Tab Electronics Guide to Understanding Electricity ... - Sciences Club

230 Chapter Eight
Although the field of linear electronics is very diversified, the basics
of linear action are common to almost all of its facets. For example, the
same techniques used to linearize (i.e., increase proportional accuracy in)
an audio amplifier are used to linearize servo systems and operational
amplifiers. An accurate understanding of the basic building blocks utilized
in linear systems will aid you in understanding a great variety of
electronic systems.
Because of a variety of factors, “discrete” (i.e., nonintegrated) circuitry
is still utilized in a significant portion of the field of audio electronics.
Therefore, this chapter focuses primarily on audio amplification circuits,
since they provide a good beginning point to study the fundamentals
of linear circuitry. In addition, the associative discussions make it a convenient
point to detail printed circuit board construction (in conjunction
with a few more “advanced” projects) and the newer
computer-automated (also -aided or -assisted) design (CAD) techniques for
designing and constructing PC boards.
Transistor Biasing and Load
Considerations
The circuit illustrated in Fig. 8-1 should already be familiar to you from
the previous discussions of transistor amplifiers. It is a common-emitter
amplifier because the output (to the speaker) is taken off from the collector,
and the input signal to be amplified is coupled to the base. C1 is a
coupling capacitor (blocking the DC bias voltage, but passing the AC
audio signal). R1 and R2 form a voltage-divider network to apply the
proper DC bias to the base. The emitter resistor (RE) increases the input
impedance, and it improves temperature and voltage stability.
Transformer T1 is an audio transformer. It serves two important functions
in this circuit. First, it isolates the DC quiescent (steady-state) current
flow from the speaker coil (speaker coils can be damaged by even
relatively small DC currents). Secondly, it provides a more appropriate
load impedance for a transistor collector than would a low-impedance
8-ohm speaker. A transistor amplifier of this configuration could not
operate very well with an extremely low collector impedance. A typical
audio transformer might have a primary impedance of 100 ohms, for
connection into the transistor circuit, and a secondary impedance of 8
ohms for connection to the speaker. Generally speaking, an audio ampli-