Question

Find the currents through the resistance in the circuits shown in figure. 

(Assume that the resistance of each diode is zero in forward bias and is infinity in reverse bias.)

Answer 1

We know that under forward bias, an ideal diode acts as a short circuit; and under reverse bias, an ideal diode acts as an open circuit. 

(a) Since both the diodes are forward biassed, they can be replaced by short circuits.
Net resistance = 2 Ω

\[i = \frac{2  V}{2  \Omega} = 1  A\]

(b) One diode is forward biassed and the other is reverse biassed; thus, the resistance of one becomes ∞. The circuit becomes open at the position of the second diode, so no current flows through this arm.

\[\therefore i = \frac{2}{2 + \infty} = 0  A\]

(c) Both of them are forward biassed. Thus, the resistance is zero.
We can replace both the resistors by short circuits.
Net resistance = 2 Ω

\[\therefore i = \frac{2}{2} = 1  A\]

(d) One of them is forward biassed and the other is reverse biassed.
Thus, the current passes through the forward-biassed diode and the other acts as open circuit.

\[\therefore   i = \frac{2}{2} = 1  A\]