Astable, Monostable and Bistable Multivibrator

A digital electronic circuit known as a multivibrator produces digital signals like pulses, square waves, and other types of signals. It often consists of passive elements like resistors and capacitors as well as digital logic gates.

What is a Multivibrator?

A multivibrator is a switching circuit that is a two-stage resistance-coupled amplifier with positive feedback from the output of one amplifier to the input of the other. The name multivibrator is derived from the fact that the square wave generated consists of a large number of sinusoidal of different frequencies. Multivibrators are crucial parts of digital circuits and computer architecture, to sum up. They are utilized for data storage, synchronization between various components, clock signal generation, timing control, and data transfer time. The three basic types of multivibrators are astable, monostable, and bistable, each with distinct properties and uses.

• A multivibrator circuit oscillates between a “HIGH” state and a “LOW” state producing a continuous output.
• In a multivibrator, the two transistors are connected in feedback so that one transistor controls the state of the other. 
• Multivibrators are used are widely used to implement two-state devices like Relaxation Oscillators, Timers, and flip-flops. 

The multivibrators are classified into three categories-

1. Astable Multivibrator.
2. Monostable Multivibrator.
3. Bistable Multivibrator.

Astable Multivibrator

An astable multivibrator, also called a free-running multivibrator, is a circuit that continuously produces square waves or pulses without the use of an external trigger. The term “astable” refers to the absence of a stable state in this particular type of multivibrator. 

• The circuit is built to alternate between two stable states, resulting in a steady oscillation. 
• By changing the values of the resistors and capacitors in the circuit, the frequency and duty cycle of the output waveform can be altered.
• In digital circuits, the astable multivibrator is frequently employed as a clock source. The timing of data transfers between various components can be synchronized using the frequency of the output waveform.

The output of an astable multivibrator does not have any stable state and it changes its state from high to low and low to high repeatedly.

Features

1. It is also known as a free-running multivibrator.
2. It has no stable state, hence the name astable.
3. It produces a continuous series of pulses with a predetermined frequency and duty cycle.
4. It requires two identical transistors two capacitors, and a few resistors.
5. It is commonly used in oscillator circuits, pulse generators, and clock circuits.

Applications

1. Used in square wave frequency generator.
2. Used as a timing oscillator in the computer system.
3. Used in flashing lights.

Monostable Multivibrator

A monostable multivibrator, also called a one-shot multivibrator, is a circuit that responds to an external trigger by producing a single pulse with a set duration. A pulse from outside causes this sort of multivibrator to flip from its stable state to an unstable one. 

• The circuit returns to its stable condition after a certain amount of time and generates a single output pulse. 
• By altering the values of the resistors and capacitors in the circuit, the output pulse’s duration can be changed.
• In digital circuits, the monostable multivibrator is frequently used for pulse shaping, debouncing, and time delay functions. 
• Other circuits can use it as a trigger as well.

The output of a monostable multivibrator has only one stable state.

Features

1. Also known as a one-shot multivibrator.
2. It has only one stable state.
3. It produces a single output pulse of a predetermined width when triggered by an input signal.
4. It requires two transistors, two capacitors, and a few resistors.
5. It is commonly used in timing circuits, delay circuits, and pulse width modulation circuits.

Applications

1.  Used for regenerating weak signals.
2. Used in the pulse generator.
3. Used in memory.

Bistable Multivibrator

A flip-flop, or bistable multivibrator, is a circuit with two stable states that can alternately exist indefinitely. 

• A signal from outside causes it to change from one stable condition to another. 
• The circuit will stay in its stable state until another trigger signal enters it. 
• A bistable multivibrator typically produces a square wave with two separate voltage levels as its output waveform.

Digital circuits frequently use bistable multivibrators for memory storage, data transport, and synchronization. They can also be utilized in shift registers and counters.

The output of a bistable multivibrator has two stable states. We have to apply external inputs in order to change the existing state of the output.

Features

1. Also known as a flip-flop multivibrator.
2. It has two stable states and can remain in either state indefinitely without any input signal.
3. It requires two transistors, two capacitors, and a few resistors.
4. It is commonly used in digital circuits as a memory element, latch, or flip-flop.
5. It is also used in applications where a simple on/off switch is required.

Applications

1. Used for changing the supply to two circuits.
2. Used in digital operation in computers.