page_title: Multivibrator
section_title: Astable multivibrator using an op-amp
hierarchical_section_title: Multivibrator / Astable multivibrator / Astable multivibrator using an op-amp
caption_attribution_description: English: Astable Multivibrator using Op-Amp Graph
context_page_description: A multivibrator is an electronic circuit used to implement a variety of simple two-state devices such as relaxation oscillators, timers and flip-flops. It consists of two amplifying devices cross-coupled by resistors or capacitors. The first multivibrator circuit, the astable multivibrator oscillator, was invented by Henri Abraham and Eugene Bloch during World War I. They called their circuit a "multivibrator" because its output waveform was rich in harmonics.
  The three types of multivibrator circuits are:
  Astable multivibrator, in which the circuit is not stable in either state —it continually switches from one state to the other. It functions as a relaxation oscillator.
  Monostable multivibrator, in which one of the states is stable, but the other state is unstable. A trigger pulse causes the circuit to enter the unstable state. After entering the unstable state, the circuit will return to the stable state after a set time. Such a circuit is useful for creating a timing period of fixed duration in response to some external event. This circuit is also known as a one shot.
  Bistable multivibrator, in which the circuit is stable in either state.
context_section_description: Assume all the capacitors to be discharged at first. The output of the op-amp Vₒ at node c is +Vₛₐₜ initially. At node a, a voltage of +β Vₛₐₜ  is formed due to voltage division where . The current that flows from nodes c and b to ground charges the capacitor C towards +Vₛₐₜ. During this charging period, the voltage at b becomes greater than +β Vₛₐₜ at some point. The voltage at inverting terminal will be greater than the voltage at the non-inverting terminal of the op-amp. This is a comparator circuit and hence, the output becomes -Vₛₐₜ. The voltage at node a becomes -βVₛₐₜ due to voltage division. Now the capacitor discharges towards -Vₛₐₜ. At some point, the voltage at b becomes less than -β Vₛₐₜ. The voltage at the non-inverting terminal will be greater than the voltage at the inverting terminal of the op-amp. So, the output of the op-amp is +Vₛₐₜ. This repeats and forms a free-running oscillator or an astable multivibrator.
  
  If VC is the voltage across the capacitor and from the graph, the time period of the wave formed at capacitor and the output would match, then the time period could be calculated in this way:
  At t =T1,
  Upon solving, we get:
  We are taking values of R, C and β such that we get a symmetrical square wave. Thus, we get T1 = T2 and total time period T = T1 + T2. So, the time period of the square wave generated at the output is:
caption: Graph showing the output waveform of the Op-Amp and the waveform formed across the capacitor C.
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