Astable vs Monostable vs Bistable Multivibrators: Differences Explained

This article explains the key differences between Astable, Monostable, and Bistable Multivibrator types. A multivibrator is essentially a two-stage RC coupled amplifier that uses positive feedback, routing the output of one amplifier back to the input of the other, as shown in Figure 1. This setup creates a switching circuit capable of generating non-sinusoidal waveforms, such as square waves, sawtooth waves, and rectangular waves.

Multivibrator circuits have many practical uses, including storing numbers, counting pulses, and synchronizing arithmetic operations.

Depending on the type of coupling network used, there are three main types of multivibrators:

• Astable (or free-running) multivibrator
• Monostable (or single-shot) multivibrator
• Bistable (or flip-flop) multivibrator

Astable Multivibrator

Figure 2 shows a typical circuit for an astable multivibrator. In this configuration, both coupling networks provide AC coupling through capacitors. Each amplifier stage introduces a 180-degree phase shift in the mid-band. This results in positive feedback because the overall phase shift is 360 degrees (or 0 degrees). It has no stable state. The circuit oscillates continuously as long as the total loop gain is greater than or equal to 1.

Monostable Multivibrator

Figure 3 depicts a common circuit for a monostable multivibrator. In this design, one coupling network provides AC coupling, while the other provides DC coupling.

This circuit features one stable state and one quasi-stable state. A triggering signal is required to initiate the transition from the stable to the quasi-stable state. Since the circuit vibrates only once per trigger, it’s also known as a uni-vibrator.

Bistable Multivibrator

Figure 4 shows a typical circuit for a bistable multivibrator. This configuration requires two triggers to return the circuit to its original state.

The first trigger causes the conducting transistor to enter cut-off mode, and the second trigger switches it back to conducting mode. This circuit does not oscillate.

In this circuit, both coupling networks provide DC coupling, and no energy storage elements are used.