Chart or info on common JFET gains??

[snoof]

Is there a chart or something similar that has the common JFET's and their gains??

[R.G.]

That's tough to do.

The thing that JFETs have that's closest to gain is called transconductance. Transconductance is the amount of current that flows in the drain for a given voltage change on the gate-source. In a normal circuit with no source resistor, the voltage gain is gm*Rdrain.

In a circuit with an unbypassed source resistor, the gain drops toward Rdrain/Rsource.

You would think that you could list gm and be done with it. But there's another item Idss. Idss is the max current that the JFET will pass if the gate and drain are shorted. So you can have a lot of gm, but a low Idss and the circuit might have a lot of gain, but only for specific Rdrain values that keep it from exceeding Idss.

You would think that you just get the highest gm value. That would be true except for Idss limiting the max drain current and Vgsoff turning off the channel current entirely.

Sorry - like I said, it's not simple.

[snoof]

Thanks for the explanation R.G.  The info I was looking for was more of a general type, such as; J201 are usually high gain, next would be MPF102 etc.  I have been tweaking some of my FET pedals for gain/dist, and I am a little fuzzy as to what the general characteristics of the common FET's that are used in our pedals here were.  Like if I replace a 201 with a 102, this would produce (generally speaking) less gain; that type of thing.  Or 2N5457's are typically "?", and so forth.  I guess I should have posed my question a bit clearer .