Question

The cut-off frequency (fβ) is basically the frequency at which the short circuit __________

(a) CB gain of transistor drops by 3 dB from its value at low frequency

(b) CE gain of transistor drops by 3 dB from its value at low frequency

(c) CC gain of transistor drops by 3 dB from its value at low frequency

(d) CC gain of transistor drops by 3 dB from its value at high frequency

This question was addressed to me during an interview.

My doubt stems from Frequency Compensation in portion Frequency Response of Electronic Devices & Circuits

Answer 1

Correct answer is (b) CE gain of transistor drops by 3 dB from its value at low frequency

The explanation: Cut-off frequency (fβ) is basically the frequency at which short circuit CE gain of transistor drops by 3dB from its value at low frequency.

Thus, the frequency range upto the cut-off frequency(fβ) is regarded as the bandwidth of circuit. Also, the current gain reduces to 70.7% of the low frequency current gain.

Likewise, the frequency at which short circuit CB gain of transistor drops by 3dB from its value next to low frequency is known as cut-off frequency (fα).

The relationship between these cut-off frequencies indicates that the 3dB frequency of CB configuration is (1 + hfe) times greater or higher as compared to that of CE configuration.