2-28
and is related to collector-to-base current gain, beta (b), of the common-emitter circuit by the
formula:
Since a given transistor may be connected in any of three basic configurations, there is a definite
relationship, as pointed out earlier, between alpha (a), beta (b), and gamma (g). These relationships are
listed again for your convenience:
Take, for example, a transistor that is listed on a manufacturer's data sheet as having an alpha of
0.90. We wish to use it in a common emitter configuration. This means we must find beta. The
calculations are:
Therefore, a change in base current in this transistor will produce a change in collector current that
will be 9 times as large.
If we wish to use this same transistor in a common collector, we can find gamma (g) by:
To summarize the properties of the three transistor configurations, a comparison chart is provided in
table 2-1 for your convenience.
Table 2-1.—Transistor Configuration Comparison Chart
AMPLIFIER
TYPE
COMMON
BASE
COMMON
EMITTER
COMMON
COLLECTOR
INPUT/OUTPUT PHASE
RELATIONSHIP
0º
180º
0º
VOLTAGE GAIN
HIGH
MEDIUM
LOW
CURRENT GAIN
LOW(!)
MEDIUM(")
HIGH(#)
POWER GAIN
LOW
HIGH
MEDIUM
INPUT RESISTANCE
LOW
MEDIUM
HIGH
OUTPUT RESISTANCE
HIGH
MEDIUM
LOW
