Parallel Wired JFET Amplifier with Transformer

Thread Starter

Sir Kit

Joined Feb 29, 2012
147
I found the circuit below hand drawn in my Dad's old notebook. There were no other notations except "JFET amplifier".

Can anyone please tell me why the JFET's are wired in parallel?

What might be the purpose of the air core transformer?

jfet-amplifier.png
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
147
Given that current in the windings oppose, that is what I was thinking. Thanks for confirming it.

The diagram also showed a switched connection from the source to ground. Any idea what that would be for? I assume it would have just killed the output.
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
147
If I read the plot correctly, the transformer imposes a slight phase delay. Does that have any practical application in terms of constructed devices?
 

MisterBill2

Joined Jan 23, 2018
13,173
It may be that the secondary in series is to provide negative feedback to reduce distortion. That was very common in the era of transformer output amplifiers. Given that I see no hint as to the turns ratio, the assumption of unity is not valid. The drawing teplates of that era always had the turns shown the same on both sides.
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
147
The information I have indicates pri and sec both have 12T on 3/4" dia. air core. I guess we can assume the circuit is really designed to "amplify" RF. Can anyone explain how JFET's wired in parallel would do this?
 

MisterBill2

Joined Jan 23, 2018
13,173
The information I have indicates pri and sec both have 12T on 3/4" dia. air core. I guess we can assume the circuit is really designed to "amplify" RF. Can anyone explain how JFET's wired in parallel would do this?
OK, I did miss that both windings were the same.

Putting multiplr active devices in parallel has been a standard method of increasing output power since early vacuum tube days. The logic behind it is that multiple devices will operate identically and so the voltage swing at the output will be the same but the current will be multiplied by the number of devices.
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
147
How much amplification, if any, might one expect from the circuit?

What would be the effect of shorting the parallel wired JFET sources to ground?
 

MisterBill2

Joined Jan 23, 2018
13,173
Three traces shown in post #4, no hint as to which points in the circuit they are.
Grounding the sources would change the class of operation and lead to a lot of distortion. because of the lack of feedback and also the transistors being cut off for at least half a cycle.
 

Bordodynov

Joined May 20, 2015
3,013
Three traces shown in post #4, no hint as to which points in the circuit they are.
Grounding the sources would change the class of operation and lead to a lot of distortion. because of the lack of feedback and also the transistors being cut off for at least half a cycle.
Honestly, I thought it was pretty clear. I named the nodes in the circuit. The same names are written on the diagram and each name has a different color. If you're not colorblind, it should be clear to you.
 

MisterBill2

Joined Jan 23, 2018
13,173
OK, the portion of the graphic with the names in color was above the edge of my display, I had to scroll down a bit to see it.

While the voltage swing is not as great, the output impedance is much lower, and so there is a power gain in the circuit.
BUT it is not a practical circuit for any common application .
 

DickCappels

Joined Aug 21, 2008
9,331
Sir Kit only asked two questions:

Can anyone please tell me why the JFET's are wired in parallel?
(Answer: This increases the drain amps per gate volts, Gm)

What might be the purpose of the air core transformer?
Still a mystery.

Please stick to the questions
 

MisterBill2

Joined Jan 23, 2018
13,173
The intended purpose of the transformer might be available from the father of the TS, if that person is available and if they recall the intention at the time.
I have a guess, which is that there was to be a third element in the transformer assembly intended to couple power to some load that we are not aware of. IF the phasing of the windings shown were such that the windings would add to the field, much more power could be delivered to a load. That is one guess
Another guess is that it was a step toward avoiding core saturation, which can be a very big challenge and a serious problem with magnetic material cores. This is another guess.
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
147
Thanks to everyone for their helpful answers. I am sorry the information I have is limited. I am also perplexed because the amplifier does not provide any voltage gain, and no precise reason for the air coil has been determined. But now I have a better idea of how the circuit might work.
 

MisterBill2

Joined Jan 23, 2018
13,173
Opposing windings would certainly tend to reduce the magnetic field is what I was thinking. And as we have no context about the circuit, that was one guess. And air core coils are sometimes easier to experiment with.
And I am quite aware that air does not megneticly saturate.
 
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