What is the quality factor (Q) of a circuit and how does it affect performance?

Quality factor Q measures a circuit's selectivity and energy storage efficiency, defined as the ratio of energy stored to energy dissipated per cycle. For series RLC: Q = (1/R)*sqrt(L/C) = XL/R at resonance. Higher Q means sharper resonance peak, narrower bandwidth (BW = fr/Q), and better frequency selectivity. Q factors above 100 are common in RF filters, while lower Q provides wider bandwidth for broadband applications.