Question

For given BJT, β=200. The applied input frequency is 20 Mhz and net internal capacitance is 100 pF. What is the CE short circuit current gain at β cut-off frequency?

(a) 200

(b) 100

(c) 141.42

(d) 440.2

The question was asked in an online interview.

My question comes from High Frequency Response in chapter Transistor Frequency Response of Analog Circuits

Answer 1

The correct choice is (c) 141.42

To elaborate: The current gain for the CE circuit is A = \(\frac{β}{\sqrt{1+(\frac{f}{f_β})^2}}\)

At f = fβ, A = \(\frac{β}{\sqrt{2}}\)

Hence A = 141.42.