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236 Engineering Fundamentals
as amplifi ers. Amplifi ers increase the power of a
signal, most commonly an audio signal. A small
amount of current applied to the base can create
a gain in collector/emitter current. Figure 11-19
shows a PNP and an NPN transistor.
Current cannot fl ow in reverse bias. See diodes
being used in a bridge rectifi er in Figure 11-18.
Bridge rectifi ers change ac to dc.
Zener Diodes
Zener diodes conduct electricity in reverse
bias. They block current until the voltage reaches
a certain level. Once this level is reached, zener
diodes conduct and help to keep the voltage at a
constant level. Because of this characteristic, they
are often used as voltage regulators, which help
smooth out variations in voltage.
Transistors
Transistors can be used as solid-state
switches and as amplifi ers. They are called solid-
state switches because they perform a switching
function with no moving parts. Bipolar transis-
tors have three junction points: emitter, base,
and collector. Current fl owing between the emit-
ter and collector can be controlled by a current
delivered to the base. Transistors can also be used
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Figure 11-18. 
Schematic of a bridge rectifier, which converts ac to dc.
Enlarged View of
NPN Transistor Construction
Collector
P-type
base
c
c
b
b
e
e
Emitter
Substrate
N-type
N-type
Enlarged View of
PNP Transistor Construction
Collector
N-type base
c
b
e
Emitter
Substrate
P-type
P-type
c
b
e
A
B
Figure 11-19. 
Bipolar transistor
construction and
schematic symbols are
shown. The three junction
points are the emitter (e),
the base (b),and the
collector (c). A—An NPN
transistor. B—A PNP
transistor.
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