Signal Processing and Integrated Circuits

Chapter 12

Two-stage CMOS Operational Amplifiers

12.1 Introduction

A critical building block in the design of analog signal processing systems is the CMOS operational amplifier (Op Amp). It may be regarded as a composite building block employing the elementary circuits and fundamental concepts given in the previous chapter. This chapter deals with the design of integrated CMOS Op Amps [22–24] and gives complete design examples. The chapter begins by a summary of Op Amp performance parameters and the fundamentals of feedback amplifier characteristics, then proceeds to give a discussion of the CMOS differential pair which is the first stage in the Op Amp. Next, the very popular two-stage CMOS Op Amp architecture is developed and a detailed account of the design considerations is given together with a summary of the design equations for easy reference and use by the reader. This is followed by a complete design example starting from the design specifications and showing in detail how to arrive at the structure and element values of the final design of an integrated CMOS Op Amp.

12.2 Op Amp Performance Parameters

Ideally, an Op Amp is a voltage-controlled voltage source as shown in Figure 12.1 whose output v_o and two inputs v₁ and v₂ are related by

12.1

where A is a constant, independent of frequency, and is very large (ideally = ∞). In practice, however, A is frequency-dependent and ...