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Transistor circuits may be
classified into three configurations based on which terminal is common
to both the input and the output of the circuit. These configurations
are:
1) the
common-emitter configuration; 2) the common-base configuration; and 3)
the common-collector configuration.
The
common-emitter
transistor configuration is shown in Figure 1. In this
configuration, the transistor terminal common to both the input and the
output of the circuit is the emitter. The common-emitter
configuration, which is also known as the 'grounded-emitter'
configuration, is the most widely used among the three configurations.
Figure 1.
Common-Emitter Transistor Configuration
The input
current and output voltage of the common-emitter configuration, which
are the base current Ib and the collector-emitter voltage Vce,
respectively, are often considered as the independent variables in this
circuit. Its dependent variables, on the other hand, are the
base-emitter voltage Vbe (which is the input voltage) and the collector
current Ic (which is the output current). A plot of the output
current Ic against the collector-emitter voltage Vce for different
values of Ib may be drawn for easier analysis of a transistor's
input/output characteristics, as shown in this
Diagram of Vce-Ic Curves.
The
common-base
transistor configuration, which is also known as the 'grounded base'
configuration, is shown in Figure 2. In this configuration, the
terminal common to both the input and the output of the circuit is the
base.
Figure 2.
Common-Base Transistor Configuration
The input
current and output voltage of the common-base configuration, which are
the emitter current Ie and the collector-base voltage Vcb, respectively,
are often considered as the independent variables in this circuit. Its
dependent variables, on the other hand, are the emitter-base voltage Veb
(which is the input voltage) and the collector current Ic (which is the
output current). A plot of the output current Ic against the
collector-base voltage Vcb for different values of Ie may be drawn for
easier analysis of a transistor's input/output characteristics, as shown
in this Diagram of Vcb-Ic Curves.
The
common-collector transistor configuration is shown in Figure 3.
In this configuration, the collector is common to both the input and
the output of the circuit. This is basically the same as the
common-emitter configuration, except that the load is in the emitter
instead of the collector. Just like in the common-emitter
circuit, the current flowing through the load when the transistor is
reverse-biased is zero, with the collector current being very small
and equal to the base current. As the base current is
increased, the transistor slowly gets out of cut-off, goes into the
active region, and eventually becomes saturated. Once
saturated, the voltage across the load becomes maximum, while the
voltage Vce across the collector and emitter of the transistor goes
down to a very low value, i.e., as low as a few tens of millivolts
for germanium and 0.2 V for silicon transistors.
Figure 3.
Common-Collector Transistor Configuration
See Also:
Bipolar Transistor;
Vce-Ic Curves;
Vcb-Ic Curves;
Active Devices; More Articles
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