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

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

Did you try google? 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...

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