OVERVIEW

The operational amplifier, which has become one of the most versatile and important building blocks in analog circuit design, is introduced in this chapter. The operational amplifier (op amp) fits into the scheme of Table 1.1-2 as an example of a complex circuit. The unbuffered operational amplifiers developed in this chapter might be better described as operational-transconductance amplifiers since the output resistance typically will be very high (hence the term "unbuffered"). The term "op amp" has become accepted for such circuits, so it will be used throughout this text. The terms "unbuffered" and "buffered" will be used to distinguish between high output resistance (operational-transconductance amplifiers or OTAs) and low output resistance amplifiers (voltage operational amplifiers). Chapter 7 will examine op amps that have low output resistance (buffered op amps).

Operational amplifiers are amplifiers (controlled sources) that have sufficiently high forward gain so that when negative feedback is applied, the closed-loop transfer function is practically independent of the gain of the op amp (see Fig. 5.6-1). This principle has been exploited to develop many useful analog circuits and systems. The primary requirement of an op amp is to have an open-loop gain that is sufficiently large to implement the negative feedback concept. Most of the amplifiers in Chapter 5 do not have a large enough gain. Consequently, most CMOS op amps use two or more stages of gain. One of the most popular op amps is a two-stage op amp. We will carefully examine the performance of this type of...