A multivibrator is switching circuits that generate the non sinusoidal signals like square wave, triangular wave, rectangular wave and saw tooth wave etc. The word of multivibrator is the combinations of two word multi mans many and vibrator means frequency. Multivibrators are use as a frequency generations, frequency dividers and generators of time delays and also as memory elements in computers etc.
The multivibrator can be made of transistor and logic gate. In transistor the output stage is connected to previous stage of amplifier and this type of connection is called Resistance coupled amplifier
What is a Multivibrator?
A multivibrator is a sequential logic circuit that oscillates between two states one is high and other is low producing a continuous output. It is a two-stage resistance or capacitor coupled amplifier with positive feedback from output of one transistor to another transistor. Hence the ON and OFF states of the whole circuit, and the time periods for which the transistors are driven into saturation or cut off are controlled by the conditions of the circuit.
The following figure shows the block diagram of a Multivibrator.
A multivibrator circuit oscillated between low state to high state and producing a continuous wave. For example, Astable multivibrators have a 50% duty cycle. It have 50% of the cycle is high output and 50% of the cycle is low output. In other word, the duty cycle for an astable multivibrator has timing pulse is 1:1.
A sequential logic circuit uses a clock pulse for synchronization. The sequential logic can also change their switching state using falling edge, rising edge, or both edges of the clock signal. The following terms are associated with the waveforms.
- Active HIGH– In active high, the change occurs from “LOW” to “HIGH”. This means the clock’s pulse rising edge.
- Active LOW– In active low, the change occurs from “HIGH” to “LOW”. This means the clock’s pulses falling edge.
- Clock Width– the time duration between rising edge and falling edge or time during when clock signal is equal to logic “1”, or HIGH.
- Clock Period– The time between successive transitions in the same direction, i.e. between two rising or two falling edges.
- Duty Cycle– It is a ratio of the clock width to clock period.
- Clock Frequency– the clock frequency is equal to the reciprocal of the clock period. Frequency = 1/clock period. ( ƒ = 1/T )
The clock generation is the combination of digital and analog circuit that produce a continuous pulse (Astable multivibrator) or pulse in a specific duration (Monostable multivibrator). The combination of two or more multivibrator is producing a desired pattern of pulses.
Types of Multivibrators
The multivibrators has two possible states. In first stage the transistor Q1 turns ON while the transistor Q2 turns OFF. In second stage, the transistor Q1 turns OFF while the transistor Q2 turns ON. The ON and OFF switching of transistors for a certain period of time is depending on the circuit conditions. The types of multivibrators are classified into three types depending upon in which two states are interchanged. And they are:
- Astable multivibrator
- Bistable multivibrator
- Monostable multivibrator
An Astable multivibrator is electronics circuit the switches between the two states without the application of any external pulse for its operation. This automatic switching produce square wave output and it is also known as free running multivibrator.
The time duration is depending upon the time constant of the circuit components use. As the Multivibrator keeps on switching, these states are known as quasi-stable or halfstable states. Hence there are two quasi-stable states for an Astable Multivibrators.
A monostable has a two state on is stable state and other is quasi-stable state. This has a trigger input to one transistor. So, one transistor changes its state automatically, while the other one needs a trigger input to change its state.
As this Multivibrators produces a single output for each trigger pulse, this is known as One-shot Multivibrator. This Multivibrator cannot stay in quasi-stable state for a longer period while it stays in stable state until the trigger pulse is received.
A Bistable has both the two states stable. It requires two trigger pulses to be applied to change the states. Until the trigger input is given, this Multivibrator cannot change its state. It’s also known as flip-flop multivibrators.
As the trigger pulse sets or resets the output, and as some data, i.e., either high or low is stored until it is disturbed, this Multivibrator can be called as a Flip-flop.
To get a clear idea on the above discussion, let us have a look at the following figure.
Applications of Multivibrators
Astable :- It is use in timing, delay circuit and communication system for receiving and transmitting a signal.
Monostable :- it is use in to regenerating a distorted pulsed signal, analog systems in order to control the frequency of the signal at the output.
Bistable :- it is use in digital counter, frequency division, latches and memory storage unit.
Multivibratotr is also use in to generate non sinusoidal wave like square triangular etc. it is also use to store binary information and synchronizing multiple operations of a digital system.