27-12-2012, 06:23 PM
The Circuit Designer’s Companion
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Introduction
Electronic circuit design can be divided into two areas: the first consists in designing a
circuit that will fulfil its specified function, sometimes, under laboratory conditions; the
second consists in designing the same circuit so that every production model of it will
fulfil its specified function, and no other undesired and unspecified function, always, in
the field, reliably over its lifetime. When related to circuit design skills, these two areas
coincide remarkably well with what engineers are taught at college
that there is no such thing as the ideal component, that printed circuits
are more than just a collection of tracks, and that electrons have an unfortunate habit of
never doing exactly what they’re told.
This book has been written with the intention of bringing together and tying up
some of the loose ends of analogue and digital circuit design, those parts that are never
mentioned in the textbooks and rarely admitted elsewhere. In other words, it relates to
the second of the above areas.
Its genesis came with the growing frustration experienced as a senior design
engineer, attempting to recruit people for junior engineer positions in companies whose
foundations rested on analogue design excellence. Increasingly, it became clear that the
people I and my colleagues were interviewing had only the sketchiest of training in
electronic circuit design, despite offering apparently sound degree-level academic
qualifications. Many of them were more than capable of hooking together a
microprocessor and a few large-scale functional block peripherals, but were floored by
simple questions such as the nature of the p-n junction or how to go about resistor
tolerancing. It seems that this experience is by no means uncommon in other parts of
the industry.
The colleges and universities can hardly be blamed for putting the emphasis in their
courses on the skills needed to cope with digital electronics, which is after all becoming
more and more pervasive. If they are failing industry, then surely it is industry’s job to
tell them and to help put matters right. Unfortunately it is not so easy. A 1989 report
from Imperial College, London, found that few students were attracted to analogue
design, citing inadequate teaching and textbooks as well as the subject being found
"more difficult". Also, teaching institutions are under continuous pressure to broaden
their curriculum, to produce more "well-rounded" engineers, and this has to be at the
expense of greater in-depth coverage of the fundamental disciplines.
Nevertheless, the real world is obstinately analogue and will remain so. There is a
disturbing tendency to treat analogue and digital design as two entirely separate
disciplines, which does not result in good training for either. Digital circuits are in
reality only over-driven analogue ones, and anybody who has a good understanding of
analogue principles is well placed to analyse the more obscure behaviour of logic
devices. Even apparently simple digital circuits need some grasp of their analogue
interactions to be designed properly, as chapter 6 of this book shows. But also, any
The Circuit Designer’s Companion
product which interacts with the outside world via typical transducers must contain at
least some analogue circuits for signal conditioning and the supply of power. Indeed,
some products are still best realised as all-analogue circuits. Jim Williams, a wellknown
American linear circuit designer (who bears no relation to this author), put it
succinctly when he said “wonderful things are going on in the forgotten land between
ONE and ZERO. This is Real Electronics”.
Because analogue design appears to be getting less popular, those people who do
have such skills will become more sought-after in the years ahead. This book is meant
to be a tool for any aspiring designer who wishes to develop these skills. It assumes at
least a background in electronics design; you will not find in here more than a minimum
of basic circuit theory. Neither will you find recipes for standard circuits, as there are
many other excellent books which cover those areas. Instead, there is a serious
treatment of those topics which are “more difficult” than building-block electronics:
grounding, temperature effects, EMC, component sourcing and characteristics, the
imperfections of devices, and how to design so that someone else can make the product.
I hope the book will be as useful to the experienced designer who wishes to broaden
his or her background as it will to the neophyte fresh from college who faces a first job
in industry with trepidation and excitement. The traditional way of gaining experience
is to learn on-the-job through peer contact, and this book is meant to enhance rather than
supplant that route. It is offered to those who want their circuits to stand a greater
chance of working first time every time, and a lesser chance of being completely redesigned
after six months. It does not claim to be conclusive or complete. Electronic
design, analogue or digital, remains a personal art, and all designers have their own
favourite tricks and their own dislikes. Rather, it aims to stimulate and encourage the
quest for excellence in circuit design.
I must here acknowledge a debt to the many colleagues over the years who have
helped me towards an understanding of circuit design, and who have contributed
towards this book, some without knowing it: in particular Tim Price, Bruce Piggott and
Trevor Forrest. Also to Joyce, who has patiently endured the many brainstorms that the
writing of it produced in her partner.
Introduction to the second edition
The first edition was written in 1990 and eventually, after a good long run, went out of
print. But the demand for it has remained. There followed a period of false starts and
much pestering, and finally the author was persuaded to pass through the book once
more to produce this second edition. The aim remains the same but technology has
progressed in the intervening fourteen years, and so a number of anachronisms have
been corrected and some sections have been expanded. I am grateful to those who have
made suggestions for this updating, especially John Knapp and Martin O’Hara, and I
hope it continues to give the same level of help that the first edition evidently achieved.
Grounding
A fundamental property of any electronic or electrical circuit is that the voltages present
within it are referenced to a common point, conventionally called the ground. (This
term is derived from electrical engineering practice, when the reference point is often
taken to a copper spike literally driven into the ground.) This point may also be a
connection point for the power to the circuit, and it is then called the 0V (nought-volt)
rail, and ground and 0V are frequently (and confusingly) synonymous. Then, when we
talk about a five-volt supply or a minus-twelve-volt supply or a two-and-a-half-volt
reference, each of these is referred to the 0V rail.
At the same time, ground is
the same as 0V. A ground wire connects equipment
to earth for safety reasons, and does not carry a current in normal operation. However,
in this chapter the word “grounding” will be used in its usual sense, to include both
safety earths and signal and power return paths.
Perhaps the greatest single cause of problems in electronic circuits is that 0V and
ground are taken for granted. The fact is that in a working circuit there can only ever be
one point which is truly at 0V; the concept of a “0V rail” is in fact a contradiction in
terms. This is because any practical conductor has a finite non-zero resistance and
inductance, and Ohm’s Law tells us that a current flowing through anything other than
a zero impedance will develop a voltage across it. A working circuit will have current
flowing through those conductors that are designated as the 0V rail and therefore, if any
one point of the rail is actually at 0V (say, the power supply connection) the rest of the
rail will
not
be at 0V.