Multivibrator Questions and Answers for Viva

Answer-1: A relaxation oscillator produces a non-sinusoidal output, such as a square wave or sawtooth. The oscillator contains a nonlinear component such as a transistor that periodically discharges the energy stored in a capacitor or inductor, causing abrupt changes in the output waveform.

Answer-2: Types of relaxation oscillator circuits include:
Multivibrator
Ring oscillator
Delay line oscillator
Rotary traveling wave oscillator

Answer-3: A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flops. A form of electronic circuit that employs positive feedback to cross-couple two devices so that two distinct states are possible, for example, one device ON and the other device OFF, and in which the states of the two devices can be interchanged either by use of external pulses or by internal capacitance coupling. When the circuit is switched between states, transition times are normally very short compared to the ON and OFF periods. Hence, the output waveforms are essentially rectangular in form.

Answer-4: There are three types of multivibrator circuit: astable, monostable, bistable

Answer-5: As the name implies, the bistable multivibrator has two stable states. As the name implies, the bistable multivibrator has two stable states. If a trigger of the correct polarity and amplitude is applied to the circuit, it will change states and remain there until triggered again.

Answer-6: If a sharp negative pulse is applied to the base of the ON transistor, its collector current decreases and its collector voltage rises. A fraction of this rise is applied to the base of the OFF transistor, causing some collector current to flow. The resultant drop in collector voltage, transferred to the base of the ON transistor, causes a further rise at its collector. The action is thus one of positive feedback, with nearly instantaneous transfer of conduction from one device to the other. There is one such reversal each time a pulse is applied to the gate of the ON transistor.

Answer-7: Multivibrators find applications in a variety of systems where square waves or timed intervals are required. The 555 timer is a popular IC chip which can be used to implement all three multivibrator modes. Other applications included early television systems, where the various line and frame frequencies were kept synchronized by pulses included in the video signal.

Answer-8: The bistable multivibrator that most technicians know is commonly known by other names: the ECCLES-JORDAN circuit and, more commonly, the FLIP-FLOP circuit.

Answer-9: If a voltmeter were connected to the output of a flip-flop, it would measure either a small positive or negative voltage, or a particularly low voltage (essentially 0 volts). No matter which voltage is measured, the flip-flop would be stable. Remember, stable means that the flip-flop will remain in a particular state indefinitely. It will not change states unless the proper type of trigger pulse is applied.

Answer-10: Flip-flops are used in switching-circuit applications (computer logic operations) as counters, shift registers, clock pulse generators, and in memory circuits. They are also used for relay-control functions and for a variety of similar applications in radar and communications systems.

Answer-11: A monostable or one-shot multivibrator has only one stable state. If one of the normally active devices is in the conducting state, it remains so until an external pulse is applied to make it nonconducting. The second device is thus made conducting and remains so for a duration dependent upon RC time constants within the circuit itself.

Answer-12: Monostable Multivibrators deliver a single output pulse when it is triggered externally only returning back to its first original and stable state after a period of time determined by the time constant of the RC coupled circuit.

Answer-13: When power is firstly applied, the base of transistor TR2 is connected to Vcc via the biasing resistor, thereby turning the transistor "fully-ON" and into saturation and at the same time turning TR1 "OFF" in the process. This then represents the circuits "Stable State" with zero output. If a trigger pulse is now applied at the result in transistor TR2 now fully "OFF". This then represents the circuit?s second state, the "Unstable State" with an output voltage equal to Vcc, transistor TR1 which results automatically returning back to its original stable state awaiting a second trigger pulse to restart the process once again.

Answer-14: Monostable Multivibrators can therefore be considered as triggered pulse generators and are generally used to produce a time delay within a circuit as the frequency of the output signal is the same as that for the trigger pulse input the only difference being the pulse width.

Answer-15: One main disadvantage of Monostable Multivibrators is that the time between the application of the next trigger pulse has to be greater than the preset RC time constant of the circuit to allow the capacitor time to charge and discharge.

Answer-16: As well as producing Monostable Multivibrators from individual discrete components such as transistors, we can also construct monostable circuits using commonly available integrated circuits. The following circuit shows how a basic monostable multivibrator circuit can be constructed using just two 2-input Logic "NOR" Gates.

Answer-17: The time of period monostable multivibrator remains in unstable state is given by t = ln(2)R₂C₁. If repeated application of the input pulse maintains the circuit in the unstable state, it is called a retriggerable monostable. If further trigger pulses do not affect the period, the circuit is a non-retriggerable multivibrator.

Answer-18: Monostable Multivibrators can produce a very short pulse or a much longer rectangular shaped waveform whose leading edge rises in time with the externally applied trigger pulse and whose trailing edge is dependent upon the RC time constant of the feedback components used.

Answer-19: The time constant of Monostable Multivibrators can be changed by varying the values of the Capacitor, C₂ the Resistor, R₂ or both.

Answer-20: Monostable Multivibrators are generally used to increase the width of a pulse or to produce a time delay within a circuit as the frequency of the output signal is always the same as that for the trigger pulse input, the only difference is the pulse width.

Answer-21: Astable, in which the circuit is not stable in either state?it continuously oscillates from one state to the other. Due to this, it does not require an input (Clock pulse or other). It is also called free-running multivibrator.

Answer-22: The astable multivibrator has capacitance coupling between both of the active devices and therefore has no permanently stable state. Each of the two devices functions in a manner similar to that of the capacitance-coupled half of the monostable multivibrator. It will therefore generate a periodic rectangular waveform at the output with a period equal to the sum of the OFF periods of the two devices.

Answer-23: Period of oscillation is very roughly, the duration of state 1 (low output) will be related to the time constant R2C1 as it depends on the charging of C1, and the duration of state 2 (high output) will be related to the time constant R3C2 as it depends on the charging of C2. The result is that when first powered up, the period will be quite long as the capacitors are initially fully discharged, but the period will quickly shorten and stabilise.

Answer-24: It is called free-running because it alternates between two different output voltage levels during the time it is on. The output remains at each voltage level for a definite period of time. The output on an oscilloscope, continuous square or rectangular waveforms. The astable multivibrator has two outputs, but NO inputs.

Answer-25: While not fundamental to circuit operation, diodes connected in series with the base or emitter of the transistors are required to prevent the base-emitter junction being driven into reverse breakdown when the supply voltage is in excess of the Veb breakdown voltage, typically around 5-10 volts for general purpose silicon transistors.

Answer-26: Some astable multivibrators must have a high degree of frequency stability. One way to obtain a high degree of frequency stability is to apply triggers. A negative input trigger to the base of Q1 (through C1) causes Q1 to go into saturation, which drives Q2 to cutoff. The circuit will remain in this condition as long as the base voltage of Q2 is positive.

Answer-27: Before the advent of low-cost integrated circuits, chains of multivibrators found use as frequency dividers. A free-running multivibrator with a frequency of one-half to one-tenth of the reference frequency would accurately lock to the reference frequency. This technique was used in early electronic organs, Other applications included early television systems, where the various line and frame frequencies were kept synchronized by pulses included in the video signal.

Answer-28: The operation of astable multivibrator is similar to the oscillator.

Answer-29: ON time for Q₁ (or OFF time for Q₂): T₁ = 0.694R2C₁ and OFF time for Q₁ (or ON time for Q₂): T₂ = 0.694R₃C₂

Answer-30: Total time period: T = T₁ + T₂ = 0.694(R₂C₁ + R₃C₂ )
(As R₂ = R₃ = R and C₁ =C₂ = C) T = 1.4 RC second.
Frequency: f = 1/ T = 0.7/ RC Hz