Switching Circuits (Multivibrators) Interview Questions and Answers:

1. What is an electric switch?

Ans. An electric switch is a device that can turn on or off current in an electrical circuit.

2. What is meant by a switching circuit ?

Ans. Switching circuit is that circuit which can turn on or off current in an electrical circuit. The switching circuit essentially consists of a switch and an associated circuitry.

3. What are saturated switches?

Ans. When the base-emitter junction of a transistor is forward biased to the extent that the transistor is saturated, maximum possible current, called saturation current I_C(sat) flows through the collector resistance making voltage drop across it almost equal to V_CC.

4. What is hard saturation?

Ans. Hard saturation means having sufficient base current to saturate the transistor at all the values of β_dc encountered in mass production of transistors.

5. What are three regions of operation of a transistor?

Ans. The three regions of operation of a transistor are :

(i) Cut-off region, (ii) Active region and (iii) Saturation region.

6. When a perfect square pulse is applied to the transistor base, the transistor does not switch on immediately. Why?

Ans. Due to junction capacitance and transit time of electrons across the junction.

7. On what factors the switching speed of a BJT and a MOSFET depends?

Ans. The switching speed of BJT depends upon the charging time of the emitter junction capacitance in going from cutoff to saturation.

The switching speed of MOSFET depends upon the internal capacitance of the device and the internal impedance of the gate drive circuits.

8. What is meant by delay time?

Ans. The time interval between the instant of the application of input pulse and the output (collector current) to attain 10 per cent of its maximum value is termed the delay time. denoted by t_d.

9. What is meant by fall time?

Ans. The fall time is defined as the time required for output (collector current) to go from 90 per cent to 10 per cent of its maximum value and is denoted by t_f.

10. What is meant by storage time?

Ans. Storage time is defined as the time interval between the end of the input pulse (trailing edge) and when the output (collector current) falls to 90% of its maximum value and denoted by t_s.

11. What is meant by rise time?

Ans. When the transistor is turned-on, a finite time elapses before the collector current I_C attains its maximum value. The time required for the output current to go from 10% to 90% of its maximum value is called the rise time.

12. Write any factor that contribute to the delay time in a transistor.

Ans. There are three factors which are responsible for the delay time and are as below.

First, when the driving signal is applied to the transistor input, a non-zero time is required to charge up the emitter-junction transition capacitance so as to bring the transistor from cut-off region to the active region.

Second, even when the transistor has been brought to the point where minority carriers have begun to cross the emitter junction into the base region, a time interval is required before these carriers cross the base region to the collector junction and be recorded as collector current.

Third, some time is required for the collector current to attain 10 per cent of its maximum value from zero due to exponential charging of collector transition capacitance to 10 % of its maximum charged value.

13. How does the commutating capacitor reduce the transition time of a transistor?

Ans. For improving the transient response of the inverter (transistor switch), the capacitor, known as commutating capacitor is connected across the base. This helps in removing minority carrier charges in the base when the signal changes between two logic states.

14. Name the different types of multivibrators.

Ans. Multivibrators are of three types viz. astable, monostable and bistable multivibrators.

15. What do you mean by quasi stable state of a multivibrator?

Ans. Quasi stable state is defined as the state in which the circuit remains for small period of time determined by components and returns back to its stable state automatically i.e., no trigger is required to switch from quasi-stable state to stable state.

16. What is free-running multivibrator?

Ans. Astable multivibrator is essentially a two-stage R-C coupled amplifier with output of one feedback to the input of the other.

17. Why an astable multivibrator is also called a square-wave generator ?

Ans. An astable multivibrator generates square wave of known periodic time. So it is also called a square-wave generator.

18. On what factors does the frequency of oscillations of an astable multivibrator depend?

Ans. Frequency of oscillations of an astable multivibrator is given as

Thus frequency of oscillations of an astable multivibrator depends upon R and C components of the circuit.

19. How an astable multivibrator can be used as a voltage-to­frequency converter?

Ans. The astable multivibrator can be used as a voltage-to-frequency converter by connecting R₁ and R₂ to an auxiliary supply voltage and varying that voltage.

