A reactive circuit (see reactance) so arranged that it is capable of resonance. In a series resonant circuit a resistor, inductor, and capacitor are arranged in series. Resonance occurs when the impedance (Z) is a minimum and the current amplitude therefore a maximum. In a parallel resonant circuit the inductance and capacitance are in parallel and resonance (with minimal current amplitude) occurs at maximum impedance. The frequency at which resonance occurs is called the resonant frequency. In a series resonant circuit Z = R + i[ωL − 1/ωC], where ω = 2πf and f is the frequency, R is the resistance, L is the inductance, and C is the capacitance. At resonance, Z is a minimum and ωL = 1/ωC, i.e. the circuit behaves as if it is purely resistive. In the parallel circuit, resonance occurs when R2 + ω2L2 = L/C, which in most cases also approximates to ωL = 1/ωC. Resonant circuits are widely used in radio to select one signal frequency in preference to others.
Z = R + i[ωL − 1/ωC],