Important Two Marks Questions with Answers of Power Amplifiers and DC/DC Converters

TWO MARKS QUESTION WITH ANSWERS

1. What is power amplifier?

Power amplifier amplifies a weak signal until sufficient power to operate output device such as loudspeaker.

2. Compare small signal and large signal amplifier?

3. What are the features of power amplifier?

i. Impedance matching to deliver maximum power to the load.

ii. Need power transistors to withstand large voltages and current.

iii. Power amplifiers are bulk.

iv. Harmonic distortions occur due to non-linear character of transistors.

4. Define circuit efficiency.

It is defined as the ratio of output ac power to the input dc power. It is also known as Figure of merit.

5. What is meant by class of amplifier?

Amplifier classes represent the amount of output signal variation over one cycle of operation when excited by a sinusoidal input signal.

6. What is class A power amplifier?

A power amplifier is known as class A amplifier if the transistor used in the circuit conducts for full cycle of the input signal. The operating point is selected approx. at the center so that the output current follows the input signal.

7. Write the merits and demerits of class A power amplifier.

Advantages

i. Simple construction

ii. Distortionless output voltage

Disadvantages

i. Very low efficiency (25%)

ii. Large power dissipation in the transistors

iii. High output impedance

8. What is meant by cross over distortion?

In Push Pull operation of Class B amplifiers, the output waveform is not a perfect sinusoidal waveform. The output waveform will be as shown in Figure.

The drive signal applied to class B transistor must reach a certain minimum level before its collector current is properly in active region

9. Differentiate push pull and complementary symmetry class B amplifier.

10. What are the types of distortion.

i. Amplitude distortion

ii. Frequency distortion

iii. Delay or phase shift distortion

iv. Cross over distortion

11. What is power MOSFET?

Power MOSFET is a special type of MOSFET designed to handle high power levels.

Power MOSFET are commonly used power devices due to the following advantages.

i. Low gate drive power

ii. Fast switching speed

iii. Parallel operation

12. Compare Class A, Class B, Class AB power amplifiers.

13. Define velocity saturation.

The high electric field in the short channel increases the velocity of charge carriers to be maximum. This is known as velocity saturation.

14. Compare power MOSFET and power BJT.

i. MOSFET provides a very good isolation between the gate and other two terminals compared to BJT

ii. MOSFET can handle more power compared to BJT

iii. MOSFET has very low power less and a high speed

iv. Voltage signals can easily operate a MOSFET, hence it is used in many digital circuits for switching applications ex: motor control circuits.

v. Power MOSFET do not suffer from second breakdown compared to BJT.

vi. Power MOSFET do not require large dc large- drive current as that in power BJTs.

vii. Higher speed of operation than power BJT,

15. What is DC to DC converter.

The DC to DC converters convert one level of DC voltage to another level. It is also known as DC chopper.

16. What are the applications of dc choppers?

i. Traction motor control in electric automobiles, trolley cars/marine hoists, trucks, minehaulers. They provide smooth acceleration control, high efficiency fast response.

ii. Regenerative breaking of de motors to return energy back into supply leading to energy saving for transportation with frequent stops.

iii. In DC voltage regulators.

17. Define Buck converter?

Buck converter is also known as step down converter and produces a lower output voltage than input voltage. The average output voltage is less than the input voltage.

18. Write the applications of Buck converter.

i. Used to drive high current loads, used in PCs, mother boards

ii. Battery chargers

iii. Solar chargers

19. Define Boost converter.

The boost regulator produces an output voltage greater than input voltage.

20. What is Buck-Boost Converter?

A Buck-Boost Regulator produces an output voltage which may be less than noitsioat boog or greater than the input voltage.

The polarity of output voltage is opposite that of input voltage. So, this regulator is also known as inverting or flyback regulator.

21. Write the advantages of Converter.

i. Requires only one transistor

ii. Simple construction

iii. High efficiency

iv. Less expensive

22. Differentiate Buck and Boost Converter.

23. Why do we need power amplifier?

When a large amount of power is needed to be delivered to the load, power amplifiers are used.

24. Define cross over distortion.

Near the zero crossing points, neither of the transistors Q₁ and Q₂ conducts which produces a gap between positive and negative half of the output signal. As a result, the output waveform is not continuous. This distortion is called cross- over distortion.

REVIEW QUESTIONS

1. Explain the operation of class A amplifiers and derive its efficiency.

2. Explain in detail transformer coupled class A power amplifier.

3. Draw the circuit of pushpull classB amplifier and explain its operation.

4. Write notes on distortions in amplifiers.

5. Explain the operation of class C amplifier with neat diagram.

6. Explain the operation of Buck converter in detail.

7. Discuss the modes of operation of Boost converter, draw its waveform and find the ripple voltage of filter capacitor.

8. Explain the operation of Buck-Boost converter in detail.