Question

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

(a) gm1 * (Rd || ro1) * gm2 * Rd1

(b) – {gm1 * Rd * gm2 * 2(Rd1 || ro2)}

(c) – {gm1 * (Rd || ro1) * gm2 * (Rd1 || ro2)}

(d) gm1 * 2Rd * gm2 * Rd1

This question was posed to me by my college director while I was bunking the class.

Query is from MOSFET Amplifier with CG Configuration topic in division FET Amplifiers of Analog Circuits

Answer 1

Right answer is (c) – {gm1 * (Rd || ro1) * gm2 * (Rd1 || ro2)}

Explanation: This is a cascade of a CG stage preceding a CS stage. For the CG stage, the voltage gain is gm * (Rd || ro1). But after this stage, the signal gets amplified by the CS stage with a factor of – {gm2 * (Rd1 || ro2)}. Hence, the overall voltage gain is the product of both the factors i.e. – {gm1 * (Rd || ro1) * gm2 * (Rd1 || ro2)}. Note that the input impedance looking into M2 is infinite ad hence the voltage gain of the CG stage is typically unaffected by the CS stage (for low frequency operations only).