Feedback Amplifier Interview Questions and Answers:

1. What is feedback in amplifiers?

Ans. The process of combining a fraction of output energy back to the input is called the feedback.

2. Define transconductance and transresistance amplifiers.

Ans. The amplifier is transconductance amplifier in which the input signal is voltage and its output signal is current. The ideal transconductance amplifier provides an output current which is proportional to the signal voltage.

The amplifier is transresistance amplifier in which the input signal is current and the output signal is voltage.

The ideal transresistance amplifier provides a output voltage which is proportional to the signal current.

3. Define the term stability.

Ans. The variations in temperature, supply voltages, ageing of components or variations in transistor parameters with replacement are some of the factors that affect the gain of an amplifier and cause it to change.

The stability of an amplifier is defined as the ability of the amplifier to maintain the voltage gain constant as possible over as long a time interval as possible.

The overall voltage gain of the amplifier can be stabilized with negative feedback.

4. What is the effect of negative feedback on the input and output impedances of a voltage-series feedback amplifier?

Ans. Series voltage negative feedback increases the input impedance and reduces the output impedance by a factor (1 +βA), where βA is the feedback factor.

5. What is the effect of negative feedback on the bandwidth of an amplifier?

Ans. Bandwidth of an amplifier is increased, when negative feedback is introduced, by a factor (1 + βA), by which its gain is reduced.

6. How does negative feedback increases bandwidth of an amplifier?

Ans. The bandwidth (BW) of an amplifier without feedback is equal to the separation between 3-dB frequencies f1 and f2. If A is the gain then gain-bandwidth product is A x BW. With the negative feedback the amplifier gain is reduced and since gain bandwidth product has to remain constant in both cases, so obviously the bandwidth will increase to compensate for the reduction in gain.

7. Why voltage-series feedback is most commonly used in cascaded amplifiers?

Ans. In cascaded amplifiers, high input impedance and low output impedance are usually the main requirements and both of these requirements can be met by voltage series feedback, so it is most commonly used in cascaded amplifiers.

8. Why an emitter follower is called so?

Ans. In an emitter follower, output and input voltages are approximately equal ill magnitude and also in phase i.e., emitter Output voltage closely follows the input and that is why it is called the emitter follower.

9. What are practical applications of emitter follower?

Ans. Because of high input impedance and low output impedance an emitter follower is capable of transferring maximum power from the high impedance source to low impedance load. When an emitter follower is employed for this purpose, it is called a buffer amplifier. Though impedance matching can be achieved by employing a step-down transformer but emitter follower is preferred. It is because emitter follower is not only more convenient than a transformer but it also provides better frequency response.

There are many instances (especially in digital electronics) where an increase in current is required but no increase in voltage is required. For such situations emitter follower, because of its high current gain, can be employed.

11. Emitter-follower circuit is also called the common-collector amplifier circuit. Why?

Ans. Since in emitter-follower circuit collector terminal is common to both input and output, the circuit is called the common collector amplifier.

12. A common-emitter circuit without bypass capacitor is called a negative current feedback circuit. Why?

Ans. In case of a common-emitter circuit without bypass capacitor the emitter resistance provides the negative current feedback, so it is called a negative current feedback.

The current Ie flowing through resistor RE produces a voltage drop IeRE across emitter resistance RE. This voltage is feedback to the input and opposes the input signal as it is in opposition. This negative voltage feedback is proportional to collector current because Ie ≈ Ic. Thus negative current series feedback is provided.

13. What is Darlington connection?

Ans. Darlington connection may be considered a device providing externally only three terminals (base, emitter and collector) and having a very high current gain and high input resistance.

14. What is feedback pair ?

Ans. The feedback pair connection is a two transistor circuit that uses a PNP transistor driving an NPN transistor and operates like the Darlington connection and provides a very high current gain.

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