Question

Which of the statement is incorrect?

(a) At unity gain frequency the CE short circuit current gain becomes 1

(b) Unity gain frequency is the same as Gain Bandwidth Product of BJT

(c) Gain of BJT decreases at higher frequencies due to junction capacitances

(d) β- cut-off frequency is one where the CE short circuit current gain becomes β/2

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

This intriguing question comes from High Frequency Response in section Transistor Frequency Response of Analog Circuits

Answer 1

Correct answer is (d) β- cut-off frequency is one where the CE short circuit current gain becomes β/2

Easy explanation: At unity gain frequency the current gain is 1 is a correct statement. The same frequency is fT = βfβ which is the gain bandwidth product of BJT. Gain of BJT at high frequency decreases due to the junction capacitance. However, at β cut-off frequency, current gain becomes \(\frac{β}{\sqrt{2}}\).