gain from the bipolar. It's been done. There was an article in Ham Radio magazine in 1970 or 1971 about a six meter converter. Just as you described, a common source JFET with a bipolar transistor as the upper element. Used a Vackar oscillator to make it tuneable. It might be this one: Six-meter converter, improved K1BQT 50 Aug 70 The date is about when I remember it, though I don't remember K1BQT being the author. Michael VE2BVW Nothing new under the sun! Guess I need to get the HamRadio collection on CRrom. |
Sure, there are _op-amps_ that have cascode input stages. There are
bootstrapped cascode input stages used in feedback amplifiers that have extremely low input capacitance and very low distortion when driven from high source impedances. One of the FM tuner manufacturers long ago used a cascode input stage. With transistor amps, you can run the input transistor at low voltage and therefore low dissipation, and have the output transistor capable of large swings, and thus get good dynamic range. I don't know that there's any particular noise advantage, per se. But it does let you pick the best transistor for each part. Another variation with transistors is a "folded cascode" where the input transistor is one polarity (e.g. NPN) and the output one is the opposite polarity (PNP). Cheers, Tom oUsama (Yuri Blanarovich) wrote in message ... Back from good old tube days, cascoded triode RF preamps were good for high gain, stability and low noise. Anything out there in transistorised version, or there is there better stuff available? Looking mainly for preamps on HF to be used with low gain antennas, like small loops or beverages. Yuri, K3BU.us |
Sure, there are _op-amps_ that have cascode input stages. There are
bootstrapped cascode input stages used in feedback amplifiers that have extremely low input capacitance and very low distortion when driven from high source impedances. One of the FM tuner manufacturers long ago used a cascode input stage. With transistor amps, you can run the input transistor at low voltage and therefore low dissipation, and have the output transistor capable of large swings, and thus get good dynamic range. I don't know that there's any particular noise advantage, per se. But it does let you pick the best transistor for each part. Another variation with transistors is a "folded cascode" where the input transistor is one polarity (e.g. NPN) and the output one is the opposite polarity (PNP). Cheers, Tom oUsama (Yuri Blanarovich) wrote in message ... Back from good old tube days, cascoded triode RF preamps were good for high gain, stability and low noise. Anything out there in transistorised version, or there is there better stuff available? Looking mainly for preamps on HF to be used with low gain antennas, like small loops or beverages. Yuri, K3BU.us |
Thanks for pointers, I will have look at references mentioned.
Another requirement would be good handling strong signals, like using 807 :-) I am trying to build the converter with strong signal handling capabilities, low noise and high gain. Next step is looking at the mixers and low noise synthesizers. Thanks a bunch! Yuri, K3BU |
Thanks for pointers, I will have look at references mentioned.
Another requirement would be good handling strong signals, like using 807 :-) I am trying to build the converter with strong signal handling capabilities, low noise and high gain. Next step is looking at the mixers and low noise synthesizers. Thanks a bunch! Yuri, K3BU |
"Yuri Blanarovich" wrote in message ... Thanks for pointers, I will have look at references mentioned. Another requirement would be good handling strong signals, like using 807 :-) I am trying to build the converter with strong signal handling capabilities, low noise and high gain. Next step is looking at the mixers and low noise synthesizers. Thanks a bunch! Yuri, K3BU You are working at it wht wrong way. The three things you mentioned do not go together. What you want is somthting with strong signal handling and low noise. You use just enough gain to override the mixer noise. Then make up the gain at the IF stages after the filters. |
"Yuri Blanarovich" wrote in message ... Thanks for pointers, I will have look at references mentioned. Another requirement would be good handling strong signals, like using 807 :-) I am trying to build the converter with strong signal handling capabilities, low noise and high gain. Next step is looking at the mixers and low noise synthesizers. Thanks a bunch! Yuri, K3BU You are working at it wht wrong way. The three things you mentioned do not go together. What you want is somthting with strong signal handling and low noise. You use just enough gain to override the mixer noise. Then make up the gain at the IF stages after the filters. |
"Ralph Mowery" ) writes:
"Yuri Blanarovich" wrote in message ... Thanks for pointers, I will have look at references mentioned. Another requirement would be good handling strong signals, like using 807 :-) I am trying to build the converter with strong signal handling capabilities, low noise and high gain. Next step is looking at the mixers and low noise synthesizers. Thanks a bunch! Yuri, K3BU You are working at it wht wrong way. The three things you mentioned do not go together. What you want is somthting with strong signal handling and low noise. You use just enough gain to override the mixer noise. Then make up the gain at the IF stages after the filters. Or, no rf stage at all. I can't remember what frequency this if for, if it was mentioned. An RF stage is only needed if there is need, like one needs front end selectivity to eliminate images or to keep the mixer from overloading, and the amplification is there to overcome the losses of the filter. Another need would be that a good low noise stage is needed because what comes later is too noisy. Michael VE2BVW |
