Question

In the following C.S. stage shown below, what is the voltage gain from the gate to the drain of M1 if λ>0?

(a) gm * ro

(b) gm * 2R1

(c) gm * R1||ro

(d) gm * R1

I got this question by my college professor while I was bunking the class.

I'm obligated to ask this question of MOSFET Amplifier with CS Configuration in chapter FET Amplifiers of Analog Circuits

Answer 1

The correct answer is (c) gm * R1||ro

Explanation: This is easily observable by performing a small signal analysis at the output of the C.S. stage. We need to turn off all voltage sources, Vcc mainly, and give a small input at the gate. Thereby, the voltage gain becomes gm * R1||ro. The presence of early effect reduces the gain a bit and deviates M1 from its ideal characteristics.