Question

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

(a) 1/g || R

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

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

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

This question was posed to me in an online quiz.

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

Answer 1

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

Explanation: The output impedance of the final stage doesn’t get affected by the previous stages since the MOSFET offers infinite impedance at the Gate. Hence, it isolates each stage from the next stage- provided the input is applied to the gate. The output impedance is simply (1/g || R) since 1/g is the impedance looking into the source of M3 while R is connected in parallel to it.