MOSFET common source amplifier difficulties

Thread Starter


Joined Sep 12, 2018
Hi! In my Analog electronics course I have this problem to solve (see photo), I know that Vthreshold = 1.5 V, K = 1.88mA/V^2, Vds = 10V, Vdd = 20V and that the power that runs from Rd - Vds - Rs is 10mW. I don't know any value of the resistors, but I know that the amplifier has a gain of -7.5 V/V and that Vg has to be 99% of Vin. I also know that the capacitors are shorted for the frequency bandwidth. I also know that R = 10k and RL = 10k. My question is : I'd like to know if Rs is of any use because of the short-circuit caused by the capacitor in parralel with Rs. Thank you very much!



Joined Sep 9, 2010
Very true. My bad on this one. Still I'm having troubles calculating all the resistances because I feel like I have too many variables. How would I get Rs?
Well, I suppose you need it low enough to discharge Cs in the time required. I don't know what that is without studying the circuit a lot more.


Joined Mar 10, 2018
Rs is used to generate bias for MOSFET, along with the gate R divider.
If it was un bypassed it would also generate - fdbk to level G.

Regards, Dana.


Joined Mar 10, 2018
3 provides - fdbk right off Drain, so raises Zin, BW, stabilizes G by minimizing
gm variation, and a few other characteristics.

4 provides - AC fdbk due to Rs (if unbypassed), if my memory serves me better bias
stability, and a few other characteristics.

If you get a good text on electronic basics all these two port characteristics are derived. Signal Model-MOSFET.pdf

Regards, Dana.
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