Syscomp CTR-201, Руководство пользователя

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3.5 NPN Transistor (BJT)
The control settings for an NPN Power Transistor (2N3055), and the results of that measurement, are shown in
figure 6.
(a) Control Settings (b) Result
Figure 6: NPN Transistor
Notes
•
The collector current of a BJT is more-or-less constant with increasing collector-emitter voltage. That
is, it behaves as a current source (or, more accurately, a current sink in the case of an NPN transistor.)
Consequently, it is best to test this characteristic by setting a base current, sweeping the collector-emitter
voltage and measuring the collector current. The base terminal is supplied with a current that increases
stepwise with each sweep.
•
The measurement can generate considerable power in the transistor. For example, at a test collector-emitter
voltage of 30 volts and collector current of 1 ampere, the transistor dissipation is 30 watts. A small signal
transistor might have a maximum power dissipation of 0.5 watts. Consequently, it’s very easy to destroy a
transistor by exceeding the power dissipation. To prevent this:
– While learning how to operate the curve tracer, use a power transistor as the device. The 2N3055 is a
good choice, since it is inexpensive, readily available, has a large semiconductor chip and is mounted
in a metal TO-3 housing.
– Do a rough calculation of the maximum expected current in the transistor. The collector current is
controlled by the base current times the current gain (DC beta
β
, or
h
F E
). A typical value for
β
is 50. Consequently, a maximum base current of 1mA will cause a maximum collector current of
50mA. With a maximum collector-emitter voltage of 5 volts, the maximum dissipation is 250mW. For
example, a 2N4401 in a TO-92 case can dissipate around 500mW at ambient air temperature of 40
◦ C,
so this measurement should be safe on a 2N4401.
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