___________________________________________________________________________ Transistors
2-74. The current gain in the CB circuit is calculated in a method similar to that of the
CE except that the input current is IE not IB and the term ALPHA ( α ) is used in place of
beta for gain. Alpha is the relationship of collector current (output current) to emitter
current (input current). Alpha is calculated using the formula:
∆IC
α=
∆I E
For example, if the input current (IE) in a CB changes from 1 mA to 3 mA and the output
current (IC) changes from 1 mA to 2.8 mA, then the current gain (a) would be 0.90 (see
below formula):
I
∆ C
α =
∆ I E
-
1.8 x 10 3
=
-
2 x 10 3
= 0.90
This is a current gain of less than 1.
2-75. Since part of the emitter current flows into the base and does not appear as
collector current, then 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 CB CONFIGURATION.
2-76. Another term for "α" is hfb. These terms (α and hfb) are equivalent and may be used
interchangeably. The meaning for the term hfb 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 CB
configuration.
2-77. Many transistor manuals and data sheets only list transistor current gain
characteristics in terms of β or hfe. To find alpha (α) when given beta (β), use the following
formula to convert β to a for use with the CB configuration:
β
α=
β +1
To calculate the other gains (voltage and power) in the CB configuration when the current
gain (a) is known, follow the procedures described earlier under the CE section.
Common Collector
2-78. The CC configuration (see Figure 2-17, view (C)) is used mostly for impedance
matching. It is also used as a current driver, due to its substantial current gain. It is
particularly useful in switching circuitry, since it has the ability to pass signals in either
direction (bilateral operation).
2-79. In the CC circuit, the input signal is applied to the base, the output is taken from
the emitter, and the collector is the element common to input and output. The CC is
equivalent to the electron-tube cathode follower. Both have high input and low output
resistance. The input resistance for the CC ranges from 2 kilohms to 500 kilohms and the
23 June 2005
TC 9-62
2-23
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