TC 9-62
For example, if the input current (IB) in a CE changes from 75 A to 100 A and the
output current (IC) changes from 1.5 mA to 2.6 mA, then the current gain (β) would be 44.
∆IC
-3
1.1 x 10
= 44
=
β=
-6
∆I B
25 x 10
This simply means that a change in base current produces a change in collector current that
is 44 times as large.
2-68. You may also see the term hfe used in place of β. The terms hfe and β are
equivalent and may be used interchangeably. This is because "hfe" means:
h = hybrid (meaning mixture)
f = forward current transfer ratio
e = common emitter configuration
2-69. The resistance gain of the CE can be found in a method similar to the one used for
finding beta:
R OUT
R =
R IN
2-70. Once the resistance gain is known, the voltage gain is easy to calculate since it is
equal to the current gain β multiplied by the resistance gain (E = βR). The power gain is
equal to the voltage gain multiplied by the current gain β (P = βE).
Common Base
2-71. The CB configuration (see Figure 2-17, view (B)) is mainly used for impedance
matching, since it has a low input resistance (30 ohms to 160 ohms) and a high output
resistance (250 kilohms to 550 kilohms). However, two factors that limit its usefulness in
some circuit applications are its low input resistance and its current gain of less than 1.
Since the CB configuration will give voltage amplification, there are some additional
applications that require both a low-input resistance and voltage amplification. Some
microphone amplifiers use a circuit configuration of this type.
2-72. In the CB configuration, the input signal is applied to the emitter, the output is
taken from the collector, and the base is the element common to input and output. Since the
input is applied to the emitter, it causes the emitter-base junction to react in the same
manner as it did in the CE circuit. For example, an input that aids the bias will increase
transistor current and one that opposes the bias will decrease transistor current.
2-73. Unlike the CE circuit, the input and output signals in the CB circuit are in phase.
To illustrate this point, assume the input to the PNP version of the CB circuit in
Figure 2-17, view (B) is positive. The signal adds to the forward bias, since it is applied to
the emitter, causing the collector current to increase. This increase in IC results in a greater
voltage drop across the load resistor RL (not shown), thereby lowering the collector voltage
VC. The collector voltage, in becoming less negative, is swinging in a positive direction
and is therefore in phase with the incoming positive signal.
2-22
TC 9-62
23 June 2005
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