20.
PARALLEL-TUNED CIRCUIT
a. If a coil and a capacitor are connected in parallel with a variable-
frequency source of ac, as in A of figure 26, the combination is called a parallel-
tuned, or parallel-resonant, circuit.
Figure 26.
Parallel-tuned circuit.
b. In a parallel-tuned circuit, R appears only in the L branch. This results
when the L and C branches are placed in parallel across the generator E. Because
of this fact, there are now two paths through which the current may flow: one path
through L and one through C.
c. If the ac source is set at a
low frequency, most of the current will flow
through the coil, since the reactance
of the coil will be small for low-frequency
ac, and the reactance of the capacitor
will be large. On the other hand, if the ac
source is set at a high frequency,
most of the current will flow through the
capacitor, since its reactance will
be small at high frequencies, while the
reactance of the coil will be large.
d. At the resonant frequency (fr), the reactance of C is equal to that of L
in most of the practical radio circuits.
Since the inductive and capacitive
reactances are equal and opposite, they cancel out, and the only limitation to
current flow at resonance within the tuned circuit is the resistance. Because
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