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INTRODUCTION TO ANALOG AND MIXED-SIGNAL ELECTRONICS

1.1 INTRODUCTION

"In the beginning, there were only analog electronics and vacuum tubes and huge, heavy, hot equipment that did hardly anything. Then came the digital-enabled by integrated circuits and the rapid progress in computers and software-and electronics became smaller, lighter, cheaper, faster, and just better all around, all because it was digital." That's the gist of a sort of urban legend that has grown up about the nature of analog electronics and mixed-signal electronics, which means simply electronics that has both analog and digital circuitry in it.

Like most legends, this one has some truth to it. Most electronic systems, ever since the time that there was anything around to apply the word "electronics" to, were analog in nature for most of the twentieth century. In electronics, an analog signal is a voltage or current whose value is proportional to (an analog of) some physical quantity such as sound pressure, light intensity, or even an abstract numerical value in an analog computer. Digital signals, by contrast, ideally take on only one of two values or ranges of values and by doing so represent the discrete binary ones and zeros that form the language of digital computers. To give you an idea of how things used to be done with purely analog systems, Figure 1.1 shows on the left a two-channel vacuum-tube audio amplifier that can produce about 70 W per channel.

Figure 1.1 A comparison: Vacuum-tube audio amplifier (left) using a design circa 1955 and class D amplifier (right) using a design circa 2008.

The vacuum-tube amplifier measures 30 cm × 43 cm × 20 cm and weighs 17.2 kg (38 lb) and was state-of-the-art technology in about 1955. On its right is a solid-state class D amplifier designed in 2008 that can produce about the same amount of output power. It is a mixed-signal (analog and digital) design. It measures only 15 cm × 10 cm × 4 cm and weighs only 0.33 kg, not including the power supply, which is of comparable size and weight. The newer amplifier uses its power devices as switches and is much more efficient than the vacuum-tube unit, which is about 50 times its size and weight. So the claim that many analog designs have been made completely obsolete by newer digital and mixed-signal designs is true, as far as it goes.

Sometimes, you will hear defenders of analog technology argue that "the world is essentially analog, and so analog electronics will never go away completely." Again, there's some truth to that, but it depends on your point of view. The physics of quantum mechanics tells us that not only are all material objects made of discrete things called atoms but many forms of energy appear as discrete packets called quanta (photons, in the case of electromagnetic radiation). So you can make just as good an argument for the case that the whole world is essentially digital, not analog, because it can be represented as bits of quanta and atoms that are either there or not there at all.

The fact of the matter is that while the bulk of today's electronics technology is implemented by means of digital circuits and powerful software, a smaller but essential part of what goes into most electronic devices involves analog circuitry. Even if the analog part is as simple as a battery for the power supply, no one has yet developed a battery that behaves digitally: that is, one that provides an absolutely constant voltage until it depletes and drops abruptly to zero. So even designers of an otherwise totally digital system have to deal with the analog problem of power-supply characteristics.

This book is intended for anyone who has an interest in understanding or designing systems involving analog or mixed-signal electronics. That includes undergraduates with a basic sophomore-level understanding of electronics, as well as more advanced undergraduates, graduate students, and professionals in engineering, science, or other fields whose work requires them to learn about or deal with these types of electronic systems. The emphasis is practical rather than theoretical, although enough theory to enable an understanding of the essentials will be presented as needed throughout. Many textbooks present electronics concepts in isolation without any indication of how a component or circuit can be used to meet a practical need, and we will try to avoid that error in this book. Practical applications of the various circuits and systems described will appear as examples, as paper or computer-simulation design exercises, and as lab projects.

1.2 ORGANIZATION OF THE BOOK

The book is divided into three main sections: devices and linear systems (Chapters 2 and 3), linear and nonlinear analog circuits and applications (Chapters 4-7), and special topics of analog and mixed-signal design (Chapters 8-12). A chapter-by-chapter summary follows.

1.2.1 Chapter 2: Basics of Electronic Components and Devices

In this chapter, you will learn enough about the various types of two- and three-terminal electronic devices to use them in simple designs. This includes rectifier, signal, and light-emitting diodes and the various types of three-terminal devices: field-effect transistors (FETs), bipolar junction transistors (BJTs), and power devices. Despite the bewildering number of different devices available from manufacturers, there are usually only a few specifications that you need to know about each type in order to use them safely and efficiently. In this chapter, we present basic circuit models for each type of device and how to incorporate the essential specifications into the model.

1.2.2 Chapter 3: Linear System Analysis

This chapter presents the basics of linear systems: how to characterize a "black box" circuit as an element in a more complex system, how to deal with characteristics such as gain and frequency response, and how to define a system's overall specifications in terms that can be translated into circuit designs. The power of linear analysis is that it can deal with complex systems using fairly simple mathematics. You also learn about some basic principles of noise sources and their effects on electronic systems.

1.2.3 Chapter 4: Nonlinearities in Analog Electronics

While linear analysis covers a great deal of analog-circuit territory, nonlinear effects can both cause problems in designs and provide solutions to other design problems. Noise of various kinds is always present to some degree in any circuit, and in the case of high-gain and high-sensitivity systems dealing with low-level signals, noise can determine the performance limits of the entire system. You will be introduced to the basics of nonlinearities and noise in this chapter and learn ways of dealing with these issues and minimizing problems that may arise from them.

1.2.4 Chapter 5: Op Amp Circuits in Analog Electronics

The workhorse of analog electronics is the operational amplifier ("op amp" for short). Originally developed for use in World War II era analog computers, in integrated-circuit form the op amp now plays essential roles in most analog electronics systems of any complexity. This chapter describes op amps in a simplified ideal form and outlines the more complex characteristics shown by actual op amps. Basic op amp circuits and their uses make up the remainder of the chapter.

1.2.5 Chapter 6: The High-Gain Analog Filter Amplifier

High-gain amplifiers bring with them unique problems and capabilities, so we dedicated an entire chapter to a discussion of the special challenges and techniques needed to develop a good high-gain amplifier design. We also introduce the basics of analog filters in this section and apply them to the design of a practical circuit: a guitar preamp.

1.2.6 Chapter 7: Waveform Generation

While many electronic systems simply sense or detect signals from the environment, other systems produce or generate signals on their own. This chapter describes circuits that generate periodic signals, collectively termed oscillators, as well as other signal-generation devices. Because oscillators that produce a stable frequency output are the heart of all digital clock systems, you will also find information on the basics of stabilized oscillators and the means used to stabilize them: quartz crystals and, more recently, microelectromechanical system (MEMS) resonators.

1.2.7 Chapter 8: Analog-to-Digital and Digital-to-Analog Conversion

Most new electronic designs of any complexity include a microprocessor or equivalent that does the heavy lifting in terms of functionality. But many times, it is necessary to take analog inputs from various sensors (e.g., photodiodes, ultrasonic sensors, proximity detectors) and transform their outputs into a digital format suitable for feeding to the digital microprocessor inputs. Similarly, you may need to take a digital output from the microprocessor and use it to control an analog or high-power device such as a lamp or a motor. All these problems involve interfacing between analog and digital circuitry. While no single solution solves all such problems, this chapter describes several techniques you can use to...