TABLE OF CONTENTS Operational amplifiers (op-amps) are perhaps the most important element in analog circuits and deserve the most attention from the circuit designer. The design of op-amps has been one of the most thoroughly covered topics in the field of integrated circuits (ICs), and a plentiful number of textbooks and handbooks are available that provide in-depth discussions and fruitful circuit design examples. Thus, the following context shall be positioned as an introductory overview, rather than a comprehensive study, of the modern complementary metal-oxide semiconductor (CMOS) op-amp topologies.
This chapter is organized as follows. Section 2.2 describes the fundamental aspects of the two-stage op-amp. Cascode-type op-amp topologies such as the telescopic and the folded-cascode op-amps are discussed in Section 2.3.
A two-stage op-amp is typically composed of two cascaded stages: the first is a fast open-loop stage, while the second is a high-gain and yet slower closed-loop stage. The total op-amp voltage gain is the product of the two stage gains. A typical compensated two-stage CMOS op-amp is shown in Figure 2.1 (the output buffer for resistive loading is not shown). The ultimate goal of compensating an op-amp is to ensure that the op-amp is not only stable (which is often reflected by phase margin and gain margin), but also capable of promptly settling its output signal to the desired value in response to a fast changing (or busy) input signal (which is reflected by unity- gain bandwidth and slew rate
