This set of Analog Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Different Types of Multivibrators – 1”.
1. What is a square wave generator?
b) Bi-stable multivibrator
c) Astable multivibrator
d) Monostable multivibrator
Explanation: An astable multivibrator is one which does not have any stable state. It has two quasi-stable output states which keep toggling at regular intervals. The output waveform is thus a square wave and this circuit is also called a square wave generator or a free running oscillator.
Explanation: The feedback factor is β = R1/R1+R2
β = 10k/30k+10k = 1/4.
3. In an astable multivibrator using an op-amp, the feedback factor is 0.5 and the supply voltage is ±12V. What are the quasi-stable output state and the input to the non-inverting terminal?
a) Quasi-stable output = ±12V, Input to non inverting terminal = ±12V
b) Quasi-stable output = ±6V, Input to non inverting terminal = ±12V
c) Quasi-stable output = ±12V, Input to non inverting terminal = ±6V
d) Quasi-stable output = ±12V, Input to non inverting terminal = 0V
Explanation: Quasi-stable output = ±VSat = ± VSupply = ±12V
Input to non inverting terminal = ±βVSat = ±6V.
4. What is the duty cycle of the output of an astable multivibrator?
Explanation: An astable multivibrator is a square wave generator. Charging and discharging times of the capacitor are equal and thus the output waveform has a duty cycle of 50%.
Explanation: TO = 2RCln (1 + β/1-β) = 2RCln(1.3/0.7) = 2x10x4x10-3x0.619 = 49.52ms
f = 1/TO = 20Hz.
Explanation: Since the two feedback resistors are same, then β=0.5.
Thus time period TO = 2RCln3 = 2x20x10x10-3x1.09 = 436ms = 0.436 seconds.
7. How can the duty cycle be changed for an astable multivibrator?
a) By adding another capacitor to the circuit
b) By adding diodes to the circuit
c) By adding an inductor to the circuit
d) The duty cycle cannot be changed
Explanation: To generate a square wave of duty cycle other than 50%, diodes can be connected so that charging and discharging now occurs through these diodes. This thus changes the time period as well as the duty cycle of the diodes.
Explanation: T = (RXC + RYC)ln(1+β/1-β)
T = 22x10x10-3xln(1.6/0.4) = 22x10x10-3x1.38 = 0.303 seconds.
9. The monostable multivibrator has one quasi-stable state and one unknown state.
Explanation: The monostable multivibrator is a circuit whose output has one stable state and one quasi-stable state. If trigger input is not applied then output remains in the stable state indefinitely and whenever the trigger is applied, the output enters the quasi-stable state. The output then changes back to the stable state on its own after a short duration.
10. Which of these is not a type of monostable multivibrator?
a) Schmitt trigger as a monostable multivibrator
b) Emitter coupled monostable multivibrator
c) Using an op-amp
d) 555 Timer as a monostable multivibrator
Explanation: A Schmitt trigger circuit is a comparator circuit, also called a regenerative comparator. It converts any waveform into a square wave. It is not a monostable multivibrator, unlike the other three options.
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