How is This Thing Even Working?

[Ripthorn]

Last night I breadboarded up a Condor Cab Sim and the Ruby both from ROG.  I haven't breadboarded anything in a while, so I had at it and dialed it in to where I thought everything was sounding great through my headphones.  However, when I went back through to make notes on the schematic mods I had made, I discovered something that has me very puzzled.  Both circuits use GSD JFETs, however, I forgot what my pinouts were for my JFETS, so I was surprised when going back through and checking things.  The inputs to both circuits are coming in on the middle pin of my JFETs, which is the source.  I tried to re-work my breadboard setup so that the inputs were coming in on the right pin, but I got nothing out of either circuit.  These are JFETs that I bought 6 or 8 years ago from mouser, so I'm pretty sure they're genuine.  My question is, how on earth is this working?  Did I do up a circuit that isn't what I thought I was building?  It sounds good, so I'm going to run with it, I'm just curious why this is even working.  I tried to take a couple pictures of the JFET stage in the Ruby, but photographing breadboard layout is hard, especially when you are as chaotic on a breadboard as I am.

[anotherjim]

I think there have been JFET's with gate in the middle. Have you diode tested them to confirm?

But, you work the FET by wiggling the gate voltage with respect to the source. What happens if you wiggle the source WRT the gate?

The lower FET has its gate worked in the usual manner, it's drain is then working the top FET by its source while its gate is fixed.
Together they wiggle Vout.

[DavidRavenMoon]

Also, the drain and the source are interchangeable on JFETs. So maybe the pinout is different on these?


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[antonis]

Also, the drain and the source are interchangeable on JFETs. So maybe the pinout is different on these?

David, that's the reason for JFET pin-out with Gate on one side (usually on left..)

It should be more than convenient a pin-out with Gate at the middle leg 'cause we'll have not to worry about orientation..

(probably manufactures don't want to make a JFETs user life easier than a BJTs one's..) 

[Ripthorn]

Thanks for the thoughts guys.  I'll just socket the transistors so that I can do whatever I need to do to get it all working.  Just very confusing why it works at all in this supposedly "wrong" state, but doesn't work when I put it in the "right" configuration. Oh well, as long as it works, right?!

[bool]

Some fets, (iirc bf244 ?? or whatever it was) also used the s-g-d pinouts.

[Ripthorn]

Yes, I am aware that some FETS are SGD, but these are a J201 and an MPF102 that I bought from Mouser a few years ago before TO92 production stopped, so I trust that they are legit and therefore should be GSD.

[amz-fx]

Google for a common gate jfet amplifier:



regards, Jack

[Ripthorn]

Jack, I had never heard of such a thing before.  Time to go research!  Thanks!

[rankot]

It seems interesting, but I found an article about that, saying that it has very low imput impedance, and it is usually used for low impedance microphone preamps. http://www.radio-electronics.com/info/circuits/fet-field-effect-transistor/common-gate-circuit-amplifier.php

[PRR]

Common-Gate makes nearly no sense in audio. (Even lo-Z mikes.)

Two compound exceptions: top of a Cascode, and backside of a Long Tail.

Common Gate does make sense in radio amplifiers, where impedances are much lower, can be transformed cheaply, and stray capacity bites.