Tab Electronics Guide to Understanding Electricity ... - Sciences Club

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166 Chapter Six
Figure 6-4
Practical example of
a common-collector
(emitter-follower)
transistor amplifier.
R1
20 k
12 V
C1
+
5 F
Q1
2N2222
C2
Input
R2
20 k
R3
1 k
5 F
Output
still appropriate, a closer estimate can probably be obtained by considering
the output impedance of most transistors to be about 80 ohms. This
80-ohm output impedance should be viewed as being in parallel with
R3, giving us a calculated output impedance of about 74 ohms (80 ohms
in parallel with 1000 ohms).
Resistors R1 and R2 have the same function within a common-collector
amplifier as previously discussed with common-emitter amplifiers. The
high negative feedback produced by R3 provides excellent temperature stability
and immunity from transistor variables. As in the case of Fig. 6-3,
the circuit illustrated in Fig. 6-4 can be a valuable building block toward
future projects.
The Common-Base Configuration
I have included the common-base transistor amplifier configuration in
this text for the sake of completeness, but the applications for it are few.
They are often used as the first RF amplifier stage, amplifying signals
from radio antennas, but are seldom seen otherwise.