Multivibrator: A Brief Introduction and its type

Multivibrator is an electronic circuit that oscillates between the two states i.e high to low and vice versa, for producing a continuous output.

A pulse generator circuit like multivibrator produces clock pulses which are square and rectangular waveform and are continuous in nature.

They are basically of three types, depending on their state:

• Astable
• Monostable
• Bistable

A brief characteristic of the various types are as follows:

Astable:

They are also called Free running multivibrator/oscillator.

1. They have no stable state
2. Switches continuously between 2 states.
3. They produces a train of SW pulses at fixed frequency.
4. Their 50 % output is high and 50% output is low.
5. The timing cycle is determined by RC time constant of the resistor capacitor- RC Network.
6. They uses 2 CMOS NAND gates connected as inverting Not gate.
7. T=2.2 RC

Application: Ideal for timing and clock pulse applications.

Monostable :

They are also called One shot multivibrator.

1. It has only one stable state.
2. They have pulses of specific duration.
3. It is triggered externally.
4. Generate a single o/p pulse either high or low with external trigger.
5. They are made from two cross coupled NAND gate.
6. T=0.69 RC

Application: Used to convert short sharp pulses into wider ones for timing applications.

Bistable:

This is also known as a “Bistable Latch“, “Toggle Latch” or simply “T-latch“.

1. It is a SR Flip flop with addition of an inverter or NOT gate to provide the necessary switching function.
2. They have two stable state either positive or negative.
3. Their circuit will remain in either state indefinitely.
4. They produces single pulse either positive or negative.
5. It passes from one state to the other “only” when a suitable external trigger pulse T is applied.Two triggering pulses are needed, to go through a full “SET-RESET” cycle.

Application: Latches, counters, frequency dividers, storage devices in computer memory.