Question

If g is the transconductance, r is the resistance due to channel length modulation and if M2 has channel length modulation but M1 and M3 doesn’t, what is the total resistance at the source of M2?

(a) (1/ g || R) || (1/g || r)

(b) (1/ g) || (1/g || r)

(c) R || (1/g || r)

(d) (1/ g || R) || 1/g

This question was addressed to me in semester exam.

My doubt is from MOSFET Amplifier with CD Configuration topic in portion FET Amplifiers of Analog Circuits

Answer 1

Correct option is (a) (1/ g || R) || (1/g || r)

To explain: The source of M2 is connected to the source of M1. Since channel length modulation isn’t present in M1, the source of M1 offers a resistance of 1/g. Again, we find the source of M2 offers a resistance of (1/g || r) due to channel length modulation. Therefore, we conclude that the total impedance at the source of M2 is (1/ g || R) || (1/g || r).