🚀 go-pugleaf

On 1998-12-27 ajh72@yahoo.com said:
 aj-  Newsgroups: sci.physics,fj.sci.physics,sci.physics.electromag,tnn.
 aj-  physics,news.electronics.basics,sci.electronics.basics,sci.ele
 aj-  Could anybody out there tell me how valves and transistors work,
 aj-  please? I would be eternally grateful.
 aj-  Adrian, UK

VALVES

Metals are made up of atoms; and atoms have electrons orbiting round a +
charged nucleus.

Warm the metal to a dull red temperature, and some of the electrons
acquire enough energy to escape the attraction of that positive nucleus.

However, that leaves the atom with an overall LACK of electrons. A lack
of - charges is what we call a + charge; and that + charge attracts them
back into the metal.

BUT if you place a cold metal plate (the anode) nearby and give it a +
charge, it will attract the free electrons.

If you connect a battery of say 20 or more volts, with its - to the hot
metal (the "cathode") and its + to the anode, you get a steady flow of
electrons from cathode to anode.

You cannot get electrons to flow the other way, so this device is useful
for allowing one-way-only flow. If you apply AC, current will only flow
during the half cycle when the anode is +. This means you have turned
the AC into a crude form of DC. This is very useful and is called
"rectifying".

Next, you place a "grid" between the anode and the cathode. If you
connect another voltage, say 3 volts, + to the cathode and - to the
grid, the grid repels the electrons and reduces the current to the
anode.

Because the grid is much closer to the cathode than the anode is, it
takes only a volt or two change on the grid, to change the anode current
as much as tens of volts change on the anode.

Over-simplifying a bit, if you connect a microphone between grid and
cathode, and a loudspeaker in series with the anode, the small voltage
change produced by the microphone causes a substantial anode current
change; and that current flowing through the loudspeaker creates a
changing voltage which is much bigger than the one from the microphone.

This is called "amplification".

If you let the sound from the speaker get back into the microphone, it
will settle down to producing a wailing note. This is called feedback;
"positive" feedback because the change going "in" at the grid increases
and adds to the change at the anode.

You can get the same result by taking a fraction of the signal from the
anode and sending it back to the grid, WITHOUT loudspeaker or
microphone. Then we call the system an "oscillator".

It is easy to decide the frequency of the oscillation. If it is above 16
kHz or so, it can be radiated into space and received some distance
away. In that case you call it a transmitter.

TRANSISTORS

I don't really understand HOW, but the device has 3 terminals called
emitter, base and collector.

Like the valve, it carries current easily one way and with great
resistance the opposite way.

If the base and emitter are connected the "difficult" way round, a very
small change in base current causes a much larger change in the current
between emitter and collector. 50 times bigger, or 500 times; even
10,000 times greater is not unknown.

So a microphone, which produces small voltage changes, is connected
between base and emitter, causing small changes in Ibe (I for current, b
and e for base and emitter).

If a loudspeaker is connected in series with a battery, and the emitter
and collector terminals, MUCH larger current changes will flow through
it, so the sound will be amplified.

I found a cartoon film many years ago, which showed HOW the small change
in base current can cause a much larger change in Ice. I thought I
understood it at the time but for most purposes I just accept that it
happens!

The transistor construction is like a sandwich: the base is usually what
we call p-type semiconductor; and the emitter and collector are n-type.

A p-type semiconductor carries current by the movement of "holes", which
are spaces in a crystal lattice where there ought to be an electron.
This stuff is made by melting silicon (which has 4 valence electrons)
and adding about 1 part in 10 million of a different element, which has
only 3 valence electrons. Thus a "vacant site" is produced; and it can
move around in the same way as an empty seat in a crowded cinema can
move around.

An n-type semiconductor has an impurity which has 5 valence electrons,
so the crystal lattice has spare electrons like someone looking for
somewhere to sit in a full cinema.

To make a good transistor one tries to get the collector and the emitter
to reach into the base as far as possible without actually touching.

Transistors can equally well be made with n-type sandwiched between
p-type; such a transistor is called "pnp".

There are great similarities between valve and transistor circuits but
one has to keep in mind the fact that a valve basically uses a VOLTAGE
to control a current; whereas a transistor uses one current to control
another one. The grid of a valve can in theory take no current at all,
so it is referred to as "high impedance". The base of a transistor can
take quite a lot of current so it is regarded as "low impedance".

If I have used any words which are unfamiliar, or said something which
does not seem to make sense, please ask. I am used to having to correct
my mistakes!

References: Basic Principles of Electronics (Vol 1 Valves 1965, and Vol
2 Semiconductors 1971) by Jenkins & Jarvis, pub. Pergamon; Library of
Congress Catalog Card Number 66-23849.
 also       A second course of electricity by A E E McKenzie, 3rd
            edition, Cambridge University Press, 1973;
            ISBN 0 521 08628 (earlier editions don't mention
            transistors)

All 3 books describe experiments to help understand how valves and
transistors are used.



Bill J
Edinburgh, UK

                       Sic friatur crustum dulce

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