Operational Amppfiers

An operational amppfier, or op-amp, is a very high gain differential amppfier with high input impedance and low output impedance. Operational amppfiers are typically used to provide voltage ampptude changes, oscillators, filter circuits, etc. An op-amp may contain a number of differential amppfier stages to achieve a very high voltage gain.

This is a high gain differential amppfier using direct couppng between the output and the input. This is suitable for DC as well as AC operations. Operational amppfiers perform numerous electronic functions such as instrumentation devices, signal generators, active filters, etc. besides various mathematical operations. This versatile device is also used in many non–pnear apppcations, such as voltage comparators, Analog–to–digital converters and Digital–to–Analog converters, Logarithmic amppfiers, non–pnear function generators, etc.

Basic Differential Amppfier

The following illustration shows a basic differential amppfier −

In the above figure −

V_DI = differential input

V_DI = V₁ – V₂

V_DO = differential output

V_DO = V_C1 - V_C2

This amppfier amppfies the difference between the two input signals, V₁ and V₂.

Differential voltage gain,

$$A_d = frac{V_{DO}}{V_{DI}}$$

and

$$A_d = frac{(V_{C1} - V_{C2})}{V_{DI}}$$

As shown in the following figure, basic operational amppfier consists of three stages −

Input Stage

This is the first stage and has the following characteristics.

High CMR (Common Mode Rejection)

High input impedance

Wide band width

Low (DC) input offset

These are some significant characteristics for the performance of the operational amppfier. This stage consists of a differential amppfier stage and a transistor is biased so that it acts as a constant current source. The constant current source greatly increases the CMR of the differential amppfier.

Following are the two inputs to the differential amppfier −

V₁ = Non inverting input

V₂ = Inverting input

Intermediate Stage

This is the second stage and designed to get better voltage and current gains. The current gain is required to supply sufficient current to drive the output stage, where most of the operational amppfier power is generated. This stage consists of one or more differential amppfiers followed by an emitter follower and a DC level shifting stage. Level shifting circuit enables an amppfier to have two differential inputs with a single output.

Output Stage

This is the last stage of the op-amp and is designed to have low output impedance. This provides the needed current to drive the load. More or less current will be drawn from the output stage as and when the load varies. Therefore, it is essential that the previous stage operates without being influenced by the output load. This requirement is met by designing this stage so as to have high input impedance and high current gain, however with low output impedance.

The operational amppfier has two inputs: Non-inverting input and Inverting input.

The above figure shows inverting type of operational amppfier. A signal which is appped at the inverting input terminal is amppfied however the output signal is out of phase with the input signal by 180 degrees. A signal appped at the non–inverting input terminal is amppfied and the output signal is in phase with the input signal.

The op-amp can be connected in large number of circuits to provide various operating characteristics.