Question

Consider the following circuit. Find the power dissipation of the transistor given that the diode breakdown voltage is 5V, R1=20kΩ, R2=100kΩ, R3=200kΩ, R4=10Ω. The source voltage VS=20V.

(a) 12.12W

(b) 9.375W

(c) 9.27W

(d) 10.575W

I have been asked this question in a job interview.

This key question is from Voltage Regulators in chapter Regulators of Analog Circuits

Answer 1

Correct answer is (b) 9.375W

For explanation I would say: Power dissipation = VCE x IC

VCE = VC – VE = VS – VOUT = 20-5(1+R2/R3) = 20-5×3/2 = 20-7.5 = 12.5V

IC ≈ IE = load current, since R2 and R3 are greater than the load, so most current flows through the load.

IL = VOUT/10 = 0.75A

P = 12.5 x 0.75 = 9.375W.