MOSFET Common Source Amplifier with Source Degeneration

SearchDS

Why does the small signal voltage gain collapse as we increase the AC input voltage?


Voltage gain = Av = (output voltage) / (input voltage). Both small signal (AC).

When you draw the small signal circuit using T-Model, you can find that
Av= (gm * Rd) / (1 + gm Rs)

gm being the the trans-conductance.

Don't need an answer that says "Because the input voltage is in the denominator"
Since as we increase the input voltage, the output voltage will increase also, but not in the same ratio.


You can see the circuit here:
http://en.wikipedia.org/wiki/Common_source

vk6kro

Why does the small signal voltage gain collapse as we increase the AC input voltage?

It doesn't. The expression Av= (gm * Rd) / (1 + gm Rs) only contains constants.

In fact, the gain gets very close to the ratio of Rd / Rs and is fairly constant as long as the signals stay sinusoidal.

Small signals mean just that. If you drive the FET into non linear regions then the signal is no longer a small signal.

The circuit in Wikipedia does not allow for any gate to source bias voltage. To do this, you need a circuit like this:

yungman

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