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One way of understanding the basic
operation of a bipolar transistor is by looking at its
Vce-Ic curves,
i.e., the plots of its collector current Ic versus the voltage Vce
across its collector and emitter for different values of base current Ib.
These curves are derived from a transistor in common-emitter
configuration, and basically describe the transistor's
common-emitter output characteristics.
A transistor
circuit is said to be in common-emitter configuration if the emitter is
the terminal common to both the input and the output. In the
analysis of transistor circuits, the input current and output voltage
are usually considered the independent variables. Thus, for
the common-emitter configuration, the independent variables are the
input current Ib and the output voltage Vce, while the dependent
variables are the input voltage Vbe and
the output current Ic. The
family of input characteristic curves may therefore be described by the function
f1 wherein Vbe = f1(Vce, Ib), while the family of output curves may be
described by the function f2, wherein Ic = f2(Vce, Ib).
The output
curves corresponding to f2 are drawn with the collector-to-emitter
voltage Vce as the abscissa, and the collector current Ic as the
ordinate. Different output curves are generated for different
values of base current Ib, which are all drawn on the same plot.
Figure 1 shows an example of a
common-emitter transistor circuit's Vce-Ic curves for different values
of Ib.
Figure 1.
The Vce-Ic Curves of an NPN transistor for different
values of Ib
(Common-emitter Collector Characteristics)
These common-emitter Vce-Ic curves are useful in
choosing the optimum operating point of a transistor used as an
amplifier. A transistor used as an amplifier must be operated in
the active region, wherein the change in the collector current (output)
is proportional to the change in the base current (input). This
linear region is ideal for amplification use because it allows the
output waveform to be an enlarged 'faithful' copy of the input waveform.
The active or linear region in the Vce-Ic curve is its flat portion on
the right side. Note that the change in collector current Ic is
most sensitive to the change in base current Ib in this region.
The slope of
the curve in this operating region represents the reciprocal of the
transistor's collector or output resistance Rc, i.e., Slope = Ic/Vce =
1/Rc. This means that in this region of the curve, the output resistance
is relatively high (between 10-50 kΩ). An average Rc value of 30 kΩ in
this region may be assumed.
Also note
that in this flat portion of the curve, the collector current Ic doesn't
change much with the collector-emitter voltage Vce for any given base current Ib.
See Also:
Bipolar Transistor;
Vcb-Ic Curves;
Active Devices; More Articles
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