Multistage Amplifier Interview Questions and Answers:

1. Differentiate single stage and multistage amplifiers.

Ans. When only one transistor with associated circuit is used for increasing the strength of a weak signal, the circuit is known as single stage amplifier while a transistor circuit containing two or more stages of amplification is known as a multistage amplifier. A practical amplifier is always a multistage amplifier.

2. Why do you cascade the amplifiers?

Ans. The voltage/power gain or frequency response obtainable from a single stage amplifier is usually not sufficient to meet the needs of either a composite electronic circuit or load device, so cascading of amplifiers is required to provide greater voltage or current amplification or both.

3. List the types of couplings.

Ans. The various coupling schemes used in cascaded amplifiers are

(i) R-C coupling (ii) transformer coupling (iii) impedance coupling and (iv) direct coupling.

4. What are the different types of configurations used in multistage amplifier circuits?

Ans. The most suitable transistor configuration for cascading is CE configuration because the voltage gain of common emitter amplifier is greater than unity while CC configuration has voltage gain less than unity and the voltage gain of CB configuration using cascading is also less than unity.

However, for input stage CC or CB configuration may be required for proper impedance matching at the cost of voltage or current gain. It is noteworthy point that for input stage, the consideration is not the maximum voltage gain but the impedance matching of the source with the input impedance of the input stage. Some driving sources may need input circuit to be an almost open circuit while others need an almost short circuit. In certain cases choice of configuration for the input stage, is the minimization of noise and maximization of signal/ noise power ratio.

5. Why the overall gain of a multistage amplifier is less than the product of gains of individual stages?

Ans. The overall gain of a multistage amplifier is less than the product of gains of individual stages because of the loading effects of the following stages.

6. Why coupling capacitor provided in a self biased CE R-C coupled amplifier is also called the blocking capacitor?

Ans. In a self biased R-C coupled CE amplifier, coupling capacitor transmits ac signal but blocks the dc voltage of the first stage from reaching the base of the next stage, so it is also called the blocking capacitor.

7. Why does R-C coupling give constant gain over mid frequency range?

Ans. In mid frequency range, the voltage gain of an R-C coupled amplifier remains almost constant. This is because in mid frequency range, with the increase in frequency, the reactance of the coupling capacitor decreases thereby increasing the gain but at the same time lower capacitive reactance causes higher loading resulting in lower voltage gain. Thus the two effects neutralize each other and uniform gain is obtained in mid frequency range.

8. Why are R-C coupled amplifiers widely used as voltage amplifiers?

Ans. R-C coupled amplifiers are widely used as voltage amplifiers because of their excellent audio fidelity over a wide range of frequency.

9. How do you improve the gain of an R-C coupled amplifier at low frequencies?

Ans. The voltage gain of an R-C coupled amplifier at low frequencies can be improved by (i) selecting very large coupling and bypass capacitors so that their reactance is small (ii) making (Rs + RB) ≪ (RC + RL) and (iii) selecting transistor with small hie.

10. What is meant by bandwidth?

Ans. The difference between the upper cut-off frequency f2 and lower cut-off frequency f1 is called the bandwidth.

11. Name the characteristics of a wideband amplifier that are improved by the compensation.

Ans. The characteristics that are improved by compensation of wideband amplifiers are:

  1. Shortening of rise time.
  2. Reduction in tilt.

12. Why a bypass capacitor is used on the bottom of each secondary winding in a transformer coupled amplifier?

Ans. In a transformer coupled amplifier, bypass capacitor is used on the bottom of each secondary winding to get an ac ground and to prevent the loss of signal power in the biasing resistors.

13. Why coupling capacitors are not required in a transformer coupled amplifier?

Ans. No coupling capacitor is required in a transformer coupled amplifier, because dc isolation between the two stages is provided by the transformer itself.

14. What type of coupling would you use for amplification of a signal obtained from a thermocouple meant for measuring temperature of a furnace?

Ans. Direct coupling.

15. Why direct coupling is not suitable for amplification of high frequency signals?

Ans. At high frequency, the gain of the direct coupled amplifier falls off due to interelectrode capacitance of the device and wiring capaci­tance, so it is not used for amplification of high frequency signals.

16. What are the main characteristics of cascode amplifier?

Ans. The main characteristics of cascode amplifier are high input impedance. the same voltage gain as that of a CE amplifier but very low input capacitance so as to provide good high-frequency operation.

17. What is differential amplifier?

Ans. An amplifier, which is designed to amplify the difference between two input signals is called the differential amplifier. The differential amplifier is also referred to as a difference amplifier.

18. What do you mean by distributed amplifier.

Ans. Lumped-circuit delay lines are essential elements in a type of pulse amplifier which is referred to as a distributed amplifier.

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