2-25
Since you have already covered what you now know to be a common-emitter amplifier (fig. 2-12),
let's take a few minutes and review its operation, using the PNP common-emitter configuration shown in
figure 2-16 view A.
When a transistor is connected in a common-emitter configuration, the input signal is injected
between the base and emitter, which is a low resistance, low-current circuit. As the input signal swings
positive, it also causes the base to swing positive with respect to the emitter. This action decreases
forward bias which reduces collector current (IC) and increases collector voltage (making VC more
negative). During the negative alternation of the input signal, the base is driven more negative with
respect to the emitter. This increases forward bias and allows more current carriers to be released from the
emitter, which results in an increase in collector current and a decrease in collector voltage (making VC
less negative or swing in a positive direction). The collector current that flows through the high resistance
reverse-biased junction also flows through a high resistance load (not shown), resulting in a high level of
amplification.
Since the input signal to the common emitter goes positive when the output goes negative, the two
signals (input and output) are 180 degrees out of phase. The common-emitter circuit is the only
configuration that provides a phase reversal.
The common-emitter is the most popular of the three transistor configurations because it has the best
combination of current and voltage gain. The term GAIN is used to describe the amplification capabilities
of the amplifier. It is basically a ratio of output versus input. Each transistor configuration gives a
different value of gain even though the same transistor is used. The transistor configuration used is a
matter of design consideration. However, as a technician you will become interested in this output versus
input ratio (gain) to determine whether or not the transistor is working properly in the circuit.
The current gain in the common-emitter circuit is called BETA (b). Beta is the relationship of
collector current (output current) to base current (input current). To calculate beta, use the following
formula:
(D is the Greek letter delta, it is used to indicate a small change)
For example, if the input current (IB) in a common emitter changes from 75 uA to 100 uA and the
output current (IC) changes from 1.5 mA to 2.6 mA, the current gain (b) will be 44.
This simply means that a change in base current produces a change in collector current which is 44
times as large.
You may also see the term hfe used in place of b. The terms hfe and b are equivalent and may be used
interchangeably. This is because "hfe" means:
