TC 9-62
output resistance varies from 50 ohms to 1,500 ohms. The current gain is higher than that
in the CE, but it has a lower power gain than either the CB or CE. Like the CB, the output
signal from the CC is in phase with the input signal. The CC is also referred to as an
emitter-follower because the output developed on the emitter follows the input signal
applied to the base.
2-80. Transistor action in the CC is similar to the operation explained for the CB, except
that the current gain is not based on the emitter-to-collector current ratio, alpha (α). Instead,
it is based on the emitter-to-base current ratio called GAMMA (γ), 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 CC. 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 CC current gain, gamma (γ), is defined as follows:
2-81.
IE
γ=
IB
and is related to collector-to-base current gain, beta (β), of the CE circuit by the formula:
γ =β + 1
2-82. Since a given transistor may be connected in any of three basic configurations,
then there is a definite relationship, as pointed out earlier, between alpha, beta, and gamma.
These relationships are listed again for your convenience:
β
Alpha (α) =
β+1
α
Beta ( β) =
1- α
Gamma (γ) = β + 1
2-83. Take, for example, a transistor that is listed on a manufacturer's data sheet as
having an alpha of 0.90, but we want to use it in a CE configuration. This means we must
find beta. The calculations are as follows:
α
0.90
0.90
β=
=
=
=9
1 - α 1 - 0.90
0.1
Therefore, a change in base current in this transistor will produce a change in collector
current that will be 9 times larger. If we want to use this same transistor in a CC, we can
find gamma by using the following formula:
Gamma = β + 1 = 9 + 1 = 10
2-24
TC 9-62
23 June 2005
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