Audio Engineering

350 Chapter 11

is inconvenient in use. Also, it does not lend itself at all well to contemporary mass-
production techniques.

In the case of silicon, which has very little conductivity in its undoped “ intrinsic ” form,
the most common dopants are boron or aluminium, which give rise to a semiconductor
with a defi ciency of valency electrons, usually referred to as holes—called a ‘ P ’ -type
material—or phosphorus, which will cause the silicon to have a surplus of valency
electrons, which forces some of them into the conduction band. Such a semiconductor
material would be termed ‘ N ’ type. Both P-type and N-type silicon can be quite highly
conductive, depending on the doping levels used.

11.2.1.2 Fermi Levels

The electron energy distribution in single-crystal P- and N-type materials is shown in
Figure 11.6 , and the mean electron energy levels, known as the Fermi levels, are shown.

11.3 Valve Audio Amplifi er Layouts ............................................................................

In its simplest form, shown in Figure 11.6 , an audio amplifi er consists of an input voltage
amplifi er stage (A) whose gain can be varied to provide the desired output signal level, an
impedance converter stage (ZC) to adjust the output impedance of the amplifi er to suit the
load, which could be a loudspeaker, a pair of headphones, or the cutting head in a vinyl
disc manufacturing machine.

In the case of headphones, their load impedance could be high enough for them to be
driven directly by the voltage amplifi er stage without a serious impedance mismatch, but
with other types of output load it will be necessary to interpose some sort of impedance
conversion device; in valve-operated audio systems this is most commonly an iron-cored

Input

Output

LS

A ZC

Figure 11.6 : An audio amplifi er block diagram.