Question

Current I if V = 5V and -5V when VB = 2V, R1 = 2K, R2 = 4K respectively are __________ (Use constant voltage drop model assumption and take VD = 0.5V)

(a) 1.3mA, 0.23mA

(b) 2.875mA, 0mA

(c) 2mA, 0mA

(d) 1.423mA, 0 mA

I had been asked this question in examination.

The above asked question is from Constant Voltage Drop Model-2 in section Diode Circuit of Analog Circuits

Answer 1

Correct choice is (b) 2.875mA, 0mA

Easy explanation: In constant voltage drop model at forward bias diode can be replaced as a cell and in reverse bias diode can be avoided by considering the terminals are open.

When V=5V, diode is in forward bias and net total voltage becomes 4.5V across R1.

Current through branch 1 will be 4.5V / 2K = 2.25mA.

Current through branch 2 will be (4.5-2)/4K = 0.625mA.

So net current is sum of these two. Therefore, net current is 2.875mA.

When V = -5V, the diode is reverse biased and net current flowing is zero.