Question

For a high frequency response of a simple C.E. stage with a transconductance of gm, what is Cin?

(a) Cµ(1 + gm*R2) – Cπ

(b) Cµ(1 + gm*R2) + Cπ

(c) Cµ(1 – 2*gm*R2) + Cπ

(d) Cµ(1 + 2*gm*R2) – Cπ

This question was posed to me in an interview for job.

My doubt stems from Effect of Various Capacitors on Frequency Response topic in portion Transistor Frequency Response of Analog Circuits

Answer 1

Correct choice is (b) Cµ(1 + gm*R2) + Cπ

For explanation I would say: The input capacitance is an equivalent of the base to emitter capacitance in parallel to the miller approximation of the base to collector capacitance. Due to miller approximation, the base to collector capacitance becomes Cµ(1+gm*R2) while the base to emitter capacitance is Cπ. Capacitors get added, when in parallel and thus Cµ(1+gm*R2) + Cπ is correct.