# Microelectronics Questions and Answers – Applications of a Diode – Set 2

This set of Microelectronics Multiple Choice Questions & Answers (MCQs) focuses on “Applications of a Diode – Set 2”.

1. What will happen to the following circuit in absence of the diode?

a) The ripple voltage decreases
b) The voltage doesn’t get affected
c) The current becomes 0
d) A pulsating voltage remains

Explanation: The capacitor voltage is stopped from reducing by the diode. The property of unilateral conduction of diodes proves very important in these applications.

2. If the input voltage is 10sin(t) and the diode ha a threshold of 0.7V, what happens to the voltage across the capacitor? Use the diode approximation.

a) It is maintained at 9.3V
b) It is maintained at 0V
c) It is maintained at 6V
d) It is maintained at 5V

Explanation: The resistance offered by D2 is extremely large. The capacitor doesn’t have a discharging path and hence it’s voltage is maintained at 9.3V. So long as the sinusoidal input is present, the voltage will stay stuck at 9.3V.

3. Each diode has a threshold of VTh while this voltage can be assumed to be constant if the resistance offered by the semiconductor regions are neglected. Find which diode remains always off.

a) D3
b) D6
c) D7
d) D1

Explanation: A branch is defined by the stretch of a conductor and the elements residing in the path of the branch. Observe the branch in where D3 is placed, let’s call it branch b1. This branch is in parallel to the branch holding diodes D5, D6 and D4, we can name that b2. This is because the two terminals of branch b1 are connected with the two terminals of branch b2. Henceforth, D6 and D5 will never turn on as D3 will maintain a constant voltage drop of VTh.

4. Each diode has a threshold of VTh while this voltage can be assumed to be constant if the resistance offered by the semiconductor regions are neglected. If D11 is on, choose the minimum Vin.

a) 7VTh
b) 6VTh
c) 8VTh
d) VTh

Explanation: To turn D11 on, we will just have to turn all the diodes which are in the same branch connecting it to Vin. So, we trace back from D11 and find that 7 diodes are present including it. This helps us to suggest that a minimum input voltage of 7VTh is required.

5. We often find a humming noise coming out of our speaker. This humming noise is due to the ripple voltage generated within the system connecting to the speaker. How can we reduce this humming sound?
a) Increase the discharging time for the capacitor
b) Reduce the discharging time for the capacitor
c) Reduce the power supply level
d) Reduce volume

Explanation: Even though reducing the power supply level and volume can reduce the humming sound, they are not very good solutions as we should expect. The amplitude of the ripple voltage is greatly reduced if we can just decrease the discharge time constant of the capacitor which, in turn, will reduce the humming effect received by the speaker.

6. Smoothening capacitors are used to generate a ____ output from an A.C. signal.
a) D.C. signal
b) Pulsating D.C. signal
c) Attenuated A.C. signal
d) No such term as smoothening capacitor

Explanation: A capacitor is referred to as a smoothening capacitor when it provides a D.C. output for an A.C. input. Such configurations are seen with diodes where the discharge path of the capacitor is blocked so that it can maintain a constant voltage across it.

7. A voltage of 10 sin(t) is applied to the following circuit where the diode has threshold of 0.7V. What happens to the height of the ripple if the frequency increases?

a) Remains constant
b) Decreases
c) Increases
d) Incomplete data

Explanation: The total ripple voltage is given by (Vo-Vth)/fRC, where f is the frequency of the signal. If the frequency decreases, the height of the ripple voltage increases.

8. In the following circuit, the threshold voltage of each ideal diode is 0.7V. What will be the maximum voltage during the positive cycle?

a) 9V
b) 4.5V
c) 6V
d) 3.33V

Explanation: The voltage across the resistors with the diodes is always Vin/3. Hence, the maximum voltage will be 3.33V. This is greater than the voltage drop across R1 by 0.7V.

9. In the following circuit, the threshold voltage of each ideal diode is 0.7V. What will be the maximum voltage during the negative cycle?

a) -3.33V
b) -9V
c) 0V
d) -5.2V

Explanation: The diode starts conducting when the negative voltage reaches about -1.4V while it stops when the voltage is -3.33V. It should be noticed that this approximation helps us in doing the analysis easily.

10. In the following circuit, V1=10sin(t). What will be the value of ripple amplitude for low frequency signals?

a) High
b) Low
c) Very low
d) Moderate

Explanation: The frequency is simply 1 Hz. The charging and the discharging constants are quite high. The capacitor can only discharge through the resistor since the diode doesn’t allow any current from n to p side. The time constant is equal to the input frequency. The ripple amplitude becomes very low since the capacitor cannot discharge much before cycle changes.

Sanfoundry Global Education & Learning Series – Microelectronics.

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