An astable multivibrator is a type of electronic circuit that does not have a stable state. Unlike a monostable multivibrator, which has one stable state and one triggered unstable state, an astable multivibrator continuously oscillates between two unstable states without external triggering. As a result, it generates a continuous square wave output. These circuits are used in oscillators and frequency generators.

Working Principle

The initial voltage across the circuit is assumed to be +Vsat. The capacitor now starts charging towards +Vsat. The op-amp is considered to be ideal and no current flows into the inverting terminal of op-amp. As soon as the capacitor starts charging, the voltage across the capacitor starts to build up. This leads to a rise in voltage at the inverting terminal of op-amp. Whenever the capacitor reaches the upper threshold voltage given by:

the voltage at the inverting node will be slightly more than the voltage at the non-inverting mode. The output of the op-amp will switch from +Vsat to -Vsat. Now the capacitor starts charging towards negative saturation voltage. As soon as the voltage goes below the lower threshold voltage given by:

the voltage at the non-inverting terminal will be slightly more than the voltage at the inverting terminal. So once again the output will switch from -Vsat to +Vsat. In this way, by charging and discharging of capacitor, a square wave is obtained at the output. 

Waveform of an Astable Multivibrator Circuit

The time period, T of the astable multivibrator depends on the R and C values and also on the upper and lower threshold values. The time period is given by:

Here, as the positive and negative threshold voltages are equal, the duty cycle of the square wave is equal to 50%. 

Click here to learn more about monostable multivibrator circuits.