In filter design, what happens to the selectivity when the Q factor increases in an LC-based band-pass filter?

• A. Increased Q leads to higher selectivity and narrower bandwidth
• B. Increases Q lowers selectivity
• C. Increases Q widens the bandwidth
• D. Q has no effect on selectivity

Answer: (A)

Higher Q means a sharper resonance in an LC band-pass filter. In resonant circuits the quality factor is tied to how narrow the passband is through the relationship Q = f0 / BW. Keeping the center frequency f0 fixed, increasing Q reduces the bandwidth BW, so the filter passes only a tighter range of frequencies around the center. That tighter passband translates directly into greater selectivity, since frequencies just outside the passband are more effectively rejected. This sharper peak happens because the energy stored in the tank is large relative to the energy lost per cycle, meaning less damping and a steeper response around resonance. Practically, you achieve higher Q by using lower-loss inductors and capacitors and minimizing series resistance, which sharpens the resonance and improves selectivity.