TRANSISTOR CONFIGURATIONS
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 VC. 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 () 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 (IC) changes from 1 mA to 2.8 mA, the current gain
() will be 0.90 or:
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 "" is hfb.
These terms (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 common- base
configuration.
Many transistor manuals
and data sheets only list transistor current gain characteristics in terms
of or hfe. To find alpha () when given beta (), use the
following formula to convert to for use with the common-base
configuration:
To calculate the other
gains (voltage and power) in the common-base configuration when the current
gain (I) is known, follow the procedures described earlier under the
common-emitter section.
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