Multivibrator - Definition, Diagram, Implementation & Applications

By Neeraj Dubey|Updated : August 30th, 2022

A multivibrator is a very important and commonly used pulse-shaping and generator circuit in applying digital and electronic systems. The term multi-vibrator refers to the oscillating response of a pulse generator circuit. For example, multivibrators are often used as a register or flip-flop, and the generated harmonic waves can be used as clock pulse signals for driving digital sequential circuits.

Any multivibrator's overall working consists of switch transistors, capacitors' charging and discharging, and resistors. We can achieve the generated pulse signal's time period, frequency, and waveform by analyzing the aspects of a multivibrator circuit. We will start with the basic definition of a multivibrator, follow along with its different types, and briefly discuss their implementations.

Table of Content

What is a Multivibrator?

A multivibrator is used to produce oscillations of different duty cycles, and they are commonly used for clock signal generation in digital circuits. It has two output levels, High and Low, and the duty cycle is determined from the period of these states; a wave having the same On and OFF time means a 50% duty cycle.

Note: The duty cycle of a wave is given by: TDuty = TON/(TON+TOFF)

Multivibrator Circuit Diagram

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Figure: Typical Multivibrator

Multivibrator Applications

There are various applications of multivibrators; a few are wave and pulse generators, Frequency dividers, common frequency sources, used in radar and tv circuits, saw-tooth generators etc.

Different States in a Multivibrator

There are two states in a multivibrator:

  • Stable State
  • Quasi-Stable State

Stable State

A stable state means the wave will continue to follow its current state until disturbed by an external source.

  • In a bistable, the state of the wave is ON after having an input at T1, and it continues to be the same until another input is given at T2.
  • In monostable, the state of the wave returns to its original state or the stable state after some time from the application of input at T1.

Quasi-Stable State

A quasi-stable state means the wave can not stay in the state at all times and will return to a stable state or keep fluctuating between the quasi-stable states.

  • In Monostable, High is the quasi-stable state, and Low is the stable state. After applying an input, the wave switched to the quasi-state and returned to a stable state without needing external help.
  • In Astable, there are no stable states; hence once the device starts, it continues to fluctuate between its quasi-stable states.

Multivibrator Types

Three types of the multivibrator are:

  • Astable
  • Monostable
  • Bistable

Astable

An Astable does not have stable states and only has two quasi-stable states. It can be constructed with different elements and configurations; however, the capacitors and resistors will still be present for the charging-discharging action and setting up the duty cycle.

Monostable

It only has one stable state and one quasi-stable state. It is also referred to as On-shot or Single-shot because it returns to the stable state after a fluctuation induced by the applied input. An external pulse is required to change the state from stable to quasi-stable.

Bistable

It has two stable states and continues to follow the present state until the next input is applied. For this reason, it is also used as a storing element since it can store a value High or Low state and can be changed when a different value state needs to be saved by applying an input trigger. Therefore, there is no sense in writing the time period for a bistable because we know the state is on hold for an indefinite time, i.e., until the next input pulse is applied. However, we will still see different circuits for it.

Implementation of Multivibrator Using BJT

Astable

There is no need to apply any input trigger in an astable, and we can take the output from the collector terminal of any of the two transistors.

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  • On-time of transistor Q1: t1=0.693R2C1(also the time for Q2 OFF)
  • Off-time of transistor Q1: t2=0.693 R3C2 (also the time for Q1 ON)
  • The total time period of the rectangular pulse wave generated is:

T= t1+t2=0.693R2C1+0.693 R3C2

Monostable

We need to provide a positive trigger in a monostable, and the output is taken from the second transistor.

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  • The ON time or the time for which the circuit will be in a quasi-stable state is:

T=0.693 R1C

Bistable

We need to provide a negative trigger pulse to set the output high or low for a bistable. Suppose we apply a negative trigger in Q1; this will set output 1 as high. We need to provide another negative trigger in Q2 again to set it to Low. We can choose any of the two transistors to get the output, which can also be backed by seeing the symmetrical nature of the circuit.

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Implementation of Multivibrators Using Op-amp

Astable

No intentional trigger is required, and the circuit will oscillate between the On and Off states depending on the charging and discharging action of the capacitor.

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  • Feedback factor β = R1/R1+R2
  • Time required for the capacitor to charge: t1 = RfC ln(1+β/1-β)
  • Time required for the capacitor to discharge: t2 = RfC ln(1+β/1-β)
  • The total time period of the rectangular pulse wave generated is:

T=2RfC ln(1+β/1-β)

Monostable

A negative trigger pulse is fed to the non-inverting input of the op-amp to set the circuit in a quasi-stable state.

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  • The ON time or the time for which the circuit will be in a quasi-stable state is:

T=2RC ln(1+R1/R2)

Bistable

A positive edge trigger is required to set the output low, and a negative trigger pulse is required to set the output high.

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Implementation of Multivibrators using Timer IC-555

Astable

The pin connections and circuitry for an astable multivibrator are shown below.

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  • Capacitor charging time/ Wave High time THIGH=0.693(R1+R2)C
  • Capacitor discharging time/ Wave Low time TLOW=0.693 R2C
  • The total time period of the rectangular pulse wave generated is:

T=THIGH+TLOW

Monostable

The pin connections and circuitry for a bistable multivibrator are shown below.

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  • The ON time or the time for which the circuit will be in a quasi-stable state is:

T=1.1 RAC

Bistable

The pin connections and circuitry for a bistable multivibrator are shown below. The operation of the circuit is controlled by two switches, where switch S1 is for SET and S2 is for RESET.

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Important Topics for GATE Exam
ResponsivityCharacteristics of Laser
SR Flip-FlopTie Set Matrix
Norton's TheoremWhat is Laser?
P N Junction DiodeTellegen's Theorem
Classless AddressingSimple Diode Circuits

 

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FAQs on Multivibrator

  • The three types of multivibrators are Astable, Monostable, and Bistable. These types are differentiated based on the number of stable states,

    • Astable has no stable states, only two quasi-stable states, and hence keeps oscillating between both levels.
    • Monostable has one stable state and one quasi-stable state. It toggles its state on the trigger but returns to its initial stable state after some time.
    • Bistable has two stable states; hence it continues to hold on to a state until it is disturbed by a trigger.
  • Multivibrators are used for clock and switch circuits in digital circuit operations, relaxation oscillators, and flip flops. An Astable multivibrator can be used in a counter for toggling states. A monostable multivibrator can be used as a delay circuit. A Bistable multivibrator can be used as a storing element to store a bit, and likewise.

  • The trigger requirements of all three types of multivibrators are-

    • Astable Multivibrators - Do not require any external triggering
    • Monostable Multivibrators – One trigger pulse is enough for each disturbance.
    • Bistable Multivibrators – One trigger pulse is required whenever the current state needs to be changed.
  • Diodes are used in each transistor collector branch to restrict the current flow as the timing capacitors are charged and discharged. What we get as a result is the improved waveform.

  • Due to capacitors, any DC signals (sometimes referred to as states) can't pass from one stage to the next, causing the stable multivibrator to have no stable DC operating point.

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