Question

If M1 suffers from channel length modulation but M2 doesn’t, what is the voltage gain?

(a) gm1 * Rd || 1/gm2 * gm2 * Rd2

(b) gm1 * Rd || 1/gm2 * gm2 * 2Rd2

(c) gm1 * Rd || 12/gm2 * gm2 * Rd2

(d) gm1 * Rd * gm2 * Rd2

I had been asked this question by my college professor while I was bunking the class.

My question comes from MOSFET Amplifier with CG Configuration in portion FET Amplifiers of Analog Circuits

Answer 1

Right answer is (a) gm1 * Rd || 1/gm2 * gm2 * Rd2

Explanation: The voltage gain due to M1 is gm1 * Rd || 1/gm2 since we find that the drain of M1 is connected to the source of M2 and the impedance looking into the source of a MOSFET is 1/gm2. Now, the voltage gain due to the 2nd CG stage is simply gm2*Rd2. Hence, the total voltage gain is a product of both the factors ie gm1 * Rd || 1/gm2 * gm2 * Rd2.