Introduction to CMOS OP-AMPs and Comparators

CHAPTER 4

CMOS OPERATIONAL AMPLIFIERS

The CMOS operational amplifier is the most intricate, and in many ways the most important, building block of linear CMOS and switched-capacitor circuits. Its performance usually limits the high-frequency application and the dynamic range of the overall circuit. It usually requires most of the dc power used up by the device. Without a thorough understanding of the operation and the basic limitations of these amplifiers, the circuit designer cannot determine or even predict the actual response of the overall system. Hence this chapter includes a fairly detailed explanation of the usual configurations and performance limitations of operational amplifiers.

The technology, and hence the design techniques used for MOS amplifiers, change rapidly. Therefore, the main purpose of die discussion is to illustrate the most important principles underlying die specific circuits and design procedures. Nevertheless, the treatment is detailed enough to enable the reader to design high-performance CMOS operational amplifiers suitable for most linear CMOS circuit applications.

4.1. OPERATIONAL AMPLIFIERS [1, Chap. 10; 2, Chap. 6]

In switched-capacitor circuits—in fact, in all linear CMOS circuits—the most commonly used active component is the operational amplifier, usually simply called the op-amp. Ideally, the op-amp is a voltage-controlled voltage source (Fig. 4.1) with infinite voltage gain and with zero input admittance as well as zero output impedance. It ...