Operational amplifier bistable multivibrator circuit

- circuit along with a description and details of the operation for a bistable multivibrator using an op amp or operational amplifier.

Bistable circuits are sometimes needed within electronic circuit designs.

Whilst they are most commonly associated with logic circuits, it is sometimes convenient to have a bistable using an op amp as the basis of the circuit.

Bistable circuits are also referred to as flip-flops or latches as a result of the way in which they operate.

When used in association with an analogue circuit, a bistable flip-flop based around an operational amplifier is an obvious choice.

What is a bistable

A bistable is an electronic circuit also referred to as a flip-flop or latch. It is a circuit that has two stable states and can be used to store state information.

A flip-flop is a bistable multivibrator and it can be made to change state by signals applied to one or more control inputs and will have one or two outputs.

Flip-flops and latches are a fundamental building block of digital electronics systems. One of their chief applications is in storing data and as such they are widely used in computers and processor systems of all sorts.

Op amp bistable

This is easy to use an operational amplifier as a bistable multivibrator. An incoming waveform is converted into short pulses and these are used to trigger the operational amplifier to change between its two saturation states. To prevent small levels of noise triggering the circuit, hysteresis is introduced into the circuit, the level being dependent upon the application required. The operational amplifier bistable multivibrator uses just five components, the operational amplifier, a capacitor and three resistors.

Circuit diagram for a basic bistable circuit based around an operational amplifier
Op amp bistable multivibrator circuit

The bistable circuit has two stable states. These are the positive and negative saturation voltages of the operational amplifier operating with the given supply voltages. The circuit can then be switched between them by applying pulses. A negative going pulse will switch the circuit into the positive saturation voltage, and a positive going pulse will switch it into the negative state.

Waveforms for the bistable multivibrator operational amplifier circuit

It is very easy to calculate the points at which the circuit will trigger. The positive going pulses need to be greater than Vo-Sat through the potential divider, i.e. -Vsat x R3 / (R2 + R3), and similarly the negative going pulses will need to be greater than +Vsat through the potential divider, i.e. +Vsat x R3 / (R2 + R3). If they are not sufficiently large then the bistable will not change state.

By Ian Poole

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