Why is a resistor often included in a parallel resonant circuit ?

In a parallel resonant circuit, the Quality Factor (Q) is determined by the ratio of the parallel resistance to the reactance (Q = R / X_L). Placing a resistor in parallel lowers the total parallel resistance of the circuit. Because the equivalent parallel resistance drops, the Q factor decreases proportionally. A lower Q factor means the circuit is less sharply tuned, which directly increases the circuit's bandwidth (since Bandwidth = resonant frequency / Q). This technique, often referred to as "swamping," is commonly used in RF circuits when a broader frequency response (wider bandwidth) is required, such as in broadband amplifiers or specific filter designs.