Bistable Multivibrator

Objective
To study and perform the Bistable Multivibrator.

Introduction

A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. It is characterized by two amplifying devices (transistors, electron tubes or other devices) cross-coupled by resistors or capacitors. The name "multivibrator" was initially applied to the free-running oscillator version of the circuit because its output waveform was rich in harmonics. There are three types of multivibrator circuits depending on the circuit operation:


Astable Multivibrator : in which the circuit is not stable in either state, it continually switches from one state to the other. It functions as a relaxation oscillator.

Monostable Multivibrator : in which one of the states is stable, but the other state is unstable (transient). A trigger pulse causes the circuit to enter the unstable state. After entering the unstable state, the circuit will return to the stable state after a set time. Such a circuit is useful for creating a timing period of fixed duration in response to some external event. This circuit is also known as a one shot.

Bistable Multivibrator : in which the circuit is stable in either state. It can be flipped from one state to the other by an external trigger pulse. This circuit is also known as a flip flop. It can be used to store a bit of information.

Bistable Multivibrator circuit acts as a basic flip-flop. The Output will stable in two states: Output High and Output Low. The switching of the output waveform is achieved by controlling the Trigger and Reset inputs which are held "HIGH" by the two pull-up resistors.
A bistable multivibrator, also known as a flip-flop, has an output which can be in one of two states, logic 0 and logic 1. The key thing is that it is stable in either state, ie. unless you do something to its inputs its output will stay the same.
The inputs may be "set" and "reset". A pulse on these inputs changes the output to a logic 1 or logic 0 respectively. This is also called a SR flip-flop.
Alternatively, it may have a single "clock" or "trigger" input, when it receives a pulse it changes state from 0 to 1 or 1 to 0.