Question

If g is the transconductance, r is the resistance due to channel length modulation and if M2 has channel length modulation but M1 doesn’t, what is the voltage gain at node x?

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

(b) (1/g || r) / (1/g + (1/g || 2r))

(c) (1/g || r) / (2/g + (1/g || r))

(d) (1/g || r) / (1/g + (2/g || r))

The question was asked by my college professor while I was bunking the class.

Asked question is from MOSFET Amplifier with CD Configuration topic in section FET Amplifiers of Analog Circuits

Answer 1

Correct choice is (a) (1/g || r) / (1/g + (1/g || r))

Explanation: The voltage gai at node x will be due to the follower stage. Since M2 has channel length modulation, we observe that the total resistance at the source of M2 is (1/g || r). Thus, from the expression of voltage gain of a follower- the voltage gain at node x is (1/g || r) / (1/g + (1/g || r)).