Table Of ContentBasic
Principles of Electronics
Volume I : Thermionics
BY
J. JENKINS
Senior Physics Master, Gordonstoun School
AND
W. H. JARVIS
Formerly Physics Master,
Gordonstoun School
PERGAMON PRESS
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Copyright © 1966 Pergamon Press Ltd.
First edition 1966
Library of Congress Catalog Card No. 66-23849
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To
F. R. Y. and J. M. 0.
Preface
THIS first volume, which is almost confined to thermionic devices,
has been written to cover the Associated Examining Board's
Electronics endorsement to A-level physics, and has been ex-
tended in scope to make it of value to national certificate students.
As such it more than covers all the electronics required for A-
and S-level physics courses. A subsequent volume will cover
semiconductors more thoroughly and will include several elec-
tronic projects.
A reasonable knowledge of the methods of calculus and of
physics at school level is assumed. Although formulae have been
quoted in the rationalized M.K.S. system of units, their counter-
parts in other systems will be immediately obvious.
The Authors must acknowledge that the treatment owes
much to their respective schools and universities; they further
acknowledge that they have been influenced by the books they
have read, and some of these are listed in the bibliography in
Appendix 5. They wish to thank Messrs. Mullard, Advance,
Rainbow Radio (Unilab), and Philip Harris Ltd., for their
invaluable help with the loan of educational apparatus relevant
to this text. They gratefully acknowledge the permission to
reprint questions from past papers of the following boards:
The Associated Examining Board (A.E.B.), University of
Cambridge Local Examinations Syndicate (C), Universities of
Manchester, Liverpool, Leeds, Sheffield and Birmingham Joint
Matriculation Board (N.), University of London (L.), Oxford
Local Examinations (O.), Oxford and Cambridge Schools Exami-
nation Board (O. and C), Southern Universities' Joint Board (S.),
City and Guilds of London Institute (C.G.L.I.), Union of
xiii
XIV PREFACE
Lancashire and Cheshire Institutes (U.L.C.I.), the Institution of
Electrical Engineers (I.E.E.), and the Institute of Physics (I. of P.).
They also gratefully acknowledge the permission of Messrs.
L. T. Agger, M. D. Armitage, E. J. Chambers, and S. Parker-
Smith, to reproduce questions from their textbooks.
The Authors are most grateful to Messrs. G. C. Dyer and
D. H. Williams for providing the answers to many of the problems,
and to Miss Pullen for most of the typing.
Finally, they apologize in advance for any errors which have
escaped correction, and they would be grateful to those who
detect them if they would inform the publishers, to whom the
Authors are very grateful for their help and patience during the
preparation of the draft.
Gordonstoun J. J.
June 1965 W. H. J.
CHAPTER I
Physical Background
1.1. "Electronics" defined
For the purposes of this book we shall define "electronics" as
that branch of science which deals with the conduction of
electricity in vacuum, gas or semiconductors; and the uses of
devices based on these phenomena. By "semiconductors" we
mean substances in which an electric current can flow under
suitable conditions ; we shall see that the manner in which it flows
is not so straightforward as in the case of conductors.
1.2. Structure of matter
The core, or nucleus, of the atom is of diameter about 10" 14m,
and is surrounded by electrons moving in orbits of diameter
about 10~10m. The nucleus carries a + charge equal to the
number of protons it contains, and usually the number of orbiting
electrons associated with it is equal to the number of protons.
Each electron carries a — charge, equal in magnitude but oppo-
site in sign to the + charge on the proton. So an isolated atom,
in its normal state, is electrically neutral.
We shall regard the charge on the electron as the fundamental
unit of charge, and the other charges will be represented as
multiples of it. We shall let — e represent the charge on the
electron.
The electrons are bound to the atom by the attraction of the
nucleus ; the strength of the bond varies over a wide range and is
strongly but not simply dependent on distance.
2 BASIC PRINCIPLES OF ELECTRONICS
1.3. Chemical combination
The number of protons in the nucleus of an atom is the only
factor which determines what the element is. Samples of the
same element, in which the atoms have varying numbers of
neutrons but the same number of protons, are called isotopes of
the same element.
The number of electrons orbiting the nucleus usually has to
be equal to the number of protons ; and the manner in which they
are arranged is subject to certain rigorous laws which can be
predicted theoretically by quantum mechanics. At this stage we
shall be content to see what some of those laws are.
