The term operational amplifier (op amp) refers to an amplifier that
performs a mathematical operation. Historically, the first op amps
were used in analog computers, where they did addition, subtraction,
multiplication, and so on. At one time, op amps were built as discrete
circuits. Now, most op amps are integrated circuits (ICs).
The typical op amp is a dc amplifier with very high voltage gain, very
high input impedance, and very low output impedance. The unity-gain
frequency is from 1 to more than 20 MHz, depending on the part
number. An IC op amp is a complete functional block with external
pins. By connecting these pins to supply voltages and a few
components, we can quickly build all kinds of useful circuits.
The input circuit used in most op amps is the differential amplifier.
This amplifier configuration establishes many of the IC’s input
characteristics. The differential amplifier may also be configured in a
discrete form to be used in communications, instrumentation, and
industrial control circuits. This chapter will focus on the differential
amplifier used in ICs.
