Actually, what determines if the transistor acts as an
Figure 7-10 shows a circuit designed as a switch.
amplifier or switch is the controlling circuit.
To make this circuit realistic, the buzzer is located a
great distance from the switch. Since the switch
Figure 7-9 shows a circuit constructed by using an
(unlike our microphone) produces no power, a battery
NPN transistor to amplify. The electrical control is
has been added to the control circuit. Because of the
provided by a microphone; a device that produces
distance to the buzzer and the resistance in the
fluctuating electrical current corresponding to
connecting wires, the battery power in the controlling
fluctuating sound waves. The microphone can produce
circuit is not great enough to activate the buzzer. But
only a small amount of power, and if hooked directly to
the surviving power does provide enough energy to
the loudspeaker, you probably wouldn't hear a sound.
operate a transistor.
But with the simple circuit you see in this figure, the
power is amplified at least 100 fold.
When the switch is closed, a small current of
electrons is withdrawn from the transistor base, a
In the example, the microphone produces a
much greater current flows in the working circuit, and
maximum of 0.5 milliwatt of power (1 milliwatt =
the buzzer sounds. This transistor is acting as a switch
1/1000 watt). The power in the main circuit produced
in the working circuit.
by the battery, however, ranges from 0 to 500
milliwatts.
Transistor Testing.--There are several different
ways of testing transistors. They can be tested while in
Suppose you create a sound wave that hits the
the circuit with a transistor tester or an ohmmeter.
microphone and produces an output of 3 milliwatts.
The microphone pumps a surge of electrons from a
Transistor testers are nothing more than the
lower voltage (the base) to a higher voltage (the
s o l i d - s t a t e e q u iva l e n t o f e l e c t r o n - t u b e t e s t e r s
emitter). As a result of the base current, a relatively
(although they do not operate on the same principle).
large current then flows across the base region from the
With most transistor testers, it is possible to test the
emitter to the collector, down the line and through the
transistor in or out of the circuit.
coil of the speaker. In this way the current flow through
Because it is impractical to cover all of the
the speaker is controlled (or amplified) in exact
different types of transistor testers and since each
proportion to the smaller microphone signal. The
tester comes with its own operator's manual, we will
signal through the speaker might be 300 milliwatts.
move on to something you use more frequently for
This means that the 3 milliwatts produced by the
testing transistors--the ohmmeter.
microphone has been amplified 100 times.
There are four basic tests required for transistors in
Consider a microphone with a 2-milliwatt sound
practical troubleshooting: gain, leakage, breakdown,
wave. Fewer electrons flow in the control circuit, so
and switching time. For maintenance and repair,
fewer are drawn from the base region; the speaker
however, it is usually not necessary to check all of
output is only 200 milliwatts. Notice that the output is
these parameters. A check of two or three parameters
still amplified 100 times. At all times, almost precise
is usually sufficient to determine whether a transistor
proportions are maintained between the control circuit
needs to be replaced. Two of the most important
and the working circuit. In other words, the power in
parameters used for testing are gain and leakage.
the working circuit is always an essential duplicate of
A basic transistor gain test can be made by using
the power in the control circuit, but greatly amplified.
an ohmmeter and a simple test circuit. The test circuit
can be made with just a couple of resistors and a
MICROPHONE
switch, as shown in figure 7-11. The principle behind
(0-5 mW)
(0-500 mW)
_
+
ELECTRON
_
ELECTRON
FLOW
FLOW
ELECTRON
FLOW
+
ELECTRON
CONTROL
FLOW
CIRCUIT
BUZZ
SWITCH
WORKING
CIRCUIT
ASf07009
ASf07010
Figure 7-9.--Basic amplifying circuit.
Figure 7-10.--Simple circuit using a transistor as a switch.
7-5
