# MOSFET common source amplifier difficulties

#### Smashmayo98

Joined Sep 12, 2018
11
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!

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#### wayneh

Joined Sep 9, 2010
17,505
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!
Suppose a charge is present on Cs. Without Rs, where can it go to discharge?

#### Smashmayo98

Joined Sep 12, 2018
11
Suppose a charge is present on Cs. Without Rs, where can it go to discharge?

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?

#### wayneh

Joined Sep 9, 2010
17,505
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
4,057
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.

#### Smashmayo98

Joined Sep 12, 2018
11
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.

What would be the difference between the third and last configurations?

Joined Mar 10, 2018
4,057
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.

https://inst.eecs.berkeley.edu/~ee105/fa14/lectures/Lecture13-Small Signal Model-MOSFET.pdf

Regards, Dana.

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