Analog Electronics: Circuits, Systems and Signal Processing

433 Pages·2002·17.97 MB·English

by  David Crecraft

Preface
This book is written primarily as a course text for the earUer parts of undergraduate courses,
BS courses in the USA and both CEng and lEng courses in the UK. It covers those topics of
analog electronics that we consider essential for students of Electrical, Electronic, Commu
nication, Instrumentation, Control, Computer and aUied engineering discipHnes. Naturally,
we recognize that this is the age of digital electronics, but we are also aware of the importance
of analog circuitry and concepts in the spectrum of skills essential for the proper education of
engineers in the 'hght-current' field. We know that the majority of the topics in this book are
covered in all good engineering programmes. They may be contained in syllabuses with the
word 'analog' in their titles, but many are equally at home in others. The book may be used to
form the basis of a full subject, course, module or unit or selectively across a number of these
and also at several levels.
Our aim is to provide a coverage which is as rigorous as possible whilst ensuring that the
engineering dimension is not lost in a mass of dry analysis. We try to bring the subject alive by
relating it to the everyday experience of the students. The distinction is made between exact
solutions, approximations and 'rules of thumb'. Exact analysis is given whenever it is appro
priate, bearing in mind the purpose and intended readership of this book, even where
approximations and 'rules of thumb' are used subsequently. We aim to provide a sufficiently
comprehensive Ust of the types of system, circuit and device, in order to acquaint students with
the full range of possible solutions. Students need not be asked to be equally familiar with
them all.
Each chapter has a selection of self-assessment questions (SAQs) inserted at appropriate
points in the text, to allow the reader to check that the preceding material has been under
stood, and can be applied to the solution of part of a reaUstic design problem. Answers to
these SAQs are given at the end of the book.
Parts of the analysis are illustrated by computer simulations, which the reader is encouraged
to perform. These use two types of software: spreadsheets and SPICE-derivative analog circuit
analysis packages. For the spreadsheets, the registered reader can download the contents from
the publisher's website, and load them into Microsoft Excel, or a compatible program,
running on a PC or a Macintosh. In the case of the circuit analysis software, few of even
the SPICE look-alikes are compatible, so we ask the reader to draw the circuit schematic, or
enter the netlist, on whichever package is available.
Chapter 1 is an introduction to electronic systems, with the aim of clarifying the roles of
analog techniques and digital techniques, putting into context the material in the following
chapters.
Preface ix
Chapter 2 is an introduction to signals and systems. This chapter covers a very wide range of
topics, often found to be the subject of complete textbooks. The coverage is complete and
rigorous, but necessarily concise and carefully selective in support of the aims of this text. It
may be used as revision or as reinforcement in conjunction with another text.
Chapter 3 takes a systems approach to ampHfiers and their properties, such as gain,
frequency response and input and output impedances. Negative feedback is introduced in
the context of operational amplifiers, since amplifiers of this type, and those with a similar
basic structure, are probably the most common users of the technique. The concept of gain-
bandwidth product is introduced. Inverting and non-inverting configurations are covered, as
is the important matter of stabiHty. Calculations of output offsets and noise are included.
Chapter 4 describes aspects of signal processing using operational amphfiers. Instrumenta
tion ampHfiers, summing amphfiers, integrators, charge amphfiers and precision rectifiers are
included.
Chapter 5 explains diode and transistor circuits, in preparation for Chapter 6. It introduces
semiconductors and the operation of diodes and transistors. The hybrid-7r equivalent circuit is
developed for the bipolar junction transistor, showing the predictable dependence of the
mutual conductance gm and other parameters on operating current, and applying it to the
analysis of a simple amplifier stage. A similar approach is used for the field-effect transistor, of
junction types and insulated-gate types (MOSFETs). The equivalent circuit is again based on a
voltage-dependent current generator, with the common approach for both bipolar and field-
effect transistors reinforcing the concept, and reflecting industry design practice.