20. What are the advantages of an emitter-coupled astable multivibrator over a collector-coupled astable multivibrator ?

Ans. An emitter-coupled astable multivibrator has the following advantages over a collector-coupled astable multivibrator.

1. Collector of transistor Q₂ is not connected to any part of the circuit, so it can be used to provide an output terminal.
2. Frequency adjustment is easy because of use of only one capacitor in the circuit.
3. Synchronisation is easily possible at base of transistor Q₁ as input at base is isolated.
4. There are no recovery transients (such as overshoot voltages) in the output.

However, the emitter-coupled AMV has the drawbacks of using more components and difficulty in obtaining proper operating conditions.

21. What are the applications of astable multivibrator?

Ans.

1. An astable multivibrator is essentially a square-wave generator.
2. An astable multivibrator is used as a clock in digital circuits.
3. An astable multivibrator can be used as a voltage controlled oscillator (VCO) or a voltage-to-frequency converter (VFC).
4. An astable multivibrator can also be used as a frequency modulator.

22. What do you understand by the block condition of an astable multivibrator?

Ans. Sometimes both the transistors in the astable multivibrator go into saturation simultaneously and remain in that state. This is called blocked condition.

23. How do we protect blocked condition in astable multivibrator?

Ans. The blocked condition in an astable multivibrator is likely to take place if the supply voltage is increased slowly from zero to full voltage V_CC. In order to protect blocked condition, the voltage is applied suddenly in astable multivibrator.

24. How a monostable multivibrator circuit can be made from an astable multivibrator circuit?

Ans. Monostable multivibrator circuit can be obtained from an astable multivibrator circuit by replacing one R-C timing circuit by dc voltage divider

25. Why monostable multivibrator is known as one shot multivibrator?

Ans. The monostable multivibrators generate one output pulse for every trigger pulse, so these multivibrators are sometimes called the one-shot multivibrators.

26. Why monostable multivibrator is called a delay circuit?

Ans. Since the MMV generates a fast transition at a predetermined time T after the input trigger, it is called a delay circuit.

27. What is the function of commutating capacitors in multivibrators?

Ans. The function of commutating capacitors is to improve the switching characteristics of the circuit.

28. Why monostable multivibrator is called a gating circuit?

Ans. A monostable multivibrator generates an output which can be used to gate other outputs. As this circuit limits the output to its window, so this circuit is called the gating circuit.

29. What are the advantages of an emitter-coupled monostable multivibrator over a collector-coupled monostable multivibrator?

Ans. Emitter-coupled monostable multivibrator (MMV) has got the following advantages over collector-coupled MMV.

1. Since in such an MMV, collector of Q₂ is not connected to base of Q₁, signal at collector of Q₂ is not directly involved in regenerative loop and hence terminal C₂ can be used freely to provide output.
2. Separate negative supply is not required as emitter junction of transistors is self-biased through R_E.
3. Since base of Q₁ is free from coupling in the circuit, trigger source cannot load the circuit, if it is connected at B₁.
4. Due to presence of R_E, circuit has good stability. The pulse width depends on the magnitude of input voltage at base of Q₁, which can be varied by potentiometer rotation.

30. What are the applications of monostable multivibrators?

Ans.

1. The falling part of the monostable multivibrator output is often used for triggering another pulse generator circuit, thus producing a pulse delayed by a time T with respect to the input pulse.
2. Monostable multivibrator is also employed for regenerating or rejuvenating old and worn-out pulses. Various pulses used in computers and telecommunication systems become somewhat distorted during use. A monostable multivibrator can be employed to generate new, clean and sharp pulses from these distorted and used pulses.

31. Why the bistable multivibrator is also called the flip-flop multivibrator?

Ans. Since in a bistable multivibrator one trigger pulse causes the multivibrator to flip from one state to the other and the next pulse causes it to flop back to its original state, it is called the flip-flop multivibrator.

32. What is meant by loading in a bistable multivibrator?

Ans. The output of a bistable multivibrator, when connected to other circuit, could cause loading on the bistable multivibrator, which in return will reduce the output.

33. What is the purpose served by collector catching diodes?

Ans. The collector catching diodes are used in a bistable multivibrator to maintain the output swing constant.

