Question

In the circuit shown below voltage Vin is 3V and VB is 1V. The resistor R1 and R2 are 1K. Assume both diodes are identical. Forward bias resistance is 10Ω. Cut-in voltage of diode is 0.7V. The voltage Vout is __________

(a) 1.14V

(b) 1.23V

(c) 0.32V

(d) 1.34V

I got this question in an online interview.

The question is from Piecewise Linear Model of Diode-2 in portion Diode Circuit of Analog Circuits

Answer 1

Correct answer is (a) 1.14V

Explanation: Assume I be the current through the circuit. By kirchoff’s voltage rule,

Vout = IR2  ————(1)

Current through R1 (Vin-Vout-VD)/1.01k = (2.3-Vout)/1010

Current through diode D2 is 0 since D2 is in reverse bias mode.

current I = (2.3-Vout)/1010

Substitute this in eq(1)

That is, Vout = (2.3-Vout)/1010 x 1000  => 1.99Vout = 2.27  => Vout = 2.27/1.99 = 1.144V.