Transistor Biasing and Stabilization Interview Questions and Answers:

1. Explain the term biasing.

Ans. The proper flow of zero signal collector current and the maintenance of proper collector-emitter voltage during the passage of signal is called the transistor biasing.

If the transistor is not biased properly, it would work inefficiently and produce distortion in the output signal. A transistor is biased either with the help of battery or associating a circuit with the transistor. The latter method is more efficient and is frequently used. The circuit used for transistor biasing is called the biasing circuit. It may be noted that transistor biasing is very essential for the proper operation of transistor in any circuit.

2. What is operating point? Explain its physical significance.

Ans. The zero signal values of I_C and V_CE are known as the operating point. It is called operating point because the variations of I_C and V_CE take place about this point when signal is applied. It is also known as quiescent point or Q point.

The operating point defines where the transistor will operate on its characteristics curves under dc conditions. For linear amplification (amplification with minimum distortion), the operating point should not be too close to the maximum power, voltage, or current rating besides avoiding the regions of cutoff and saturation.

3. What is faithful amplification?

Ans. The process of raising the strength of a weak signal without any change in its general shape is referred to as faithful amplification.

4. What is operating point? Why is it necessary to stabilize operating point of a transistor amplifier?

Ans. The zero signal values of I_C and V_CE are known as the operating point. It defines where the transistor will operate on its characteristics curves under dc conditions.

The stabilization of operating point is necessary because of (i) temperature dependence of collector current I_C (ii) individual variations and (iii) thermal runaway.

5. How does I_CBO vary with the temperature?

Ans. I_CBO is exactly like the reverse saturation current of a reverse biased diode, which, theoretically, increases by 8 per cent per °C for silicon and 11 per cent per °C for germanium. But from experimental data it is found that the reverse saturation current increases 7 per cent for both silicon and germanium. This is because a surface leakage current component of reverse saturation current is independent of temperature. Since (1.07)¹⁰ = 2.0, the reverse saturation current approximately doubles for every 10°C rise in temperature.

6. What is meant by thermal runaway in a transistor? Explain?

Ans. The collector current l_C, being equal to βI_B + (1 + β) I_CO, increases with the increase in temperature. This leads to increased power dissipation with further increase in temperature. Being a cumulative process it can lead to thermal runaway resulting in burn-out of the transistor. The self destruction of an unstabilized transistor is called the thermal runaway.

7. Explain bias stabilization.

Ans. Only the fixing of a suitable operating point is not sufficient but it is also to be ensured that the operating point remains stable i.e., it does not shift due to change in temperature or due to variations in transistor parameters (due to replacement of transistor). Unfortunately it is not possible in practice unless special efforts are made to achieve it.

The maintenance of the operating point stable (independent of temperature variations or variations in transistor parameters) is known as stabilization.

8. What is meant by midpoint biasing?

Ans. When an amplifier circuit is so designed that the operating point Q lies at the centre of the dc load line, the amplifier is said to be midpoint biased.

9. Which two methods make the collector current constant? Explain them.

Ans. Operating point stability may be achieved by adopting either stabilization techniques or compensation techniques.

10. Stabilization technique makes use of a resistive biasing circuit that permits such a variation of base current (biasing current) I_B as to maintain collector current l_C almost constant inspite of variations in reverse saturation current I_CO, base-emitter voltage V_BE and β.

11. Compensation techniques make use of temperature sensitive devices such as diodes, transistors, thermistors etc. Such devices produce compensating voltages and currents so as to make operating point stable.

12. What is the requirement of good biasing circuit?

Ans. Requirements of Biasing Circuit. A biasing network associated with a transistor should fulfill the following requirements :

1. Establish the operating point in the middle of the active region of the characteristics, so that on applying the input signal the instantaneous operating point does not move either to the cutoff region or to the saturation region, even at the extreme values of the input signal.
2. Stabilize the collector current I_C against temperature variations.
3. Make the operating point independent of transistor parameters so that replacement of transistor by another transistor of the same type in the circuit does not shift the operating point.

13. Explain why bias stabilization is done in a bipolar junction transistor circuit.

Ans. Only the fixing of a suitable operating point is not sufficient but it is also to be ensured that the operating point remains stable i.e., it does not shift due to change in temperature or due to variations in transistor parameters (due to replacement of transistor). Unfortunately it is not possible in practice unless special efforts are made to achieve it.

The maintenance of the operating point stable (independent of temperature variations or variations in transistor parameters) is known as stabilization.

The stabilization of operating point is essential because of (i) temperature dependence of collector current l_C (ii) individual variations and (iii) thermal runaway.

14. Define stability factor.

Ans. Stability factor is defined as the rate of change of collector current l_C w.r.t. reverse saturation current I_CO with V_BE and β constant.

15. What is the significance of stability factor in transistor operation?

Ans. The stability factor indicates the change in collector current l_C due to change in collector leakage current I_CO. Thus a stability factor of high value say 50 means that l_C changes 50 times as much as any change in I_CO. In order to achieve greater thermal stability, it is desirable to have as low stability factor as possible. Thus importance of stability factor is with regard to stability of the transistor.

16. Why fixed bias circuit, inspite of its simplicity, is not much used in practice?

Ans. Rise in temperature causes increase in leakage current I_CO, increase in current amplification factor β and decrease in base-emitter voltage V_BE, so operating point is not stabilized. This is the reason that fixed bias circuit is not commonly used.

17. Explain why collector-to-base bias circuit is seldom used?

Ans. The base resistor R_B not only provides a dc feedback for the stabilization of the operating point, but it also causes an ac feedback in a collector-to-base bias circuit. This is the reason that this circuit is not much used.

18. Why bias compensation is required?

Ans. In certain cases loss in signal gain due to negative feedback may be intolerable and so use of compensation techniques, in order to reduce the drift of the operating point, becomes imperative.

19. In the active region, as temperature increases the current in BJT increases.

Ans. In BJT, collector current is given as Collector current, I_C = β I_B + (1 + β) I_CO

Leakage current l_CO increases with the increase in temperature (for 1°C rise in temperature l_CO increases by 7% and for 10°C rise in temperature I_CO becomes double). Current gain β also increases with the increase in temperature of BJT. Because of these two reasons, the collector current I_C increases as the temperature increases.

20. Which value of β is to be used in the analysis of a biasing circuit?

Ans. Normally, the transistor specification sheet lists minimum and maximum values of β (i.e., β_min and β_max). In this case geometric mean of the two values should be used i.e., β = √β_min x β_max.

If the transistor data sheet lists only one value of β, that value should be used.

21. Define thermal resistance for BJT.

Ans. The steady-state temperature rise at the collector junction is proportional to the power dissipated at the junction i.e., T_j – T_A = θP_D

where T_j and T_A are the junction and ambient temperatures respectively in degrees centigrade and P_D is the power in watts dissipated at the collector junction. The constant of proportionality θ is called the thermal resistance. Thermal resistance may also be defined as the resistance to heat flow between two temperature points. Its value depends on (i) the size of the transistor (ii) convection or radiation to the surroundings (iii) forced air cooling if used and (iv) thermal connection of the device to a metal chasis or to heat sink.