Question

Two Zener diodes (A and B) having breakdown voltages of $ 6V$ and $ 4V$ respectively, are connected as shown in the circuit below. The output voltage $ {V}_{O}$ variation with input voltage linearly increasing with time is given by $ \left({V}_{input}=0V at t=0\right)$ (figures are qualitative).

• A.

  

• B.

  

• C.

  

• D.

  

Answer 1

The correct option is D

The explanation of the correct option -

Option (D)-

Zener diode:

• Zener diode, invented by Clarence Melvin Zener, is a diode specially designed to allow backward the flow of current, when acertain voltage called Zener voltage is reached.
• It works on the principle of Zener effect , an electric breakdown taking place in the reverse p-n junction.
• It can be used as a voltage regulator.

Effect on output voltage:

• When the input voltage is $0$ at $t=0$, the output voltage will also be $0$.
• After that, as the input voltage increases linearly, the output voltage will also increase in the same trend with input voltage until it reaches $4V$.
• As theinput voltage reaches $4V$, the diode with breakdown voltage of $4V$ will break and stop functioning.
• With further increase in input voltage, the output voltage will also increase gradually. But this time with a slower rate, as one of the diodes has stopped functioning.
• As the input voltage reaches $6V$, the diode with breakdown a voltage of $6V$ will also break and stop functioning.
• When the input voltage is increased further, the output voltage remains constant with time, because there are no diodes functioning.

Graph:

• So, the graph of output voltage with time will be:

The explanation of incorrect options -

Option(A)-

1. There is a gradual increase in input voltage therefore there would be a gradual increase in output voltage too
2. But this graph shows that there is a sudden increase in output voltage from $4Vto6V$

Option(B)-

1. At breakdown voltages, the graph becomes constant, therefore inclines a bit
2. Here, voltage breakdown at $4V$ is not shown

Option(C)-

1. At breakdown voltages, the graph becomes constant, therefore inclines a bit
2. Here, voltage breakdown at $6V$ is not shown

Hence, the correct option is option D.