Parallel Resonance

Consider an AM radio tuner comprising a parallel connection of a resistor, capacitor, and inductor, which collectively contribute to the circuit's input admittance.

At resonance, the imaginary part of the admittance is zero, causing the resonant frequency to vary inversely with the square root of the product of inductance and capacitance.

At the resonant frequency, the parallel combination of the inductor and capacitor acts as an open circuit. As a result, the resistor draws minimal current, and energy oscillates between the inductor's magnetic field and the capacitor's electric field.

The frequency response indicates that the current magnitude initially decreases, reaches a minimum at the resonance frequency, and increases as the frequency increases.

At the half-power frequencies, the current is 1.4142 times the minimum current.

The bandwidth is calculated by determining the difference between the higher and lower half-power frequencies.

The quality factor, which indicates the sharpness of the resonance curve, can be expressed in terms of the reactances.

A higher quality factor yields half-power frequencies symmetrically distributed around the resonant frequency and a narrower bandwidth.