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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