Astable Multivibrator: Diagram, Working, Waveform, Applications

By Neeraj Dubey|Updated : September 19th, 2022

Astable Multivibrators are automatically triggered multivibrators. Multivibrators are non-sinusoidal oscillators capable of generating waveforms other than sine waves. Relaxation oscillators with regenerative switching circuits, such as the Astable Multivibrator, are the most widely used because they are easy to build and provide a constant square wave output.

Due to their ability to oscillate independently without external inputs or assistance, Astable multivibrators are also known as free-running multivibrators. Astable Multivibrators, unlike Monostable Multivibrators or Bistable Multivibrators, are automatically triggered, so they toggle between their two unstable states, set and reset continuously, using a built-in trigger pulse.

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What is an Astable Multivibrator?

In the case of an Astable Multivibrator, it is a free-running oscillator that oscillates between two states, continually producing two square wave output waveforms. This device is a two-stage amplifier with positive feedback from one amplifier to the other. As a result of this feedback, the transistor on one end of the circuit is driven to saturation (On state), and the transistor on the other ends up in cut-off(OFF state). After a certain amount of time, the circuit conditions reverse, with the saturated transistor turning off and the cut-off transistor turning on.

Astable Multivibrator PDF

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Astable Multivibrator Diagram

Astable Multivibrator Diagram


Design and Working of Astable Multivibrator

There are two symmetrical CE amplifier stages, each providing feedback to the other. Because each stage has a 180º phase shift, the feedback ratio between the two is positive and unity. In case, R2C1=R3C2 is the collector load, and R2=R3 is the biasing resistor.

The output of the transistor further increases the input of transistor Q2 and the output of transistor Q2 into transistor Q1. Q1 and Q2 supply the square wave output by driving the transistors to saturation or cut-off.

Circuit Diagram of Astable Multivibrator

There are two switching transistors, a cross-coupled feedback network, and two time-delay capacitors in the stable circuit so that the state can change without external triggering.

Astable Multivibrator circuit

With this circuit, one stage conducts "fully-ON" (saturation). At the same time, the other is switched "fully-OFF"(cut-off), which creates an extremely high level of mutual amplification between the two transistors. The conductivity is transferred from one stage to another through the discharging action of a capacitor through a resistor.

Operation in Astable Multivibrator

In applying Vcc to Q1 and Q2, collector current starts flowing in both transistors simultaneously. Additionally, the coupling capacitors C1 and C2 also start charging up. As Q1's collector current rises, its positive output is applied to C1's base, creating a reverse bias on Q2, thereby decreasing its collector current. In the presence of C2, the collector of Q2 connects to the base of Q1, resulting in Q1 becoming more forward-biased, further increasing the collector current in Q2. In this way, the circuit continues to drive Q1 until saturation occurs, and Q2 is cut-off. Consequently, Vcc appears across R2C1 and R3C2appears to be at no voltage. In Q1 and Q2, the charges developed across C1 and C2 suffice to maintain saturation and cut-off conditions.

These conditions are represented by the time interval 'bc' in the below figure.


Despite this, tAs a result, the capacitor will not retain the charges forever; instead, they will discharge through respective circuit paths. Where C2 and R3 determine the duration determines the time when the forward bias at Q2 is re-established after C1 discharges, causing the collector current to start at Q2. As a result, Q2 will be driven to saturation quickly due to the increasing positive potential at the collector of Q2. This is done through capacitor C2. Therefore, the base of Q1 will become more positive than Q2. As a result, Q2 will remain in saturation for a period of time, and Q1 will remain at the cut-off. C2 R3 and determine the duration.

Waveform of Astable Multivibrator

A pair of cross-coupled grounded emitter transistors produces the square wave output of an astable multivibrator. As a common emitter amplifier in the multivibrator, both transistors, NPN and PNP, have a bias for the linear operation and are 100% positive feedback transistors.


Frequency and Time Calculations of Astable Multivibrator

At each collector, the circuit keeps changing states in this way, producing a square wave. Since the relevant capacitor takes approximately 0.69CR to charge enough for a state to occur, the frequency of oscillation can be calculated as follows:


When C1=C2=C and R1=R2=C, the mark-to-space ratio will be 1:1, and the frequency of oscillation will be:

f0 = 1/(1.4 RC)

Duty Cycle

It is the ratio of time Tc during which the output is high to the total time period T of the cycle.


Based on this output rate, Duty Cycle = TON/(TOFF+TON) when the transistor has a collector output.

Applications of Astable Multivibrator

Astable multivibrators can be used for many applications, such as pulse position modulation, frequency modulation, etc., because they are simple, reliable, and easy to build.

  • We use the astable multivibrator to generate waves.
  • It is used to convert voltage to frequency.
  • Synchronization of pulses is achieved using it.
  • Due to its square wave production, it produces harmonic frequencies of higher order.
  • This multivibrator is used in the construction of voltmeters and SMPS.
  • In addition to operating at a wide range of frequencies, an astable multivibrator can also function as an oscillator.
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FAQs on Astable Multivibrator

  • An astable multivibrator is one of the most commonly used relaxation oscillators because it produces a constant square wave output waveform while being simple, reliable, and easy to construct.

  • The free-running oscillator switches routinely between its two unstable states without any external signal applied. With no external signal applied, the transistors alternatively switch from cutoff to saturation states at a frequency determined by the coupling circuits RC time constant.

  • In an astable multivibrator, the state of the circuit changes randomly after the circuit has been turned on for a certain period of time determined by the RC time constants.

  • A relaxation oscillator is an oscillator without a stable state, oscillating continuously without any external excitation. It has two quasi-stable states between which the transistors oscillate independently.

  • Some of the common applications of an astable multivibrator are listed below:

    • Astable multivibrators are used to generate square waves.
    • In a computer system, it serves as a timing oscillator or clock.
    • It can also be used for circuits that flashlights, switch, and supply electricity.

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