Introduction

The subject of impedance matching is frequently surrounded by an undeserved aura of mystery. When a circuit fails to work as well as expected, impedance mismatch is often blamed in a rather vague sort of way, but correcting the problem may be regarded more as witchcraft than science. The purpose of this chapter is to outline the principles and practice of impedance matching and, in so doing, to attempt to dispel any associations of mystery and magic!

Input impedance

Any electrical device which requires a signal for its operation has an input impedance. Just like any other impedance (or resistance in d.c. circuits), the input impedance of a device is a measure of the current drawn by the input with a certain voltage across it.

For example, the input impedance of a 12 V light bulb rated at 0.5 A is 12/ 0.5Ω, or 24 Ω. The bulb is a clear example of impedance because we know that there is nothing but a filament to consider. The input impedance of a circuit such as bipolar transistor amplifier might seem to be more complicated. At first sight, the presence of capacitors, resistors and semiconductor junctions in a circuit makes the input impedance difficult to assess. However, any input circuit, however complicated, may be resolved into the simple impedance shown in fig. 5.1.

Anyone who is interested in technology is aware of the importance of electronics. Despite its all-pervading influence, however, electronics retains a strong element of mystery for many people who are otherwise well informed on technical matters. In this book, I aim to provide a practically-based explanation of the subject to try to dispel the mystery.

In the study of electronics, I have always found practical experience to be an invaluable stimulant and confidence-builder. There is enormous satisfaction in ‘lashing up’ a new circuit on the test bench and seeing it work for the first time. As an experimental physicist, I have never regarded electronic design as an end in itself, but rather as a valuable tool in research and development. The practical approach is therefore of primary importance in the electronics content of the UMIST Pure and Applied Physics degree course. Experience of teaching electronics to people of various backgrounds has encouraged me in the belief that a book based on a practical viewpoint can be valuable at all levels from school, through college or university, to the industrial laboratory. This book aims to cover a wide range of circuit building bricks, analogue and digital, discrete and integrated. It is from these bricks that an elaborate system is constructed, whether it be a colour television set or a computer. Sufficient practical information is given to enable the reader to construct the circuits and test them in the laboratory.

Introduction

The thermionic valve was the first active (amplifying) element in electronics. Although obsolete for most small-scale amplification, the valve still finds a place where high voltages must be handled or high-power high-frequency signals are involved. In addition, the inherent characteristics of valve audio amplifiers are popular with many audiophiles. In particular, the graceful behaviour of the circuits near overload can give a subjectively clean sound at high levels. It is therefore useful for the electronic engineer to have at least a rudimentary knowledge of the valve and its circuitry. This chapter gives a brief account of valve (vacuum tube) circuits, including a description of the one thermionic device which is still extensively used: the cathode-ray tube.

Thermionic emission

In the early 1880s, Thomas Edison, having developed the carbon-filament lamp, turned his attention to the blackening of the glass bulb which occurred after some hours of use. In an attempt to intercept some of the blackening particles, he sealed a metal plate inside one of his lamps and was surprised to find that, if the plate was made positive with respect to the filament, it drew a current. For twenty years, no one was to know that this ‘Edison effect’ current was due to electrons emitted by the hot filament being captured by the positively charged plate. The term thermionic emission was coined to describe this thermal liberation of free particles, literally thermal ions.

Introduction

Our introduction to the transistor in chapter 1 used the device as a switch to turn a light bulb on and off. This is the simplest possible example of amplification, where the signal has only two voltage levels coresponding to the ‘on’ and ‘off’ conditions. Such a signal is shown in fig. 10.1; here, the two levels are zero and +5 V, the transition between the levels being virtually instantaneous so that the waveform is rectangular. Signals from radiation detectors such as the Geiger–Müller tube are of this form, whilst the enormous field of digital electronics is concerned entirely with trains of rectangular pulses. It is now common practice to transmit and record ordinary analogue signals in digital form because the signal has then only two levels and the various distortions which may occur in transmission and recording are readily corrected. This chapter is concerned with the handling of pulses and the modifications and distortions which may occur to them in various circuits.

‘Squaring’ a waveform

Any repetitive waveform may be converted to a rectangular wave merely by driving a simple voltage amplifier so hard that it is driven alternately into cut-off and saturation. Fig. 10.2 shows a circuit that will do this. If the input is fed with a sine wave of at least 5 V r.m.s. amplitude, the output will be a rectangular wave suitable for use with the various pulse-shaping experiments in this chapter.

