Question

If a C.E. stage with early effect has a load Rl and transconductance gm, what is the factor by which the capacitance between the base and the collector at the output side, gets multiplied?

(a) 1 + 2/gm*(Rl || ro)

(b) 1 – 1/gm*(Rl || ro)

(c) 1 + 1/gm*(Rl || ro)

(d) 1 – 2/gm*(Rl || ro)

This question was addressed to me in my homework.

Query is from Effect of Various Capacitors on Frequency Response topic in chapter Transistor Frequency Response of Analog Circuits

Answer 1

The correct choice is (c) 1 + 1/gm*(Rl || ro)

The explanation: If the early effect is considered, the low frequency response of the C.E. stage becomes gm*(Rl || ro). Thereby, miller approximation shows that the capacitance between the base and the collector, looking from the output side, will be increased by a factor of 1 + 1/gm*(Rl || ro). Hence the correct option is 1 + 1/ gm*(Rl || ro).