In Chapter 6, with all the necessary background material in place, we now tackle the full
design of a simple operational amphfier (op amp) circuit, starting with a 'top-down' approach
to the circuit architecture. The required d.c.-couphng, open-loop frequency response for
closed-loop StabiHty, temperature compensation, common-mode rejection and rail rejection
lead to choices of three stages, powered via temperature-compensated current-sources. These
are then designed. The slew rate is calculated. The chapter is completed by a description of
available integrated-circuit op amps, and techniques used in video-frequency and r.f. op amps,
including a discussion of voltage-feedback ampHfiers and current-feedback amplifiers.
Chapter 7 deals with the bridge between the analog and the digital worlds. It starts with the
fundamental issues of quantization and sampHng. Analog comparators are described here
and, of course, the various converter configurations, including the over-sampling types. The
discussion of the errors found in converters can easily be broadened to an explanation of
errors in ah types of instrumentation.
Chapter 8 describes the design of low-distortion audio-frequency power ampHfiers. The
circuit architecture is based on that of the op amp. Much of the analysis concentrates on the
design of complementary-symmetry push-pull output stages; the double emitter-follower and
the Darlington and Sziklai compound pairs. Analysis of their output impedances, and the way
they vary with load current, is complemented by software simulations of the total harmonic
distortion, leading to choices of circuit parameters and operating conditions for minimum
distortion.
Chapter 9 introduces modulator and demodulator circuits for ampHtude, frequency, phase
and digital modulation. Analysis of the different types of modulation leads to their frequency
spectra. This is complemented by spreadsheets showing the Hne spectra for modulating signals
consisting of just one or two sine waves. The registered reader can download these spread
sheets, to perform 'what if investigations of different modulation indexes and more complex
modulating signals, a technique of especial value in the analysis of frequency modulation. The
X Preface
chapter concludes with a description of tuned r.f. and superhet receivers, including an analysis
of second-channel interference and its influence on superhet design.
Chapter 10 provides a comprehensive coverage of both active and passive analog filter
design and implementation. All the well-known configurations are included, but the treatment
enables the selective teaching of just some of these. In addition, SAW and switched capacitor
implementations are also covered. Digital filters are mentioned but not covered in detail.
Sensitivity analysis and related advanced aspects of design are considered to lie outside the
scope of this text.
Chapter 11 deals with a wide range of signal generation techniques. The operation of the
well-known oscillator circuit configurations is explained together with the more advanced
techniques of frequency synthesis. Specialist topics such as oscillator stabihty, phase noise, etc.
are not included in this treatment.
Chapter 12 deals with the problems of signal transmission via metaUic or fibre optic
interconnections. The formal analysis of transmission Hnes forms a substantial part of this
chapter. Although the section starts with the description of the general case, it is equally valid
to start the teaching of the topic using the more particular case of sinusoidal signals. The
authors consider it important to provide an understanding of the physical mechanism of
energy propagation as well as the derivation of the various relationships. The use of Smith
charts is mentioned, but they are not described in detail. This chapter also includes a discus
sion of the important topic of interference and the design techniques for its control.
Chapter 13 deals with power supplies which are required for all electronic equipment. This
chapter includes a discussion of batteries as well as mains (Hne) power suppUes in recognition
of the ubiquity of battery operated devices. The section on mains supplies also provides an
explanation of the operation of transformers for students who either have not met this topic in
other parts of their education, or wish to refresh their knowledge. The coverage of switch-
mode supplies includes a description of the various basic d.c. to d.c. converter configurations.
This could form a useful foundation for students who proceed to specialize in power electronics.
