Question

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

(a) – {gm1 * (Rd || 1/gm2) * (Rd1/ (1/gm2 + Rd))}

(b) – {gm1 * (Rd || 1/gm2) * gm2 * 2Rd1}

(c) – {gm1 * Rd * gm2 * Rd1}

(d) – {gm1 * 1/gm2 * gm2 * Rd1}

The question was posed to me during a job interview.

Origin of the question is MOSFET Amplifier with CG Configuration in division FET Amplifiers of Analog Circuits

Answer 1

Right answer is (a) – {gm1 * (Rd || 1/gm2) * (Rd1/ (1/gm2 + Rd))}

Explanation: This is a cascade of CS stage preceding a CG stage. The voltage gain of the first stage is – (gm1 * Rd || 1/gm2). This is because the drain of M1 is connected to the source of M2 and the impedance looking into the source of M2 is 1/gm2. Now, the source of the CG stage is connected to Rd and hence the voltage gain due to this stage is affected by source degeneration and the voltage gain is Rd1/ (1/gm2 + Rd). The overall voltage gain is – {gm1 * (Rd || 1/gm2) * (Rd1/ (1/gm2 + Rd))}.