Page 209 - Complete Wireless Design

Amplifier Design

208 Chapter Three

Figure 3.111 Capacitive coupling between two stages.

RC coupling is a simple method to transfer energy from one circuit to
another, but has great difficulty matching the stage’s impedances. And,
unless we employ a low-value, but series-resonant, coupling capacitor at the
stage’s input and output, this coupling method does not prevent harmonics
from being transferred from stage to stage and being further amplified. (This
small resonant coupling capacitor will also help to stabilize the amplifier
chain. It accomplishes these tasks by attenuating the lower and higher unde-
sired RF frequencies with its low capacitance value and series-resonance
operation—but easily passes the frequencies of interest.) An L, T, or pi net-
work is normally added for harmonic attenuation and impedance-matching
requirements.
Inductive coupling, also referred to as impedance coupling (Fig. 3.112), is
found in AC and RF circuits only, and is comparable to RC coupling, but
instead of exploiting a resistor in the collector circuit, it has a collector induc-
tor. Inductive coupling has the advantage in that the collector inductor wastes
little DC power because of its very small DC series resistance, thus permitting
far more efficient amplifier operation. This high-value inductor works as a
transistor’s collector load because of its high reactance to the alternating col-
lector current, which produces an AC voltage drop. This action will subtract
from or add to the voltage from the transistor’s emitter-collector. However,
inductive coupling is practical only over a relatively narrow band of frequen-
cies, since X changes directly with frequency—and stage gain would thus
L
change as well.
Direct coupling (Fig. 3.113), also referred to as DC coupling, is valuable for
very low frequency and DC amplification. R functions as a collector resistor
3

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