Introduction

The integrated circuit (IC) is clearly the building brick of electronic circuits. We have already had a flavour of IC applications when looking at power supply regulators. Now we turn towards the full range of IC capabilities. A glance through a component distributor's catalogue reveals a seemingly limitless range of ICs for virtually every conceivable function. An IC may contain from a dozen transistors to a million depending on the application, together with all necessary resistors, diodes etc. The intimate thermal connection achieved by fabricating all the components on one chip of silicon generally leads to excellent stability and predictability in use.

The understanding of discrete components gained from earlier chapters will be found essential to the proper interfacing of ICs: in fact even today very few analogue circuit applications dispense totally with discrete semiconductors. However, the IC designer has today relieved the circuit designer of much of the ‘donkey work’. In addition, the small size and low power consumption of ICs has made possible products like the Camcorder and hand-held GPS position-fixing satellite receiver.

This chapter deals with applications of linear ICs. They are designed to handle analogue signals, which carry their information in terms of amplitude and waveshape. Most audio and radio signals come into this category; they are distinct from the standard binary pulses of digital circuits which are discussed in chapter 13.

Power sources

The necessary d.c. supplies for electronic circuits may be drawn from batteries or obtained by rectification of the a.c. mains. Batteries have the advantage of portability and complete absence of a.c. components in their output. There is, however, a danger of leakage if exhausted batteries are accidentally allowed to stay too long in equipment; this may endanger many hundreds of pounds worth of circuitry through corrosion damage.

The e.m.f. of a battery is not usually constant throughout its life, that of the common low cost zinc chloride and alkaline batteries, falling from 1.6 V to 1.3 V over the useful life of the cell. Mercury oxide cells have a much better e.m.f. characteristic, remaining at 1.3 V over virtually the whole of their life and then falling off rapidly so that there is no doubt when the end of their life is reached; they are, however, expensive. Silver oxide cells have a similarly constant e.m.f. of 1.55 V.

Rechargeable nickel–cadmium (NiCd) cells are available in an enormous range of sizes, ranging from tiny ‘button’ cells to the large batteries used for electric traction. The smaller sizes are usually hermetically sealed so that there is no risk of leakage and no need for topping up. NiCd batteries and the newer nickel-metal-hydride (NiMH) type make an ideal power source for portable electronics, since the need for battery replacement is avoided; the charger may be incorporated into the instrument giving facilities for mains or battery operation.

General

Most of the op amp circuits in this book are designed round the popular ‘industry standard’ 741 IC. Some applications might, however, require a different type of op amp, such as one with very low input bias current or high slew rate. This appendix gives the major electrical characteristics of the 741 and 748, together with those of three other op amps of different specifications.

The LM308 is basically similar to the 741, but with lower input bias current. The NE5534 offers a very high slew rate and low-noise audio performance, maintaining full-output up to 90 kHz, whereas the 741 has restricted output swing above 10 kHz. The TL081 is typical of IC op amps with a FET input. Input bias and offset currents are exceedingly low, and input resistance very high. Its high slew rate also makes it useful for high quality audio work, especially in its low noise version, TL071. Dual (072/082) and quad (074/ 084) versions are useful for compact layouts.

There have of course been many advances in electronics since the preparation of the first edition and this has been a welcome opportunity to bring the text up to date. The decade leap is reflected particularly in the two new chapters (13 and 14) on digital techniques and computers. Here I have aimed to present the important relationship of the microcomputer chip to other circuits, both digital and analogue, in a digestible form with plenty of experiments.

In the remainder of the book, many detailed changes have updated it without destroying the logical structure which has by now become familiar to students on many college and university courses. The approach throughout remains a practical one and is still based on my experience of teaching electronics in the Department of Pure and Applied Physics at UMIST. In recent years my work has been in industrial electronic design, further experience which has helped me to keep the text relevant to real applications.

In acknowledging gratefully the help I have received in preparing this second edition, I must first thank those readers of the first edition who wrote in with encouraging suggestions and amendments. Thank you too to my eldest son Christopher for his help in devising and testing the computer-based experiments. Alan Jubb was a great help in checking the manuscript of the new material.

Finally thank you again to my wife Sylvia not only for her typing skill but also for her encouragement and patience.

