Question

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

(a) Ccs – Cµ*(2 + 1/gm*R2)

(b) Ccs + Cµ*(1 + 2/gm*R2)

(c) Ccs – Cµ*(1 + 1/gm*R2)

(d) Ccs + Cµ*(1 + 1/gm*R2)

This question was posed to me during an internship interview.

The above asked question is from Effect of Various Capacitors on Frequency Response in division Transistor Frequency Response of Analog Circuits

Answer 1

Right choice is (d) Ccs + Cµ*(1 + 1/gm*R2)

Easy explanation: We have a capacitor from the collector to substrate, Ccs, which comes in parallel to the miller approximation of the capacitance from base to collector. The miller approximation defines the latter as Cµ*(1 + 1/gm*R2). Since capacitors gets added, when in parallel, the correct option is Ccs + Cµ*(1+ 1/gm*R2).