"Ralph Mowery" ) writes:
"Yuri Blanarovich" wrote in message ... Thanks for pointers, I will have look at references mentioned. Another requirement would be good handling strong signals, like using 807 :-) I am trying to build the converter with strong signal handling capabilities, low noise and high gain. Next step is looking at the mixers and low noise synthesizers. Thanks a bunch! Yuri, K3BU You are working at it wht wrong way. The three things you mentioned do not go together. What you want is somthting with strong signal handling and low noise. You use just enough gain to override the mixer noise. Then make up the gain at the IF stages after the filters. Or, no rf stage at all. I can't remember what frequency this if for, if it was mentioned. An RF stage is only needed if there is need, like one needs front end selectivity to eliminate images or to keep the mixer from overloading, and the amplification is there to overcome the losses of the filter. Another need would be that a good low noise stage is needed because what comes later is too noisy. Michael VE2BVW |
Ralph wrote:
You are working at it wht wrong way. The three things you mentioned do not go together. What you want is somthting with strong signal handling and low noise. You use just enough gain to override the mixer noise. Then make up the gain at the IF stages after the filters. Not really. What you say is the general "wisdom". I want low noise, high (adjustable) gain preselector. Something with below 0.1 uV. There are situations when that is needed (small loops receiving antennas, Beverages on low bands, etc.) The gain should be adjusable so it is (band noise) just above the noise threshold of the mixer for particular antenna/band situation. At the same time have high dynamic range to handle strong signals S9+60dB without overload. Way back I modified Drake R4B by putting 6EH7 as preselector with separate RF gain control. It worked very well and it is still hard to beat. Yuri, www.K3BU.us |
Ralph wrote:
You are working at it wht wrong way. The three things you mentioned do not go together. What you want is somthting with strong signal handling and low noise. You use just enough gain to override the mixer noise. Then make up the gain at the IF stages after the filters. Not really. What you say is the general "wisdom". I want low noise, high (adjustable) gain preselector. Something with below 0.1 uV. There are situations when that is needed (small loops receiving antennas, Beverages on low bands, etc.) The gain should be adjusable so it is (band noise) just above the noise threshold of the mixer for particular antenna/band situation. At the same time have high dynamic range to handle strong signals S9+60dB without overload. Way back I modified Drake R4B by putting 6EH7 as preselector with separate RF gain control. It worked very well and it is still hard to beat. Yuri, www.K3BU.us |
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The tuner I mentioned did the cascode specifically to achieve low
noise and the ability to handle big signals with low distortion. Somewhere in my disorganized archives I think I have more info on it, but don't ask me to look! ;-) As I recall, they used a transistor billed for CATV service with low noise and good linearity on the input, and an RF power mosfet on the output--one of the old Siliconix VMOS ones?? Haven't looked at it in 30 years, so I hope you'll excuse my lapses of memory. In general to get low distortion, expect to have to use fairly high current levels. Do you have specific requirements? You may well be able to do fine with a different, simpler design. I'd submit your requirement is not for a cascode, but for specific TOI and perhaps harmonic performance and noise level and input and output impedances. Cheers, Tom oUsama (Yuri Blanarovich) wrote in message ... Thanks for pointers, I will have look at references mentioned. Another requirement would be good handling strong signals, like using 807 :-) I am trying to build the converter with strong signal handling capabilities, low noise and high gain. Next step is looking at the mixers and low noise synthesizers. Thanks a bunch! Yuri, K3BU |
The tuner I mentioned did the cascode specifically to achieve low
noise and the ability to handle big signals with low distortion. Somewhere in my disorganized archives I think I have more info on it, but don't ask me to look! ;-) As I recall, they used a transistor billed for CATV service with low noise and good linearity on the input, and an RF power mosfet on the output--one of the old Siliconix VMOS ones?? Haven't looked at it in 30 years, so I hope you'll excuse my lapses of memory. In general to get low distortion, expect to have to use fairly high current levels. Do you have specific requirements? You may well be able to do fine with a different, simpler design. I'd submit your requirement is not for a cascode, but for specific TOI and perhaps harmonic performance and noise level and input and output impedances. Cheers, Tom oUsama (Yuri Blanarovich) wrote in message ... Thanks for pointers, I will have look at references mentioned. Another requirement would be good handling strong signals, like using 807 :-) I am trying to build the converter with strong signal handling capabilities, low noise and high gain. Next step is looking at the mixers and low noise synthesizers. Thanks a bunch! Yuri, K3BU |
