TABLE OF CONTENTS Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.
Archimedes
The operational amplifier was originally developed as the active element of analog computers (Korn and Korn 1956). The name derives from their ability to carry out a range of mathematical operations which were essential to the solution of differential equations. These operations are such as addition, subtraction, integration and differentiation. They are now indispensable devices in electronic systems though they are not commonly used in analog computers as such, but the functions performed are still the same. The performance of the various functions depends not only on their inherent properties but particularly on the application of negative feedback. We will consider this in some detail both here and in Section 3.10 as this is an instructive application to consider the benefits and the inevitable penalties and difficulties faced in the use of feedback. We will first consider an ideal operational amplifier and then let some reality intrude to show what we have to make do with in practice.
The ideal operational amplifier requires the following properties:
infinite open-loop gain;
infinite input impedance;
infinite bandwidth;
zero output impedance, offset, drift and noise.
The early vacuum tube operational amplifier had only a single input terminal owing to the nature of the drift-correcting chopper amplifier (Goldberg 1950). All standard operational amplifiers now have differential input terminals to give us the simple feedback configuration as shown in Fig. 5.3.1.
