Question

For a cascode stage, with input applied to the C.B. stage, the input capacitance gets multiplied by a factor of ____

(a) 0

(b) 1

(c) 3

(d) 2

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Asked question is from Effect of Various Capacitors on Frequency Response topic in division Transistor Frequency Response of Analog Circuits

Answer 1

Correct choice is (d) 2

Easy explanation: The small signal gain, of the C.B. stage, in a cascode stage is approximately equal to the ratio of the transconductances of the two B.J.T.’s. Since they are roughly same, the gain is 1. Miller multiplication leads to multiplying the capacitance, between base and collector, by a factor of (1 + small signal gain) which is 2. Hence, the correct option is 2.