Question

Consider the following circuit. IDSS = 2mA, VDD = 30V. Find R, given that VP = – 2V.

(a) 10kΩ

(b) 4kΩ

(c) 2kΩ

(d) 5kΩ

This question was addressed to me in an interview for job.

My question is taken from Biasing of JFET and MOSFET in section FET Amplifiers of Analog Circuits

Answer 1

Correct choice is (b) 4kΩ

Easy explanation: IDSS = 2mA

IDS = (VDD – 15)/50k = 0.3mA

VGS = VP[1 – \(\sqrt{\frac{I_{DS}}{D_{SS}}}\)]

VGS = -2 x (1 – \(\sqrt{.15}\)) = – 1.22V

Thus, VGS + IDS x (R) = 0

R = 1.22/0.3mA = 4kΩ.