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JFET "Amplifier". How Would This Work?

epsolutions

Sep 7, 2019
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I found this circuit hand drawn in my Dad's old notebook. There were no other notations except "amplifier".

Can anyone tell me anything about its possible intended purpose, or how it functions electronically?jfet-amplifier.png
Why are the JFET's wired in parallel, and what load resistance could this arrangement handle?

What would be the purpose of the air core transformer?

Thanks for any advice along these lines.
 

danadak

Feb 19, 2021
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"Normally" one parallels devices to increase current delivery / power handling capability.

Whats confusing with the drawing is test signal is not RF, yet the air wound coil, few turns,
and fact 2N5670 is a small signal JFET implies its a RF amp.

Then there is transformer with no polarity markings so not sure whats going on there.

Drawing simply too little info........for me at least.


Regards, Dana.
 

epsolutions

Sep 7, 2019
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Thanks for your reply. That is all the info I had on this circuit. Maybe the "not RF" note is wrong.

I am still wondering what the purpose of the transformer might be. The primary is 12VDC and the secondary is the source-follower output, but their currents oppose. For discussion, let's assume that there is some degree of coupling. Is it supposed to oscillate or affect the phase of the output?
 

danadak

Feb 19, 2021
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Depending on polarity could be + fdbk (oscillate) or - fdbk, amplify.

If it was oscillator why the need for input signal.....produce a damped ringing.....

Too little info = too much guessing....:)

No other clues in his notebook......? Like what was it used in, a toaster or a
tricorder......



Regards, Dana.
 

epsolutions

Sep 7, 2019
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Not sure what you mean by "polarity". The currents in pri and sec are inverted from each other as shown in the diagram.

It may have been an amplifier that was designed to produce oscillation as a carrier. Maybe some kind of broadband jammer.

Sorry, I am guessing as well. I thought someone here might recognize the circuit, or at least the coil-coupled Vcc.
 

danadak

Feb 19, 2021
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I see the fdbk is negative as you point out.

Here is a crude sim -

1668217729496.png

I dropped Rl to 50 ohms, gate starting to go +, clamping, not good for JFET.
At Rl = 200 ohm problem goes away.


Regards, Dana.
 
Last edited:

aph

Jul 4, 2023
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I found this circuit hand drawn in my Dad's old notebook. There were no other notations except "amplifier".

Can anyone tell me anything about its possible intended purpose, or how it functions electronically?View attachment 56880
Why are the JFET's wired in parallel, and what load resistance could this arrangement handle?

What would be the purpose of the air core transformer?

Thanks for any advice along these lines.
Hi epsolutions,
While trying to source some 2N5670 myself I stumbled across your question about the arrangement of the three JFETs. It reminded me of this circuit and I wondered if what you have is a related to this? I'm not an electronics expert so am not in a position to assess whether they are the same thing, it just looked similar.
1688455003911.png
 

AnalogKid

Jun 10, 2015
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Hi epsolutions,
While trying to source some 2N5670 myself I stumbled across your question about the arrangement of the three JFETs. It reminded me of this circuit

Please post a link to the source of that circuit.

ak
 
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