Introduction to feedback principles

The concept of negative feedback is fundamental to life. A simple experiment will illustrate this point: close your eyes and then bring your index fingers together so that they touch at the tips. You will probably miss. By closing your eyes you have broken a feedback loop which is vital to most human actions; in order to perform an operation accurately we must be able to see what we are doing and thus apply any small corrections as and when necessary. In effect, we are taking the output (the action) and feeding it back to the input (the mental ‘instruction’ or intention) in such a way that the output is made equal to the input. In other words, the action is forced to correspond exactly with the intention.

Examples of negative feedback can also be found in the field of mechanical engineering. One of the clearest examples is the governor which is used to control the speed of rotating machinery. The most spectacular form of governor used to be found on the old steam engines which were the prime source of motive power in the 19th century. The governor is shown in basic form in fig. 4.1 and consists of a vertical shaft geared up to the main flywheel shaft of the engine, carrying weights on a flexible linkage.

As the speed of the engine, and hence of the governor shaft, increases, centrifugal force causes the weights to fly outwards on their linkage.

Introduction

So far in our discussion of amplifiers, a vital component in the design has been the coupling capacitor which transmits the a.c. signals but removes the steady d.c. voltage present at the input and output of each stage. This is necessary in order to avoid one stage upsetting the operation of adjacent ones.

A two-stage capacitor-coupled amplifier is shown in fig. 8.1 together with quiescent d.c. voltages. It is clear that C2 is isolating the collector of T1 (which needs to sit at 4.5 V for correct operation) from the base of T2, which is only 0.6 V above the grounded emitter, being a forward-biased junction. Making a direct connection between stages, omitting C2, would have the unfortunate result of clamping the collector of T1 only about 0.6 V above 0 V and passing a 2 mA base current into T2 through T1 collector load, permanently bottoming T2. The design would not be a success!

Coupling capacitors can, however, be eliminated by special d.c. amplifier design which is employed in virtually all present-day circuitry. There are two main reasons for this. The first, very practical, reason is that large capacitors cannot be fabricated on ICs, the maximum being a few tens of picofarads. The second reason is that the coupling capacitor inevitably leads to attenuation and phase shift at low frequencies: after all, there is no clear distinction between low-frequency a.c. and slowly changing d.c. and it is impossible to provide isolation from the latter without affecting the former.

The popularity of the first and second editions has been very gratifying. This third edition uses the same practical approach to provide an update relevant to today's exciting electronics scene whilst retaining the character of the basic text.

It is a sign of the relentless advance of electronics that key applications today such as the CD player, cellular telephone and fax machine were nowhere on the scene at the original publication date of 1977. The second edition in 1985 then recognised the burgeoning microcomputer industry, though some home computers popular at that time have now disappeared without trace.

Today, a decade later, the industry has seen remarkable progress, especially in computer processing speed, memory capacity and component packing density. Surface-mount technology is the norm and more and more signal processing and recording is carried out digitally. Fortunately for the student and the experimenter, the basic component and circuit elements explained in the book continue to be just as relevant as ever. Components are still widely manufactured in the wire-ended packages best suited to experimental work; the favourite BC107 transistor remains commonplace. In those circuits where components have become obsolete, suitable updates have been introduced.

In some areas, developing technology has changed the relative importance of particular topics. For example switch-mode power supplies are now deserving of greater coverage, as is the whole theme of analogue to digital conversion. Some other topics no longer as relevant have been quietly dropped.

Computer operations

The word computer is one we naturally associate with highly complicated calculations carried out at a speed immensely faster than human thought. Indeed, the speed of today's ‘supercomputer’ is limited only by the velocity of light itself, which ultimately determines how fast the binary logic signals can travel round the circuits. In this chapter, we shall see that a computer is much more than a fast calculator. In addition to performing arithmetic, most computer operations are dominated by the rather dull but vital business of shifting, sorting and matching data. Whether the end result is the diagnosis of a patient's medical condition, a foreign language translation or even a video game missile shooting across the screen, most of the impressive results achieved in this computer age are achieved simply by moving numbers around as required and comparing them with one another.

Although actual arithmetic is a much less dominant function in most computers than is popularly believed, the arithmetic and logic unit (ALU) lies at the centre of every computer. This is an arrangement of gates capable of carrying out the logical functions discussed in the last chapter together with elementary arithmetic. It is appropriate first therefore to look further at electronic arithmetic circuits.

Electronic arithmetic

Addition

In chapter 13, simple combinational logic experiments led quickly to the addition of two binary numbers in the full adder (section 13.8).