Question

Given Vout = 5V, β=100, I1=10mA, R1=100KΩ. Find the output resistance.

(a) 8 kΩ

(b) 8.163 kΩ

(c) 7.582 kΩ

(d) 8.4 kΩ

This question was posed to me during a job interview.

My question is based upon Transistor Bias Configuration in chapter Transistor Biasing and Low Frequencies of Analog Circuits

Answer 1

Correct choice is (b) 8.163 kΩ

To explain I would say: The circuit is a current mirror, whose output resistance, ROUT = (VA + VCE)/IOUT

Here, IOUT = I1 / (1+ 2/β) = 10mA / (1 + 2/100) = 9.8 mA

VCE = VOUT = 5V

VA = Early voltage = 75 V (obtained from the graph)

ROUT = (VA + VCE)/ IOUT = (75 + 5) / 9.8 * 10^-3

ROUT = 80,000/9.8 = 8.163 kΩ.