2-26
h = hybrid (meaning mixture)
f = forward current transfer ratio
e = common emitter configuration
The resistance gain of the common emitter can be found in a method similar to the one used for
finding beta:
Once the resistance gain is known, the voltage gain is easy to calculate since it is equal to the current
gain (b) multiplied by the resistance gain (E = bR). And, the power gain is equal to the voltage gain
multiplied by the current gain b (P = bE).
Common Base
The common-base configuration (CB) shown in figure 2-16, view B is mainly used for impedance
matching, since it has a low input resistance (30 ohms-160 ohms) and a high output resistance (250
kilohms-550 kilohms). However, two factors limit its usefulness in some circuit applications: (1) its low
input resistance and (2) its current gain of less than 1. Since the CB configuration will give voltage
amplification, there are some additional applications, which require both a low-input resistance and
voltage amplification, that could use a circuit configuration of this type; for example, some microphone
amplifiers.
In the common-base configuration, the input signal is applied to the emitter, the output is taken from
the collector, and the base is the element common to both 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 common-emitter
circuit. For example, an input that aids the bias will increase transistor current, and one that opposes the
bias will decrease transistor current.
Unlike the common-emitter circuit, the input and output signals in the common-base circuit are in
phase. To illustrate this point, assume the input to the PNP version of the common-base circuit in figure
2-16 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), thus lowering the collector voltage V
C. The collector voltage, in becoming less negative,
is swinging in a positive direction, and is therefore in phase with the incoming positive signal.
The current gain in the common-base circuit is calculated in a method similar to that of the common
emitter except that the input current is I
E
not IB and the term ALPHA (a) 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:
For example, if the input current (IE) in a common base changes from 1 mA to 3 mA and the output
current (I
C) changes from 1 mA to 2.8 mA, the current gain (a) will be 0.90 or:
