2.1 INTRODUCTION

The operational amplifier, often referred to as the op amp, is one of the most versatile building blocks in linear circuits applications. The availability of high-performance, inexpensive op amps has had a dramatic influence in the electronic industry. Op amps are the workhorses in linear circuit applications and play a role comparable to that of microprocessors in digital circuitry. Integrated circuit op amps of relatively high performance cost less than $0.50 (1980) and are composed of small capacitors, resistors, and many transistors. In fact, in certain op amp packages one may find four identical op amps in one package (e.g., LM 348; see Appendix C). Their cheap price, off-the-shelf availability, and good performance justify their use in industry and emphasis in this book.

Basically, op amps are direct-coupled differential amplifiers with extremely high gain that are generally used with external feedback for gain-bandwidth control. Until recently, typical op amps were realized as silicon integrated circuits composed of bipolar transistors. These circuits are very high quality, small, and quite inexpensive. Bipolar op amps [P1] typically consume < 3 mm ² of silicon area. To further reduce the cost of linear circuits and combined linear-digital subsystems, there has been considerable economic motivation to develop op amp technologies which are compatible with well-established large-scale integrated (LSI) circuit technologies, such as the N-channel metal-oxide-semiconductor (NMOS) and complementary-metal-oxide-semiconductor (CMOS) technologies. Currently available NMOS operational amplifiers [P3, P4] do not compete well with the performance characteristics of their bipolar transistor counterparts;