As the Q-factor of a resonant circuit increases, what happens to the resonance bandwidth and selectivity?

• A. Bandwidth increases and selectivity decreases.
• B. Narrower bandwidth with higher selectivity; resonance frequency unchanged.
• C. Bandwidth remains the same and selectivity improves.
• D. Resonance frequency shifts upward.

Answer: (B)

When the Q-factor goes up, the circuit stores energy more efficiently compared to how much it loses each cycle. This makes the resonance peak sharper. Since Q is defined as f0 divided by the bandwidth Δf, a higher Q at the same center frequency f0 means the bandwidth Δf gets smaller. That smaller bandwidth translates into better selectivity, because the circuit responds strongly only very close to the resonance frequency and attenuates frequencies farther away more effectively. The resonance frequency itself is set by the values of the reactive elements (like the LC tank) and is not changed by increasing Q, which mainly reduces losses and narrows the peak. So the effect is a narrower bandwidth with higher selectivity, while the resonance frequency remains unchanged.