Question

If channel length modulation is present, what is the voltage gain?

(a) ro1 / {(1/gm1 || ro2) + ro1}

(b) ro2 / (1/gm1 + ro1)

(c) ro2 / (1/gm1 + 3ro2)

(d) ro2 / (2/gm1 + ro2)

The question was posed to me in exam.

Origin of the question is MOSFET Amplifier with CD Configuration in portion FET Amplifiers of Analog Circuits

Answer 1

Right option is (a) ro1 / {(1/gm1 || ro2) + ro1}

For explanation I would say: M2 behaves as a Source follower i.e. it is being used in a C.D. configuration. M1 provides a output resistance of ro1  but M2 provides an impedance of (1/gm2||ro2). Thus, from the voltage gain of a follower stage, we conclude the overall voltage gain is ro1 / {(1/gm1 || ro2) + ro1}.