Question

Find net voltage gain, given hfe = 50 and hie = 1kΩ.

(a) 27.68

(b) -22

(c) 30.55

(d) -27.68

This question was addressed to me by my school principal while I was bunking the class.

This intriguing question comes from Low Frequency Response and Miller Effect Capacitance topic in section Transistor Frequency Response of Analog Circuits

Answer 1

Correct option is (d) -27.68

Explanation: Apply millers theorem to resistance between input and output.

At input, RM=100k/1-K = RI

Output, RN=100k/1-K^-1 ≈ 100k

Internal voltage gain , K = -hfeRL’/hie

K = – 50xRc||100k/1k = – 50x4x100/104 = – 192

RI = 100k/1+192 = 0.51kΩ

RI’ = RI||hie = 0.51k||1k = 0.51×1/1.51 = 0.337kΩ

Net voltage gain = K.RI’/RS+RI’ = – 192 x 0.337/2k + 0.337k = -27.68.