___________________________________________________________________________ Transistors
Common Emitter
2-63. The CE configuration (see Figure 2-18, view (A)) is the arrangement most often
used in practical amplifier circuits. This is because it provides good voltage, current, and
power gain. The CE has a somewhat low input resistance (500 ohms to 1,500 ohms)
because the input is applied to the forward-biased junction. The CE has a moderately high
output resistance (30 kilohms to 50 kilohms or more) because the output is taken off the
reverse-biased junction. Since the input signal is applied to the base-emitter circuit and the
output is taken from the collector-emitter circuit, then the emitter is the element common to
both input and output.
2-64. Using the PNP CE configuration (see Figure 2-18, view (A)), let us review the CE
amplifier. When a transistor is connected in a CE 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 that 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. This 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.
2-65. Since the input signal to the CE goes positive when the output goes negative, the
two signals (input and output) are 180 degrees out of phase. The CE circuit is the only
configuration that provides a phase reversal.
2-66. The CE 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 the following:
output
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)
in order to determine whether or not the transistor is working properly in the circuit.
2-67. The current gain in the CE circuit is called BETA (β). Beta is the relationship of
collector current (output current) to base current (input current). Use the following formula
to calculate beta:
∆I C
β=
(∆ is the Greek letter delta, it is used to indicate a small change)
∆I C
23 June 2005
TC 9-62
2-21
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