Audio Amplifi ers
John Linsley HoodSolid-state device technologies, which are available to the amplifi er designer, fall, broadly,
into three categories: bipolar junction transistors (BJTs) and junction diodes; junction fi eld
effect transistors (FETs); and insulated gate FETs, usually referred to as MOSFETs (metal
oxide silicon FETs), because of their method of construction. These devices are available
in both P type—operating from a negative supply line—and N type—operating from a
positive supply line. BJTs and MOSFETs are also available in small-signal and larger
power versions, whereas FETs and MOSFETs are manufactured in both enhancement-
mode and depletion-mode forms. Predictably, this allows the contemporary circuit designer
very considerable scope for circuit innovation, by comparison with electronic engineers of
the past, for whom there was only a very limited range of vacuum tube devices.
In addition, there is a very wide range of integrated circuits (ICs), which are complete
functional modules in some (usually quite small) individual packages. These are designed
both for general-purpose use, such as operational amplifi ers, and for more specifi c
applications, such as voltage regulator devices, current mirrors, current sources,
phase-sensitive rectifi ers, and an enormous variety of designs for digital applications,
which mostly lie outside the scope of this book.
In the case of discrete devices, I think it is unnecessary for the purposes of audio
amplifi er design to understand the physical mechanisms by which the devices work,
provided that their would-be user has a reasonable grasp of their operating characteristics
and limitations and, above all, a knowledge of just what is available.
9.1 Junction Transistors ..............................................................................................
These are nearly always three-layer devices, fabricated by the multiple and simultaneous
vapor phase diffusion and etching of small and intricate patterns on a large, thin slice of
CHAPTER 9