Single Jfet amp How to calc gain?

[Zero Cool]

I am looking to calculate the gain of this single Jfet amp stage. I found this previous thread:
http://www.diyaudio.com/forums/soli...ate-gain-typical-small-jfet-preamplifier.html

and using the simplified version of John Curls calculations: Vg=R(1)/{R(2)+1/Gm} or R(1)/R(2) approximately

R1/R2 = gain

I find that we have about 6K/12K = gain of 2 or 6db Correct? seems to make sense.


Zc

 

[Zero Cool]

I am hoping someone can confirm my math here?

 

[richie00boy]

Yes, 6k in the source and 12k in the drain would yield you gain of 2 when output is taken from the drain and loaded with a reasonably high impedance (10k would be fine).

 

[Zero Cool]

Thank you!

 

[richie00boy]

Remember that the output will be inverted with respect to the input, so you may want to include another stage or simply live with it.

 

[Zero Cool]

This is actually part of a GK 1001RB bass amp. This Jfet is in there for flavor only from what i gather. there is an op-amp before and an op-amp after so this Jfet really doesn't DO anything other then overload and change the sound. But once i started looking at the circuit i got curious as to how much gain it was providing. I didn't figure much if any at all.

 

[h_a]

That's a simple common source stage, for the basic euquations look in wikipedia:

Common source - Wikipedia, the free encyclopedia

R1/R2 = gain is a bit too simplified as the jfet itself does not matter at all and of course there are high gain (transconductance) and low gain parts 😉

Also, 6k in the source are a little...uncommon. You will be driving any small signal jfet near pinch-off voltage limiting signal swing. Better try smaller source resistor and smaller load resistor.

Have fun, Hannes

EDIT: just thought over your last post: sure, that's ok if you want to add sound to the circuit 😉