Question

If channel length modulation is present. what is the overall output impedance of the following CG stage?

(a) Rd || {Rs*(1+gm * ro) + ro}

(b) Rd || {Rs*(1+gm * ro) + 2ro}

(c) Rd || {Rs*(1+gm * ro) – ro}

(d) Rd || {Rs*(2+gm * ro) + ro}

I had been asked this question by my college director while I was bunking the class.

This interesting question is from MOSFET Amplifier with CG Configuration topic in division FET Amplifiers of Analog Circuits

Answer 1

Correct option is (a) Rd || {Rs*(1+gm * ro) + ro}

Easy explanation: The impedance looking into the drain of M1 would be similar to that of a CS stage with source degeneration. That implies the impedance is  Rs*(1+gm * ro) + ro. But this impedance is parallel to Rd. Hence, the overall output impedance becomes Rd || {Rs*(1+gm* ro) + ro}.