Valve (Tube-Based) Amplifi ers 349taken in the manufacture of valves to maintain the fi rmness of the mounting of the grids
and other electrodes. This is done by the use of rigid supporting struts whose ends are
located in holes punched in stiff mica disc-shaped spacers, which, in turn, are a tight fi t
within the valve envelope.
Since a microphonic valve will pick up vibration from any sound source, such as a
loudspeaker system in proximity to it, and convert these sounds into (inevitably distorted)
electrical signals, which will be added to the amplifi er output, this can be a signifi cant,
but unsuspected, source of signal distortion, which will not be revealed during laboratory
testing on a resistive dummy load. Because it is diffi cult to avoid valve microphony
completely, and it is equally diffi cult to sound proof amplifi ers, this type of distortion will
always occur unless such valve amplifi er systems are operated at a low volume level or
the amplifi er is located in a room remote from the loudspeakers.
11.2 Solid-State Devices ...............................................................................................
11.2.1 Bipolar Junction Transistors
11.2.1.1 ‘ N ’ - and ‘ P ’ -Type Materials
Most materials can be grouped in one or other of three classes, insulators,
semiconductors, or conductors, depending on the ease or diffi culty with which electrons
can pass through them. In insulators, all of the electrons associated with the atomic
structure will be fi rmly bound in the valency bands of the material, whereas in good,
usually metallic, conductors many of the atomic electrons will only be loosely bound and
will be free to move within the body of the material.
In semiconductors, at temperatures above absolute zero (0°K or –273.15°C), electrons
will exist both in the valency levels where they are not free to leave the atoms with which
they are associated and in the conduction band in which they are free to travel within
the body of the material. This characteristic is infl uenced greatly by the “ doping ” of the
material, which is normally done during the manufacture of the semiconductor material
by introducing carefully controlled amounts of specifi c impurities into the molten mass
from which the single semiconductor crystal is grown. The most common semiconductor
material in normal use is silicon because it is inexpensive, readily available, and has
good thermal properties. Germanium, the material from which all early transistors were
made, has electrical characteristics that are infl uenced greatly by its temperature, which