In a Sallen-Key low-pass filter, the op-amp typically acts as what?

In a Sallen-Key low-pass filter, the op-amp typically acts as what?

• A. Inverting amplifier
• B. Buffer or non-inverting follower
• C. Integrator
• D. Comparator

Answer: (B)

The main idea is that a Sallen‑Key low-pass uses the op-amp as a buffer. It’s wired as a non‑inverting follower (unity-gain buffer) so the RC network sees a high-impedance source and the op-amp, in turn, can drive the next stage with low output impedance. This buffering preserves the intended pole placements of the RC network and prevents loading effects from the following stage, which is essential for obtaining the second‑order low‑pass response. The op-amp’s feedback in this configuration can also set or slightly adjust the damping (the Q) if a bit of gain is added, but the core role remains buffering and providing a low‑impedance drive while keeping the signal from inverting or integrating, which would distort the desired filter characteristics.

The main idea is that a Sallen‑Key low-pass uses the op-amp as a buffer. It’s wired as a non‑inverting follower (unity-gain buffer) so the RC network sees a high-impedance source and the op-amp, in turn, can drive the next stage with low output impedance. This buffering preserves the intended pole placements of the RC network and prevents loading effects from the following stage, which is essential for obtaining the second‑order low‑pass response.

The op-amp’s feedback in this configuration can also set or slightly adjust the damping (the Q) if a bit of gain is added, but the core role remains buffering and providing a low‑impedance drive while keeping the signal from inverting or integrating, which would distort the desired filter characteristics.