Astable Multivibrator

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 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.

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.


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.

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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