Figure 25.

Series-tuned circuit.

tuned circuit at resonance is the smallest possible value which the circuit

constants will permit, because the opposition to current flow from the generator is

the smallest value at resonance.

Thus we say the impedance of a series-resonant

circuit is minimum at resonance.

b. The voltage drop across the inductor and the capacitor can be many times

the applied voltage, because E = I x X.

This increase in voltage is sometimes

called the resonant rise of voltage, since the rise is greatest at resonance.

c. At frequencies below resonance, the capacitive reactance predominates and

the circuit acts like a capacitor in series with a resistor. At frequencies above

resonance, the inductive reactance predominates and the circuit behaves like an

inductor in series with a resistor.

d. If the graph of line current is plotted against frequency, as in B of

figure 25, the resultant curve is called a response curve.

In a tuned circuit

having a small resistance value, the current rises sharply near the resonant

frequency (fr), whereas a large circuit resistance produces a curve that is

relatively broad and flat.

The circuit represented by curve A of the figure is

said to be much more selective, or have more selectivity than that represented by

curve B. This circuit can sharply discriminate against frequencies on either side

of resonance.

The height of the peak of the curve indicates the ability of the

tuned circuit to produce an output voltage across either the inductance or

capacitance. The circuit represented by curve A is said to be more sensitive, or

have more sensitivity than that represented by curve B.

Selectivity and

sensitivity are two important characteristics of tuned-circuits.