common-base configuration cannot be discounted completely, because the power gain
is substantial and often adequate.
Because power gain is usually a major
requirement for high-frequency applications, the relatively low power gain of the
common-collector circuit detracts greatly from its usefulness.
b. Impedance Matching.
Maximum power gain can be realized only by perfect
impedance matching between the output of one stage and the input of the succeeding
stage. Since the difference between the input resistance and the output resistance
is least for the common-emitter circuit, matching stages of common-emitter
amplifiers is not so difficult.
By contrast, the marked difference between the
input and output resistance of common-base and common-collector circuits makes
impedance matching quite difficult for these types.
Thus, the common-emitter
amplifier circuit has a decided advantage with regard to impedance matching for
power transfer.
c. Internal Feedback. The common-emitter amplifier, however, has more feedback
than the common-base amplifier.
Because the common-base amplifier has the least
amount of internal feedback, it does not require neutralization and is preferred in
applications where stability, accurately controlled gain, and interchangeability
are required. The internal feedback for the common-collector configuration is far
greater than that of the other two configurations; thus, the common-collector
amplifier is a poor selection if interaction between the input and output circuits
is undesirable.
d. Circuit Selection.
All things considered, the common-emitter circuit is
generally chosen for use as both RF and IF amplifiers. The common-base circuit is
used when feedback must be kept to an absolute minimum. Hence the low power gain
and excessive feedback of the common-collector configuration makes it virtually
unsuitable for RF and IF amplifiers.
1-2.
COUPLING SCHEMES
Both RF and IF amplifier circuits are generally designed for a relatively narrow
band of frequencies. Consequently, tuned coupling circuits are used to achieve the
desired selectivity and power transfer. An IF amplifier circuit is designed for a
particular center frequency, whereas an RF amplifier is designed so that it can be
tuned over a range of frequencies.
Discounting this difference, RF and IF
amplifiers are essentially alike. The same coupling schemes can be used for both
types of circuits.
a. Connections. Tuned coupling circuits for narrow-band amplifiers are shown in
figure 1-1. The points 1, 1' and 2, 2' of each circuit correspond to the points of
connection 1, 1' and 2, 2' of the block diagram in the figure.
The output
impedance of amplifier M1 is connected between 1 and 1'.
The input impedance to
amplifier M2 is connected between 2 and 2'.
b. Output Impedance.
All of the circuits in figure 1-1 match a high output
impedance to a low input impedance. Since a parallel resonant circuit is connected
between points 1 and 1' in each case, a high impedance is realized between these
points for each of the circuits.
c. Coupling for Common-Base Circuits. In circuits A and B, the input impedance
of M2 is inserted directly into the parallel resonant circuit. Either of these two
arrangements is suitable for a common-base amplifier which has a very low input
resistance. Since the inserted resistance is
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