This chapter describes the fundamental principles of basic opamp design. To illustrate many of these principles, the design of a traditional opamp—namely, the two-stage CMOS opamp—is used first. This example illustrates compensation techniques needed to ensure stability in closed-loop amplifiers as well as a number of other important design techniques, such as ensuring zero systematic input-offset-voltage and process-insensitive lead compensation.
Although the two-stage CMOS opamp is a classic circuit used in many integrated circuits, other architectures have gained popularity. A few of the most important applications of opamps in analog integrated circuits are illustrated in Fig. 6.1. Note that in many cases, the loads being driven by the opamp are purely capacitive. This fact may be used to advantage by designing opamps to have high output impedances, providing large voltage gain in a single stage with relatively low power consumption. Opamps exemplifying this approach are described later in the chapter. However, one should not overlook the potential of the two-stage opamp, which is well-suited to lowvoltage applications since it does not require a cascode output stage. Fully-differential opamps are also described.
6.1 TWO-STAGE CMOS OPAMP
The two-stage circuit architecture has historically been the most popular approach to opamp design. When properly designed, the two-stage ...