Audio Engineering

128 Chapter 4

4.1.4 Transistors

Transistors are totally different from resistors, capacitors, and diodes. The latter are what
are termed passive components, performing a singular function as we’ve seen, useful
certainly, but not active in the electronic sense. A transistor is a truly active device. It can
take a signal and amplify it. A number of support components are needed to make the
transistor into a working amplifi er—you guessed it, using a few resistors and capacitors
again. Depending on the designer’s talent, transistors can be confi gured into an endless
string of circuits, amplifi ers, oscillators, fi lters, alarms, receivers, transmitters, and so on.
The versatility of transistors knows no bounds.

Although I do not include transistor-based circuits in this book—the reason being that
integrated circuit projects are so much more well behaved and therefore simpler to
design—I do provide a brief overview on transistors, as integrated circuits are really just
a huge collection of transistor-based circuits. Transistors are three-terminal devices; the
terminals are known as the emitter, the base, and the collector. Figure 4.7 shows transistor
details. Transistors come in two “ fl avors ” so to speak: the more common NPN type
operates with a positive supply voltage, and hence, it is very compatible with integrated
circuits, which almost always run on a positive supply. The less common transistor type
is the PNP device, which, as you might have guessed, requires a negative supply voltage
(not so commonly found in circuits).

Transistors are defi ned as active devices because they have the capability, given the
appropriate support components, to perform useful functions; the most common of these
is amplifi cation, but the other is oscillation. A simple, common emitter amplifi er can

Collector Collector

NPN

transistor

PNP

transistor

Emitter Emitter

Base Base





Figure 4.7 : Transistor terminals.