Astable Multivibrator vs Monostable Multivibrator vs Bistable Multivibrator
This page on Astable Multivibrator vs Monostable Multivibrator vs Bistable Multivibrator covers basic difference between Astable, Monostable and Bistable Multivibrator types.
A multivibrator is basically a two stage RC coupled amplifier with positive feedback from output of one amplifier to the input of the another amplifier. This is shown in the figure-1.
It is basically a switching circuit which generates non-sinusoidal waves such as square, sawtooth and rectangular etc. Multivibrator circuits are widely used in storing numbers, counting of pulses, synchronization of arithmetic operations and so on.
Depending upon type of coupling network used, there are three types of multivibrators.
• Astable or free running multivibrator
• Monostable or single shot multivibrator
• Bistable or flipflop multivibrator
Figure-2 depicts circuit used for astable multivibrator. In this configuration, both coupling networks provide AC coupling through coupling capacitors. Each amplifier stage provides phase shift of 180degree in midband. This results into positive feedback due to overall phase shift of 360 degree or 0 degree. It has no stable state. Circuit oscillates as long as total loop gain >= 1.
Figure-3 depicts circuit used for monostable multivibrator. In this configuration, one coupling network provides AC coupling while the other provides DC coupling. The circuit has one stable state and one quasi stable state. In this circuit, a triggering signal is required to induce transition from stable to quasi stable state.
Since the circuit vibrates once for a trigger, it is called a uni-vibrator or monostable multivibrator.
Figure-4 depicts circuit used for bistable multivibrator. This configuration requires application of two triggers to return the circuit to its original state. The first trigger causes conducting transistor to cut-off mode and second trigger causes it back to conducting mode. This circuit does not oscillate. In this circuit, both coupling networks provide DC coupling and no energy storage element is used.