What is the Sallen–Key topology typically used for in analog filter design?

• A. A type of oscillator
• B. A high-power buffer
• C. A comparator
• D. A second-order active filter using an op-amp with RC network providing feedback; used to realize low-pass, high-pass, or band-pass responses with simple component count and good Q control.

Answer: (D)

Sallen–Key is an active second‑order filter topology that uses an op‑amp as a buffer/amplifier and a feedback RC network to shape the response. The RC network around the output sets the poles of the filter, while the op‑amp provides gain and isolates the load so the filter’s behavior isn’t degraded by what comes after it. With this arrangement you can realize low‑pass, high‑pass, or band‑pass responses using only a few components, which is why it’s valued for simple yet flexible designs. The op‑amp’s gain in this topology lets you tailor the damping and hence the quality factor, giving you predictable Q control to achieve a desired response (Butterworth, Chebyshev, etc.). That combination of compact parts, versatile response types, and adjustable Q is what makes the Sallen–Key topology so typical for second‑order active filtering. It isn’t intended as an oscillator, a high‑power buffer, or a comparator.