Large Signal or Power Amplifiers Interview Questions and Answers:

1. What is meant by power amplifier?

Ans. Power amplifier is meant to raise the power level of the input signal and may be defined as a device that converts dc power into ac power and whose action is controlled by the input signal.

2. How do you classify amplifiers? What are small signal amplifiers? How are they different from large signal amplifiers?

Ans. When the input signal is so weak as to produce small fluctuations in the collector current compared to its quiescent value, the amplifier is known as small signal amplifier. On the other hand, when the fluctuations in collector current are large i.e., beyond the linear portion of its characteristics, the amplifier is called the large signal amplifier. In case of a small signal amplifier, the transistor can be represented by a linear circuit model, and a simple analytical method can be used for determination of various performance characteristics of the amplifier. But in case of a large signal or power amplifier, the equivalent linear circuit model of the transistor and analytical method cannot be applied and graphical method is preferred because of involvement of certain nonlinear operation in it. Voltage amplifiers and audio power amplifiers are the small signal power amplifiers while radio power amplifiers are the large signal or power amplifiers.

3. Why a power amplifier is always preceded by a voltage amplifier.

Ans. Signal voltage at the input is quite small but a large voltage input signal is necessitated so as to provide large power at the output. So voltage amplifier is required for raising the voltage level of the weak input signal before it is fed to the power amplifier.

4. Why is a power amplifier called a large signal amplifier? Explain.

Ans. Power amplifiers are required to handle large voltage signals so as to deliver large power at the output. That is why power amplifiers are called large signal amplifiers.

5. What is the collector junction temperature for Ge and Si in case of transistor power amplifier?

Ans. The collector junction temperature for Ge lies in the range of 150°C to 225°C while in case of silicon it ranges 60°C to 100°C.

6. What is meant by power dissipation capacity of a power transistor?

Ans. Power dissipation capacity of a power transistor is its capability to dissipate the heat developed in it.

7. Why a class A power amplifier is cooler in the presence of signal than in the absence of signal?

Ans. With the zero signal applied at the input of the class A power amplifier, ac timer developed across the load reduces to zero and, therefore, all the power drawn from collector supply V_CC is wasted in the form of heat. Thus, a power transistor dissipates maximum power under zero signal condition. Thus the device is cooler in the presence of signal than in the absence of signal.

8. What is the maximum collector circuit efficiency of transformer coupled class A power amplifier?

Ans. 50 per cent.

9. Why do we use transformer in the output stage of an amplifier?

Ans. For transfer of maximum power from amplifier to the output device matching of amplifier output impedance with the impedance of the output device is necessary and this is accomplished by using a step-down transformer of suitable turn ratio at the output.

10. What principal factor contributes to the doubling of the conversion efficiency in a transformer coupled load amplifier compared with an amplifier with a direct coupled resistive load ?

Ans. Negligible power loss in the transformer primary (in the collector load) is the major factor that contributes to the doubling of the conversion efficiency in a transformer coupled load amplifier compared with an amplifier with a direct coupled resistive load.

11. What advantages does the transformer provide in a transformer coupled power amplifier?

Ans. Transformer provide the following advantages in a transformer coupled power amplifier.

1. Power loss in the primary of the transformer, used for coupling the load to the amplifier, is negligible and therefore conversion efficiency is doubled in a transformer coupled-load amplifier compared with a direct coupled resistive load.
2. Output transformer makes the transfer of maximum power to the load through impedance matching.
3. Output transformer prevents a large dc current from flowing in the load, which otherwise could be harmful if the load were a loudspeaker.

12. Under what conditions would the dc load line in a transformer coupled amplifier not be drawn vertically on the output characteristics? Which way would it slope?

Ans. If the resistance of the transformer primary is significant and biasing stabilizing resistance is included, the dc load line will slope slightly to the left.

13. Why may an amplifier be operated with a load resistance lower than the value that would provide maximum output power ?

Ans. From the power output and distortion load resistance curves, it is seen that the load providing minimum total harmonic distortion is lower than the load that provides maximum power output. That is why, an amplifier is operated with a load resistance lower than the value that would provide maximum output power so that output with least distortion is obtained.

14. Comment on the maximum efficiency of class B operation.

Ans. The maximum possible value of efficiency for class B operation is 78.5%. The efficiency of class B operation exceeds that of class A operation because with zero excitation there is no current in class B operation whereas power is drawn from supply source in class A operation.

15. Although a class-B single ended power amplifier has high efficiency, yet it is less used in practical circuits. Explain why?

Ans. Severe distortion occurs, because of total absence of negative half cycles from the output in a class-B single ended power amplifier so it is less used in practical circuits.

16. What is the use of class C amplifiers?

Ans. The use of class C amplifiers is limited for a fixed frequency, as occurs in communication circuits.

17. Why class D power amplifiers are becoming more and more popular?

Ans. Class D power amplifiers are becoming popular because of their very high efficiency (above 95%).

18. What is class A push-pull amplifier?

Ans. Class A push-pull amplifier is that in which current flows in the output of each transistor for the whole of the input cycle.

19. Why does the output stage employ push-pull arrangement?

Ans. Push-pull arrangement is used in the output stage in order to have high output power at high efficiency with less distortion.

20. What is crossover distortion? How can it be minimized?

Ans. Crossover distortion occurs in class B push-pull amplifiers as a result of one transistor cutting off before the other begins conducting. The distortion introduced is called the crossover distortion because it occurs during the time operation crossover from one transistor to the other in the push-pull amplifier.

For elimination of crossover distortion, it is necessary to add a small amount of forward bias to take the transistors to the average of conduction or slightly beyond. Technically the operation of transistors will lie between class B and class A mode (i.e., class AB operation).

21. The complementary symmetry push-pull amplifiers are called so. Why?

Ans. The complementary symmetry push-pull amplifier uses two transistors having complementary symmetry (one N-P-N and another P-N-P). The term complementary arises from the fact that one transistor is the N-P-N and the other is P-N-P. They have the symmetry as they are made with the same material and technology and are of same maximum ratings.

22. What is the maximum collector circuit efficiency of a comple­mentary push-pull class B power amplifier?

Ans. Maximum collector circuit efficiency of a class B complemen­tary push-pull power amplifier is 78.5%.

23. What are the drawbacks of complementary symmetry push-pull class B power amplifier?

Ans. One obvious drawback of this circuit arrangement is the requirement for two supply voltages. Another drawback of this circuit is difficulty of obtaining matched complementary transistors. If there is an unbalance in the characteristics of the two transistors, even harmonics will no longer be cancelled and considerable distortion will be introduced. Very often negative feedback is used in power amplifiers to reduce nonlinear distortion.

24. Why the power rating of a power transistor is required to be reduced when it is operated above ambient temperature of 25°C ?

Ans. The heat flow from the transistor junction to the surroundings depends on the difference of junction temperature and the ambient temperature. As the ambient temperature increases, rate of heat flow and so the power rating of the transistor decreases. Thus the power rating of a power transistor is required to be reduced when operated above ambient temperature of 25°C.