Introduction to electronic
systems
1.1 The roles of analog electronics and digital electronics
The purpose of this chapter is to clarify the distinction between analog and digital electronics,
and to provide examples of systems in which both types of electronics are used, so as to set in
context the analog circuit analysis and signal processing in the rest of the book. 'Electronics' is
one of those words which most of us recognize, but which is quite hard to define. However, we
usually mean apparatus and systems which use devices which amplify and process electrical
signals, and which need a source of power in order to work. Most of the devices which do this
are transistors. In the early days of electronics, the devices were vacuum-tubes ('valves') in
which a stream of electrons was emitted from a heated cathode, via a control grid, towards an
anode. This is probably where the name 'electronics' came from. (Beams of electrons in a
vacuum are still used in the cathode-ray tubes used for displays in television receivers and
computer monitors, and in microwave devices called magnetrons and travelling-wave tubes.)
The electrical signal from a microphone is an example of an analog signal; its waveform
(graph of voltage against time) has a similar shape to the waveform of the sound waves which
it 'picks up'. The converse process, that of converting an electrical signal into a sound wave,
also involves analog signals. The electrical analog signal is fed to a loudspeaker, which
produces a sound waveform which is an analog of the original sound.
An example of a digital signal is that recorded on a compact disc (CD). If an analog signal
from a microphone is to be recorded, then it must first be converted to digital form. This
involves sampling the analog signal at a frequency much higher than the highest analog signal
frequency, and then converting the sample amplitudes into corresponding digital codes
represented by a series of electrical pulses. Further coding is used, first to 'compress' the total
data and then to convert it into longer sequences for immunity against corruption. These
sequences are stored on the disc as tiny 'blips' representing binary data.
All of the digital circuits use transistors, in sub-circuits called gates and flip-flops. So, both
types of electronics have transistors in common. The design of the gate and flip-flop circuits,
for ever-higher speed and lower power dissipation, depends heavily on the same circuit
concepts as the analog circuits designed for higher frequencies and lower power dissipation.
These are concepts such as the equivalent circuit of the transistor, stored charge, stray
capacitance and inductance, input and output impedance, electrical noise and the Uke. Inter
connections between sub-assemblies, whether for digital signals in the form of pulses, or for
analog signals, must be designed with a knowledge of transmission line theory when high
2 Analog Electronics
frequencies or high data rates (which incur high frequencies) are present. So, a great deal of
the analog material of this book forms also the basic material of digital circuit design. The
analog signal processing in the book is mirrored in digital signal processing, much of which
is modelled on analog prototypes, and uses the same design theory, modified for digital
implementation.
Thus, a good grounding in the theory of analog electronics is not only useful in its own
right, but provides much of the background skills for the design of digital systems. The
following examples illustrate the roles of analog and digital electronics in various systems.
S 1.2 Hi-fi and music amplifiers
We start with one of the most familiar uses of electronics, the amplification of speech and
music. Figure 1.1 shows a block diagram of a typical set-up used by a group of musicians
during a live performance. Each microphone converts sound waves into an electrical voltage,
or electrical signal, which represents the sound. The electrical signals are conveyed by cables to
an ampUfier which boosts, or amplifies, the signals before passing them by cables, or radio, to
the loudspeakers. The loudspeakers convert the electrical signals back into sound waves
which, ideally, are the same as the original speech or music but at much higher intensities.
In Figure 1.1, the form of the sound wave at a microphone is shown in a graph of the sound
pressure (p) variation with time (/), called the waveform of the sound. The waveform of the
electrical voltage (v) generated by the microphone has an almost identical shape. It is
analogous to the sound waveform, so it is called an analog waveform, or analog signal. The
microphone waveform has a typical peak voltage of some millivolts.
The typical peak voltages needed to provide enough sound from the loudspeakers in a
concert hall are a few volts, or tens of volts. So it is quite obvious that the microphone signal
voltages need to be amplified by a factor of about one thousand or more. The amplifier has to
have a voltage gain of about one thousand or more.
sound
waves
audio loudspeaker
power
amplifier
microphones
(a)
v/mV A
sound waveform microphone output loudspeaker
at microphone voltage: analog input voltage
of the sound
(b) (c) (d)
Fig. 1.1 Block diagram of a purely-analog sound amplification system.