# JFET question

Discussion in 'Electronic Design' started by Bo, Apr 21, 2008.

1. ### BoGuest

I have a common source n channel JFET circuit--

with R1/R2 bias divider R1 = 420K, R2 = 180K

RD = 2.7K
Rs = 2.7K
RL = 4K

cap Cs in || with Rs.

Vp = -4
early voltage = 125
Idss = 12mA

How can I find small signal voltage gain @ mid freq?

I try to compute gm-- but need Vgs to do so. Do I assume Vs = 0 since
Cs short @ mid freq and then use dc value of Vg (6V) to say Vgs = 6v?

I'm thinking there has to be additional info to figure this out...

Any clues appreciated...

Bo

2. ### Helmut SennewaldGuest

Hello Bo,

It's easy to calculate the gain.

G = gm*RD||RL||rd

Just forget the output resistance rd, because
it's much higher than RD||RL.

Calculate the DC-current Id.
Go to the datasheet and look for gm(Id).

G = gm*RD||RL

That's all

Helmut

3. ### Jon SlaughterGuest

http://www.ele.auckland.ac.nz/archives/design/tools/ampfet.htm
http://www2.eng.cam.ac.uk/~dmh/ptialcd/spice/simulation.htm
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/fet.html

and many more...

4. ### BoGuest

Thanks to all for your replies...

I finally found something via google that helped me. Ended up
calculating Vgs from a quadratic equation...

I could not look up in datasheet because P/N is unknown...

Thanks again,

Bo