Question

A circuit with Si and Ge diodes is shown in the

accompanying figure. The barrier potential for

Silicon is VSi=0.7 V and that of Germanium is

VGe=0.3 V. If the input voltage is 20 V, then

the voltage VA measured across the resistance

RL = 1 kΩ is

(A) 19.7 V (B) 19.3 V

(C) 19.0 V (D) 18.7 V​

Answer 1

Answer: (A) 19.7 V

Explanation:

1) We are given a circuit as shown in figure.

We have input resistance,

Vo = 20 V.

For the case of 2nd (Ge) diode, potential across that diode is negative .

=> no current flows across that.

2) For the case of 1st and 3rd diode,

Barrier Potential across Germanium diode is smaller than that of silicon diode.

So, firstly germanium diode gets on and hence current across the silicon diode doesn't flows.

3) So, potential across the GE diode will be 0.3V .

=> Potential across the resistor 100 oh my will be

(20-0.3) = 19.7 V.

We can also find current across the resistor by ohms law.