Signal Processing and Integrated Circuits

Chapter 13

High Performance CMOS Operational Amplifiers and Operational Transconductance Amplifiers

13.1 Introduction

Although the basic two-stage Op Amp discussed in the previous Chapter has a performance which is satisfactory in many applications, it suffers from a number of disadvantages which were pointed out. Improvements in the performance with regards to one or more of theses non-ideal effects, such as obtaining higher gain, better PSRR, reduced offset voltage, lower noise, better settling time and slew rate, require special techniques which may entail modification of the Op Amp structure. These techniques are discussed in this chapter [22–24]. We also give integrated circuit realizations of operational transconductance amplifiers (OTAs) which are used with great advantage in high-frequency applications, including submicron and deep submicron integrated circuit design [25, 26].

13.2 Cascode CMOS Op Amps

Figure 13.1 shows the first stage of a cascode Op Amp constructed with the main objective of increasing the gain. The two common-gate transistors Q_1c and Q_2c form the cascode for the differential pair Q₁, Q₂. The output resistance at Q_2c is

13.1

which is much higher that the value without the cascode transistors. Naturally, in order to make full use of the high output resistance, we must also increase the active load resistance; thus a Wilson current mirror is used in Figure ...