TRANSISTOR CONFIGURATIONS
Common Collector
The common-collector
configuration (CC) shown in figure 2-16 view C is used mostly for impedance
matching. It is also used as a current driver, because of its substantial
current gain. It is particularly useful in switching circuitry, since it has
the ability to pass signals in either direction (bilateral operation).
In the common-collector
circuit, the input signal is applied to the base, the output is taken from
the emitter, and the collector is the element common to both input and
output. The common collector is equivalent to our old friend the
electron-tube cathode follower. Both have high input and low output
resistance. The input resistance for the common collector ranges from 2
kilohms to 500 kilohms, and the output resistance varies from 50 ohms to
1500 ohms. The current gain is higher than that in the common emitter, but
it has a lower power gain than either the common base or common emitter.
Like the common base, the output signal from the common collector is in
phase with the input signal. The common collector is also referred to as an
emitter-follower because the output developed on the emitter follows the
input signal applied to the base.
Transistor action in the
common collector is similar to the operation explained for the common base,
except that the current gain is not based on the emitter-to-collector
current ratio, alpha (). Instead, it is based on the emitter-to-base
current ratio called GAMMA (), because the output is taken off the emitter.
Since a small change in base current controls a large change in emitter
current, it is still possible to obtain high current gain in the common
collector. However, since the emitter current gain is offset by the low
output resistance, the voltage gain is always less than 1 (unity), exactly
as in the electron-tube cathode follower
The common-collector
current gain, gamma (), is defined as
and is related to
collector-to-base current gain, beta (), of the common-emitter circuit by
the formula:
Since a given transistor
may be connected in any of three basic configurations, there is a definite
relationship, as pointed out earlier, between alpha (), beta (), and gamma
(). These relationships are listed again for your convenience:
Take, for example, a
transistor that is listed on a manufacturer's data sheet as having an alpha
of 0.90. We wish to use it in a common emitter configuration. This means we
must find beta. The calculations are:
Therefore, a change in
base current in this transistor will produce a change in collector current
that will be 9 times as large.
If we wish to use this
same transistor in a common collector, we can find gamma () by:
To summarize the
properties of the three transistor configurations, a comparison chart is
provided in table 2-1 for your convenience.
Table 2-1. - Transistor
Configuration Comparison Chart
AMPLIFIER TYPE |
COMMON BASE |
COMMON EMITTER |
COMMON COLLECTOR |
INPUT/OUTPUT PHASE RELATIONSHIP |
0° |
180° |
0° |
VOLTAGE GAIN |
HIGH |
MEDIUM |
LOW |
CURRENT GAIN |
LOW() |
MEDIUM() |
HIGH() |
POWER GAIN |
LOW |
HIGH |
MEDIUM |
INPUT RESISTANCE |
LOW |
MEDIUM |
HIGH |
OUTPUT RESISTANCE |
HIGH |
MEDIUM |
LOW |
Now that we have analyzed
the basic transistor amplifier in terms of bias, class of operation, and
circuit configuration, let's apply what has been covered to figure 2-12. A
reproduction of figure 2-12 is shown below for your convenience.
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