In a Sallen-Key second-order filter, which parameter is commonly adjusted to control the quality factor Q?

• A. Supply voltage
• B. Output impedance
• C. Component values and feedback factor
• D. Input impedance

Answer: (C)

Quality factor Q is about how the resonance of a second-order filter is damped. In a Sallen-Key second-order filter, the damping comes from the RC network values together with how much positive feedback the op-amp provides in the feedback path. Those are the knobs that set the pole locations and thus Q. If you change only the supply voltage, you’re mainly affecting the op-amp’s output swing, not the pole positions in the linear region. Similarly, input or output impedance can affect loading, but they don’t set the intrinsic damping unless they noticeably load the network. By selecting the resistor and capacitor values and adjusting the feedback factor (the op-amp gain in the feedback loop), you tune both the natural frequency and Q. For equal components, Q becomes 1/(3 − K), showing clearly that both component values and the feedback factor control Q.