Question

If channel length modulation is present, what is the impedance looking into node X?

(a) (1/gm2 || ro2 || 1/gm1)

(b) (1/gm2 || 1/gm1 || ro1)

(c) (1/gm2 || ro2 || 12/gm1 || ro1)

(d) (1/gm2 || ro2 || 1/gm1 || ro1)

I had been asked this question in an internship interview.

The question is from MOSFET Amplifier with CD Configuration topic in portion FET Amplifiers of Analog Circuits

Answer 1

The correct option is (d) (1/gm2 || ro2 || 1/gm1 || ro1)

The best explanation: The source of M1 and M2 are connected at node X. The impedance looking into the source of a MOSFET is (1/gm || ro). If we look at node X, M1 and M2 are connected from Node X to ground. Hence, they are parallel to each other and the overall impedance becomes (1/gm2 || ro2 || 1/gm1 || ro1).