Question

Consider the biasing circuit shown. The β for the circuit is large. R3 = 1kΩ, R4 = 2kΩ. The stability factor varies between 10 and 11. Find the maximum and minimum values of R2.

(a) Minimum = 16.36kΩ, Maximum = 20kΩ

(b) Minimum = 16.36kΩ, Maximum = 18kΩ

(c) Minimum = 10 kΩ, Maximum = 20kΩ

(d) Minimum = 6 kΩ, Maximum = 10kΩ

I got this question during an online interview.

Enquiry is from Transistor Bias Configuration in section Transistor Biasing and Low Frequencies of Analog Circuits

Answer 1

Right option is (a) Minimum = 16.36kΩ, Maximum = 20kΩ

For explanation I would say: Circuit is a self bias circuit.

Base resistance = RB = R1*R2 / (R1+R2)

Since β is large, stability factor, S = 1 + RB/RE = 1 + RB/R3

1 + RB/R3 = 10

RB/R3 = 9 => RB = 9k => R2 = 16.36 kΩ

For S = 11

RB/R3=10 => RB = 10k => R2 = 20kΩ.