Single Stage Transistor Amplifiers Interview Questions and Answers:

1. What is an amplifier?

Ans. The device that amplifies the amplitude of the input signal is called the amplifier. An amplifier may be defined as a device that increases the current, voltage or power of an input signal with the help of a transistor by furnishing the additional power from a separate source of supply.

2. What is meant by small signal amplifier?

Ans. When the input signal is quite weak and produces small fluctuations in the output current in comparison to its quiescent value, the amplifier is called the small signal or voltage amplifier.

3. Write the characteristics of an ideal voltage amplifier.

Ans. Main characteristics of an ideal voltage amplifier is to present a high impedance to the input source so as to minimize the loading effects and to provide a large enough voltage signal to the output device like loudspeaker.

4. Explain how phase reversal of the signal takes place when it is amplified by a single stage CE voltage amplifier.

Ans. In a CE configuration, the output voltage increases in the negative direction when the input signal voltage increases in the positive direction and vice versa. This is called the phase reversal and causes a phase difference of 180° between the input signal voltage and output voltage.

5. What is an ac emitter resistance?

Ans. The dynamic resistance of the emitter-base junction diode is called the ac emitter resistance. It is given as 25mV/IE where IE is the dc emitter current at Q-point.

6. What is the effect of source resistance on voltage gain of a common base transistor amplifier?

Ans. The voltage gain of a CB transistor amplifier will decrease if source resistance is considered because in such a case there will be a voltage drop across the source resistance and output voltage will decrease.

7. Why is coupling capacitor used to connect a signal source to an amplifier?

Ans. An electrolytic capacitor, called the input capacitor, Cin of capacity of about 10 μF is used to couple the signal source to the transistor base. In the absence of this capacitor, the signal source resistance will come across resistor R2 of the biasing network and thus change the bias. This capacitor allows only ac signal to pass but isolates the signal source from R2.

8. What is the function of bypass capacitor?

Ans. The capacitor CE, called the emitter bypass capacitor, is connected in parallel with the emitter resistance Rin order to provide a low reactance path to the amplified ac signal. In the absence of this capacitor, amplified ac signal will cause a voltage drop across it which in turn will feedback the input side and reduce the output voltage.

9. State what will happen to the voltage gain of an amplifier if the bypass capacitor is open circuited.

Ans. Removal of bypass capacitor in a CE amplifier circuit causes excessive degeneration in the amplifier circuit and, therefore, reduction in voltage gain.

10. Why common collector amplifier is called an emitter follower?

Ans. The CC circuit amplifier is called an emitter follower because in this circuit the output voltage at the emitter terminal follows the input signal applied to the base terminal.

11. What configuration of bipolar transistor is called as emitter follower? For what purpose it is used.

Ans. The common collector configuration of bipolar transistor is called as emitter follower because in this circuit the output volt­age at the emitter terminal follows the input signal applied to the base terminal. This configuration finds wide application as a buffer amplifier between a high impedance source and a low impedance load.

12. What are the main purposes for which a common collector amplifier may be used?

Ans. For a common collector amplifier, current gain is as high as for CE amplifier, voltage gain is less than unity, input resistance is the highest and the output resistance is the lowest of all the three (CE, CC and CB) configurations. This circuit finds wide application as a buffer amplifier between a high impedance source and a low impedance load.

13. 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 he represented by a linear circuit model, and a simple analytical method can he used for determination of various performance characteristics of the amplifier. But in case of a large signal power amplifier, the equivalent linear circuit model of the transistor and analytical method cannot he 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 power amplifiers.

14. What is meant by distortion in amplifier?

Ans. Under ideal conditions, amplified output signal must have exactly the same waveform as the input signal, But in practical amplifiers, this ideal condition is never achieved. Some changes in the waveform may take place in addition to the increase in the amplitude. Such a change is called the distortion and is undesirable because it may change the intelligence (useful information) carried by the signal, as illustrated in Fig. 1.

15. Define harmonic distortion.

Ans. The frequency-domain of nonlinear distortion (harmonic distortion) is depicted in Fig. 2. The input is a single-frequency (f1) signal but the output consists of dc component and different harmonics. These harmonics are integral multiples of the input signal frequency. The result is that we obtain distorted output. The distortion magnitude depends on the strength and number of these harmonics.

In audio amplifiers employed for amplification of speech or music, the less harmonic distortion, the better performance. For speech harmonic distortion should not exceed 10% otherwise intelligibility will suffer. High fidelity or Hi-Fi amplifiers have harmonic and intermodulation (IM) distortion of less than 1 percent.

16. Why distortion in amplifier is considered harmful?

Ans. Because distortion may change the intelligence (useful information) carried by the signal.