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Operational Amplifiers Summary

Operational Amplifiers Summary

The following is a summary of the different types of Operational Amplifiers and their configurations discussed in this tutorial section.
The Operational Amplifier, or Op-amp as it is most commonly called, is an ideal amplifier with infinite Gain and Bandwidth when used in the Open-loop mode with typical d.c. gains of well over 100,000, or 100dB.
The basic Op-amp construction is of a 3-terminal device, 2-inputs and 1-output.
An Operational Amplifier operates from either a dual positive ( +V ) and an corresponding negative ( -V ) supply, or they can operate from a single DC supply voltage.
The two main laws associated with the operational amplifier are that it has an infinite input impedance, ( Z∞ ) resulting in "No current flowing into either of its two inputs" and zero input offset voltage "V1 = V2".
An operational amplifier also has zero output impedance, ( Z = 0 ).
Op-amps sense the difference between the voltage signals applied to their two input terminals and then multiply it by some pre-determined Gain, ( A ).
This Gain, ( A ) is often referred to as the amplifiers "Open-loop Gain".
Closing the open loop by connecting a resistive or reactive component between the output and one input terminal of the op-amp greatly reduces and controls this open-loop gain.
Op-amps can be connected into two basic configurations, Inverting and Non-inverting.

The Two Basic Operational Amplifier Circuits

The Open-loop gain called the Gain Bandwidth Product, or (GBP) can be very high and is a measure of how good an amplifier is.
Very high GBP makes an operational amplifier circuit unstable as a micro volt input signal causes the output voltage to swing into saturation.
By the use of a suitable feedback resistor, ( Rƒ ) the overall gain of the amplifier can be accurately controlled.

Gain Bandwidth Product

For negative feedback, were the fed-back voltage is in "anti-phase" to the input the overall gain of the amplifier is reduced.
For positive feedback, were the fed-back voltage is in "Phase" with the input the overall gain of the amplifier is increased.
By connecting the output directly back to the negative input terminal, 100% feedback is achieved resulting in a Voltage Follower (buffer) circuit with a constant gain of 1 (Unity).
Changing the fixed feedback resistor ( Rƒ ) for a Potentiometer, the circuit will have Adjustable Gain.
The Differential Amplifier produces an output that is proportional to the difference between the 2 input voltages.

Differential and Summing Operational Amplifier Circuits

Reproduced with permission from Wayne Storr

Written by Wayne Storr

Wayne Storr