I am trying to design a simple common source jfet amplifier using a J109 transistor. I'm using the J109 datasheet and equations from the book Electronic Devices by Thomas L. Floyd. The issue is that I do not obtain the right values of resistors to achieve maximum voltage gain.
Vdd = 12V Id = 0.5Idss = 0.02 A Vgs ≈ Vp/3.4 ≈ -1.75 V Vd = 0.5Vdd =6V
Components values :
Rs = |Vgs/Id| = 1.75/0.02 ≈ 88Ω
Rd = (Vdd-Vd)/Id = (12-6)/0.02 = 300Ω
I arbitrarely choosed 100µF for the bypass capacitor since I don't really care about the cutoff frequency in this case.
I built the circuit on a breadboard and it worked except the gain was only about 1.67 (unloaded coutput), which I found quite low. (I fed a 1KHz sine wave from a function generator to the input and measured the output with a scope).
I was using potentiometers for Rs and Rd and after adjusting those, I managed to get a gain of ten with values of Rs = 265Ω and Rd = 390Ω.
I just don't understand why I get such a small gain using the results from the equations. What is wrong with my calculations ?