We find that the electron orbits may be grouped together,
giving "shells" which can contain up to In2 electrons, where n
is an integer, not necessarily the number of the shell. All the
electrons in one shell have energies very close together. The
potential energy of an electron at rest an infinite distance from
the nucleus is taken arbitrarily as zero, and the energy of the
electrons in shells around the nucleus is compared to it. Since
the nucleus attracts electrons, work must be done to remove an
electron to infinity; so an electron in a shell near the nucleus is
regarded as having large negative energy, and one further away
has less negative energy.
The most drastic stipulation of quantum theory is that electrons
cannot possess any energy, but that they are restricted to certain
allowed amounts of energy, rather as air in a pipe is restricted to
vibrating in resonance only with certain notes. Furthermore, the
permitted energy levels are not absolutely definite, but cover
narrow bands. The width of the bands of allowed energy can be
affected by the proximity of other atoms, by heat, and by electric
and magnetic fields. This is further discussed in Chapter 3.
In any single isolated atom there are a certain number of
electrons which must be in orbit around its nucleus, each one
possessing energy within one of the allowed bands. Experiment
has shown that only two electrons can enter orbits in the allowed
energy band nearest the nucleus, regardless of the nature of the
PHYSICAL BACKGROUND 3
nucleus. (A theoretical explanation for this and other shell rules
has since been evolved.) We say that the innermost, or first, shell
is full when it contains 2 electrons. But the next shell can hold up
to 8 electrons (another case of 2n2); and so can the third shell.
So the second and third shells can only be termed "filled" if they
each hold 8 electrons. Figure 1.1 shows a carbon atom (atomic
number 6, i.e. 6 protons and electrons). The first shell is full and
the second holds 4, with room for 4 more.
+ = Proton
-θ^>^ = Vacancy for
electron in «hell
-θ-^ =' Vacancy'
occupied by electron
FIG. 1.1. Carbon atom
The successive electron shells are still referred to by their old
spectroscopic notations: the innermost is called the A^-shell, the
next the L-shell, and so on.
In chemical combination, each atom arranges itself along with
its neighbours in such a way that its outermost electron shell is
effectively full. This it may achieve either by collecting an
electron from another atom (ionic combination), or by sharing
electrons with other atoms (convalent combination). It is only
necessary for the outermost shell of a combining atom to be thus
filled; it is not uncommon for shells further in to be incomplete
even in a stable compound. Certain elements, called the "rare
gases", already have only full shells, and therefore show no desire
to enter into chemical combination with any other element.
They are referred to as "inert", but recently, under extreme
conditions, they have been known to react.
4 BASIC PRINCIPLES OF ELECTRONICS
1.4. Ionic combination
We shall not be concerned with compounds of this type, but
one example may be helpful. Ordinary table salt, sodium
chloride, consists of molecules which are formed by the combina-
tion of one chlorine atom with one sodium atom. The chlorine
atom has 17 electrons: 2 in the inner shell, 8 in the second, and 7
in the third. Since the third shell is full when it has 8 electrons,
CL nucleus Na nucleus
FIG. 1.2. Ionic compound
the chlorine atom attempts to combine so as to gain the extra
electron. However, the sodium atom has 11—2 in the first, 8 in
the second, and 1 in the third. If it had one less, the third shell
might just as well not have been started, and it would have just
two complete shells. In the sodium chloride molecule the eleventh
electron from the sodium atom moves into orbit around the
chlorine nucleus, so that both atoms have, effectively, nothing but
full electron shells. But now we have a sodium atom which has
lost a negative charge—that makes it a positive sodium ion—and
a chlorine atom which has gained a negative charge—the negative
chlorine ion. So the two ions are bound together by electrostatic
forces. This is illustrated in Fig. 1.2.
PHYSICAL BACKGROUND 5
1.5. Covalent combination
In this type of compound, atoms become bonded by "sharing"
electrons in such a way as to leave each atom with effectively full
outer electron shells. An example is the oxygen molecule, which
consists of two oxygen atoms. Each atom carries 8 electrons,
These 4 electrons
orbit both nuclei
FIG. 1.3. Covalent bond
there being 2 in the first shell and 6 in the second. Each atom
gives up two electrons, these 4 then orbiting both nuclei so that
both have effectively full electron shells. The situation is sketched
in Fig. 1.3.
1.6. Crystalline structure
The number of electrons in the outermost shell of an element
determines what is called its "valency". This is a number which
states either how many electrons are required to complete the
outer shell, or how many electrons would have to be given up to
leave the previous shell filled. In the foregoing examples, chlorine
has a valency of 1 since it needs 1 electron to leave it with the
outer shell filled. Sodium has a valency of 1 since it needs to give
up 1 electron to achieve filled-shell status. But oxygen has a
valency of 2, since each atom contributes 2 electrons in the sharing
