Question

In the following circuit, what is the voltage at the source of the M1?

(a) Vin * Rs / (Rs + 1/gm)

(b) Vin * Rs / (Rs + gm)

(c) Vin * Rs / (Rs – 1/gm)

(d) Vin * Rs / (Rs + 2/gm)

This question was addressed to me in an interview for job.

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

Answer 1

Correct option is (a) Vin * Rs / (Rs + 1/gm)

Easiest explanation: The impedance looking into the source of M1 is 1/gm. Vin experiences a potential divider before entering the source of M1. The voltage drop across Rs is typically the voltage at the source and hence, it is equal to Vin * Rs / (Rs + 1/gm).