| Operational amplifiers have been in use for many years. Originally they were built using discrete transistor circuits, but the development of the integrated circuit (IC) has revolutionized analogue circuit design. The operational amplifier was one of the first analogue integrated circuits, because of its usefulness as a building block in many circuit designs. The popularity of the operational amplifier has resulted in a shortened name ‘op-amp’ to be commonplace. The term op-amp will be used extensively in this book.
The op-amp’s popularity stems from its versatility. It is a high-gain DC amplifier that has differential inputs; the output voltage is the voltage difference between the two inputs multiplied by the gain. Passive components can be used to provide feedback, and this controls the gain and function of the opamp circuit overall. Passive negative feedback components result in a linear response, i.e. the output is proportional to the input. Passive positive feedback results in switching or oscillation. Sometimes active components such as transistors and diodes are used in the feedback loop to give a non-linear response; typical applications are logarithmic amplifiers or precision rectifiers.
My interest in op-amp circuits began while I was an apprentice technician. One of the first books that I bought was Clayton’s Operational Amplifiers (first edition). It is therefore fitting that I should be asked by the publisher to edit the fifth edition. In my previous employment as a circuit design engineer for British Telecom, and now as a field applications engineer for Supertex Inc., I have used op-amps in hundreds of circuits. For me, one valuable application is in active filter circuits (refer to Chapter 9 and to my book, Analog and Digital Filter Design, ISBN 0–7506–7547–0).
In this fifth edition of Operational Amplifiers I have added more on active filters, especially gyrator and frequency-dependent negative resistance circuits. Throughout the book I have updated and added material, where appropriate. This includes the important practical guidelines about passive components used in op-amp circuits. Although placed near the end of the book, in Chapter 10, this information is important and should not be overlooked. |