By the way, did you check the "A High-Dynamic-Range MF/HF Receiver front end " by Jacob Makhinson N6NWP (QST February 1993 p. 23-28, with corrections in June 1993 p. 73) ? It contains push-pull preamplifier designs with feedback with nearly +33 dBm input IP3 and gain compression starting around S9+80 dB. Paul OH3LWR The mixer section is what is REALLY impressive about that front end. There was another article (also in the handbook) about a similar mixer that used a siliconix quad dmos fet that was even better. |
By the way, did you check the "A High-Dynamic-Range MF/HF Receiver front end " by Jacob Makhinson N6NWP (QST February 1993 p. 23-28, with corrections in June 1993 p. 73) ? It contains push-pull preamplifier designs with feedback with nearly +33 dBm input IP3 and gain compression starting around S9+80 dB. Paul OH3LWR The mixer section is what is REALLY impressive about that front end. There was another article (also in the handbook) about a similar mixer that used a siliconix quad dmos fet that was even better. |
Paul OH3LWR:
While you definitely need the lowest noise figure and a high gain when using small magnetic loops, I very much doubt that you are going to need the strong signal handling capabilities for these antennas. Why not use a dedicated preamplifier constantly attached to these antennas, possibly with some protection circuitry to avoid damage when transmitting through your nearby transmitter antenna ? The idea is to have one preamp or RF stage for whole range of antennas 160 -10 or 6 m. No outboard boxes, preamps, but one good "in house" adjustable gain amp. No step or switchable attenuators either. Normal city mortals have high band noise levels, but when you go to the ocean front, use RX antennas or beam north on high bands, you need sensitivity and of course high dynamic range for the other end of the signals spectrum, like operating in Multi/multi environment. Thanks for N6NWP article info, I will look it up. Yuri, K3BU.us |
Paul OH3LWR:
While you definitely need the lowest noise figure and a high gain when using small magnetic loops, I very much doubt that you are going to need the strong signal handling capabilities for these antennas. Why not use a dedicated preamplifier constantly attached to these antennas, possibly with some protection circuitry to avoid damage when transmitting through your nearby transmitter antenna ? The idea is to have one preamp or RF stage for whole range of antennas 160 -10 or 6 m. No outboard boxes, preamps, but one good "in house" adjustable gain amp. No step or switchable attenuators either. Normal city mortals have high band noise levels, but when you go to the ocean front, use RX antennas or beam north on high bands, you need sensitivity and of course high dynamic range for the other end of the signals spectrum, like operating in Multi/multi environment. Thanks for N6NWP article info, I will look it up. Yuri, K3BU.us |
Do you have specific requirements? You may well be able to do fine with a different, simpler design. I'd submit your requirement is not for a cascode, but for specific TOI and perhaps harmonic performance and noise level and input and output impedances. Cheers, Tom Low noise, high dynamic range, strong signal handling. Again, useable for low signal RX antennas and handling strong in-band signals like in multi/multi situations, when you want to tune/scan the same band while transmitting on that band. Filtering and protection is another story. Low noise mixer is to follow, with low phase noise synthesizer. Trying to beat K2 and Orion front ends. Yuri, K3BU |
Do you have specific requirements? You may well be able to do fine with a different, simpler design. I'd submit your requirement is not for a cascode, but for specific TOI and perhaps harmonic performance and noise level and input and output impedances. Cheers, Tom Low noise, high dynamic range, strong signal handling. Again, useable for low signal RX antennas and handling strong in-band signals like in multi/multi situations, when you want to tune/scan the same band while transmitting on that band. Filtering and protection is another story. Low noise mixer is to follow, with low phase noise synthesizer. Trying to beat K2 and Orion front ends. Yuri, K3BU |
On 4 Apr 2004 12:07:18 -0700, (Tom Bruhns) wrote:
Do you have specific requirements? You may well be able to do fine with a different, simpler design. I'd submit your requirement is not for a cascode, but for specific TOI and perhaps harmonic performance and noise level and input and output impedances. Cheers, Tom I really suspect that it is outlined an amplifier which it is not expereinced whether it is needed or not, I believe it is no need for extra amplification below 15 or 20MHz, and since the frame antenna is relatively narrow banded it is no need for particularly good IP3 characteristics on any frequencies Somebody once mentioned that a transistor with 50mA collector current equalled a valve amplifier with 5mA anode current in respect of IP3, wonder such is likely to be true? Suggested valve is 6761 with gm=50mmho 73 Jan-Martin LA8AK http://home.online.no/~la8ak/e.htm ---- Jan-Martin, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/ |
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