2-27
This is a current gain of less than 1.
Since part of the emitter current flows into the base and does not appear as collector current,
collector current will always be less than the emitter current that causes it. (Remember, IE = IB + IC)
Therefore, ALPHA is ALWAYS LESS THAN ONE FOR A COMMON-BASE CONFIGURATION.
Another term for "a" is hf. These terms (and hf) are equivalent and may be used interchangeably. The
meaning for the term hf is derived in the same manner as the term hfe mentioned earlier, except that the
last letter "e" has been replaced with "b" to stand for common- base configuration.
Many transistor manuals and data sheets only list transistor current gain characteristics in terms of b
or hfe. To find alpha (a) when given beta (b), use the following formula to convert b to a for use with the
common-base configuration:
To calculate the other gains (voltage and power) in the common-base configuration when the current
gain (a) is known, follow the procedures described earlier under the common-emitter section.
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 (a). Instead, it is
based on the emitter-to-base current ratio called GAMMA (g), 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 (g), is defined as