34. How does self-bias binary differ from fixed-bias binary?

Ans. In a self-bias binary, the emitter of both the transistors are com­mon and a resistance R_F is connected from the common emitter to ground. The voltage drop across the emitter resistance R_E provides the self-bias to keep one of the transistors in the off state.

35. What is the use of commutating capacitors in a bistable multivibrator ?

Ans. The commutating capacitors are provided to improve the switch­ing characteristics of the circuit by allowing flow of high-fre­quency components of square wave through them.

36. What do you mean by transition time, settling time and resolu­tion time ? What is relation between them?

Ans. Transition time is defined as the time duration after the applica­tion of trigger pulse, during which the conduction is transferred from the off transistor to the on transistor.

Once the transfer of conduction from one transistor to another is over i.e., transition phase is complete, the voltages of the commutating capacitors must also change corresponding to the other stable state. Thus, the additional time required to complete the charge of the commutating capacitors after the completion of the commutation process is known as settling time.

Resolution time of a binary is the minimum time interval re­quired between successive trigger pulses to be reliably able to drive the binary from one stable state to the other.

The resolution time is the sum of transition time and settling time.

37. How the resolution time of a binary can be improved?

Ans. The resolution time of bistable multivibrator can be improved by

• Reducing all stray capacitances orjunction capacitances. By doing so, charging time can be reduced.
• Reducing the values of R₁, R₂ and R_C. Reduction in the values of R₁ and R₂ will reduce the charging time of speed­up capacitors resulting in improvement in transition speed. Reduction in the values of resistors also reduces the recovery time.
• By not allowing the transistors to go into saturation. This will reduce the storage time resulting in fast change from on to off state.

38. What are the applications of bistable multivibrators?

Ans. The bistable multivibrator is employed in many digital opera­tions such as counting and storing of binary information, as a frequency divider in timing circuits and can also be used for generation of pulses.

39. What is meant by triggering?

Ans. Triggering is a process of applying an external signal to induce transition from one state to another.

40. Give the applications of unsymmetrical triggering.

Ans. Unsymmetrical triggering is used in logic circuitry and as a generator of a gate whose width equals the interval between triggers.

41. Differentiate between symmetrical and unsymmetrical triggering?

Ans. Symmetrical triggering is a process in which each successive triggering signal induces a transition regardless of the state in which the binary happens to be. It requires only one source to produce transition and is used in binary counting circuits.

Unsymmetrical triggering is a process in which the signal is effec­tive in inducing transition only in one direction. It needs two sepa­rate sources to produce transition and is used in logic circuitry (in electronic registers, coding etc.) and is also used as a generator of gate whose width equals the intervals between registers.

42. What are advantages and disadvantages of symmetrical and unsymmetrical triggering?

Ans. Unsymmetrical triggering has the advantage of inducing transition in only one direction taking care of the binary state and is used in logic circuitry. But it has the drawback of requiring two separate sources for transition to occur.

Symmetrical triggering has the advantage of requiring only one source to produce transition and is used in binary counting circuits and other applications.

43. What is Schmitt trigger? Mention some of its applications.

Ans. Schmitt trigger is a bistable circuit and the existence of only two stable states results from the fact that positive feedback is incorporated into the circuit, and from the fact that the loop gain of the circuit is greater than unity.

It differs from the basic collector-coupled bistable multivibrator in that the coupling from the output of the second stage to the input of first stage is missing and the feedback is obtained through a common emitter resistor R_E.

The Schmitt trigger forms an important BMV and is also called the emitter-coupled binary.

Its important uses are:

• As a squaring circuit because it can convert inputs of any waveshape into output pulses of rectangular or square waveshapes.
• As an amplitude comparator or level detector because in Schmitt trigger circuit a change of state occurs whenever the input crosses a trigger point.
• As a flip-flop circuit.
• For reshaping the worn-out pulses by giving them sharp leading and trailing edges.

44. Define UTP and LTP in a Schmitt trigger.

Ans. The input voltage at which the output voltage of a Schmitt trigger goes high is called the upper trip point UTP while the input voltage at which the output voltage goes low is called the lower trip point (LTP).