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One way of understanding the basic
operation of a bipolar transistor is by looking at its
Vcb-Ic curves,
i.e., the plots of its collector current Ic versus the voltage Vcb
across its collector and base for different values of emitter current Ie.
These curves are derived from a transistor in common-base
configuration, and basically describe the transistor's
common-base output characteristics.
A transistor
circuit is said to be in common-base configuration if the base 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-base configuration, the independent variables are the
input current Ie and the output voltage Vcb, while the dependent
variables are the input voltage Veb and
the output current Ic. The
family of input characteristic curves may therefore be described by the function
f1 wherein Veb = f1(Vcb, Ie), while the family of output curves may be
described by the function f2, wherein Ic = f2(Vcb, Ie).
The output
curves corresponding to f2 are drawn with the collector-to-base
voltage Vcb as the abscissa, and the collector current Ic as the
ordinate. Different output curves are generated for different
values of emitter current Ie, which are all drawn on the same plot.
Figure 1 shows an example of a
common-base transistor circuit's Vcb-Ic curves for different values
of Ie.
Figure 1.
The Vcb-Ic Curves of an NPN transistor for different
values of
Emitter Current
(Common-Base Output Characteristics)
These
common-base Vcb-Ic curves are useful in distinguishing between the
different regions of operation of a bipolar transistor, namely, the
active, saturation, and cut-off regions. The active or linear
region, wherein the collector junction is reverse-biased while the
emitter junction is forward-biased, is characterized by the flat or
horizontal portions of the curves. The saturation region, in which
both the collector and emitter junctions are forward-biased, lies in the
portions of the curves wherein Vcb is just very slightly above 0 V and
Ie > 0. Lastly, the cut-off region, wherein both the collector and
emitter junctions are reverse-biased, is where the curves are below Ie
=0.
See Also:
Bipolar Transistor;
Vce-Ic Curves;
Active Devices; More Articles
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