Page 88 - Understanding Automotive Electronics

2735 | CH 3 Page 75 Tuesday, March 10, 1998 11:03 AM

ELECTRONICS FUNDAMENTALS 3

PNP transistor and NPN transistor. Their functions as amplifiers and switches
are the same.
During normal opera- The base-emitter junction of a transistor acts like a diode. Under normal
tion, current flows from operation for an NPN transistor, current flows forward into the base and out
the base to the emitter in the emitter, but does not flow in the reverse direction from emitter to base. The
an NPN transistor. The arrow on the emitter of the transistor schematic symbol indicates the forward
collector-base junction is direction of current flow. The collector-base junction also acts as a diode, but
reverse biased, so that supply voltage is always applied to it in the reverse direction. This junction does
–6
only a very small have some reverse current flow, but it is so small (1 × 10 to 1 × 10 –12 amperes)
amount of current flows that it is ignored except when operated under extreme conditions, particularly
between the collector temperature extremes. In some automotive applications, the extreme
and the base when there temperatures may significantly affect transistor operation. For such applications,
is no base current flow. the circuit may include components that automatically compensate for changes
in transistor operation.
Under normal linear (analog) circuit operation, the collector-base junction
is reverse biased as mentioned previously; however, when used as a switch in the
on condition, the collector-base junction can become forward biased. In normal
operation, the current that flows through the collector-emitter terminals is
controlled by the current flowing through the base terminal. Relatively small
base currents control relatively large collector currents through a complicated
physical process that is beyond the scope of this book. However, for the purposes
of the present discussion, it is correct to think of the transistor as a current-
amplifying device in which the base current controls the collector current. The
current amplification of a typical transistor is on the order of 100. That is, a base
current controls a collector current that is about 100 times larger.

Transistor Model
To aid in circuit analysis, Figure 3.2c shows the diagram of a commonly
used transistor model for an NPN transistor. The base-emitter diode is shown
in solid lines in the circuit, while the collector-base diode is shown in dotted
lines because generally it can be ignored.
The base-emitter diode does not conduct (there is no transistor base
current) until the voltage across it exceeds V volts in the forward direction. If
d
the transistor is a silicon transistor, V equals 0.7 volt just as with the silicon
d
diode. The collector current I is zero until the base-emitter voltage V exceeds
BE
c
0.7 volt. This is called the cutoff condition, or the off condition when the
transistor is used as a switch.
When V rises above 0.7 volt, the diode conducts and allows some base
BE
current I to flow. The collector current I is equal to the base current I times
c
b
b
the transistor current gain h . The current gain (h ) can range from 10 to 200
fe
fe
depending on the transistor type. It is represented by a current generator in the
collector circuit of the model. This condition is called the active region because
the transistor is on and amplifying. It also is called the linear region because
collector current is (approximately) linearly proportional to base current. The
UNDERSTANDING AUTOMOTIVE ELECTRONICS 75