Active filter implementation: When op-amps are used to realize standard low-pass and high-pass active filter stages in audio and instrumentation (for example, Sallen–Key or similar topologies), which amplifier configuration is most commonly employed?

Introduction:
Active filters use op-amps with R–C networks to realize precise frequency responses. Choosing the amplifier configuration impacts input impedance, noise gain, and ease of component selection.

Given Data / Assumptions:

• Standard low-pass and high-pass responses at modest orders (e.g., 2nd-order).
• Common practical topologies such as Sallen–Key, which buffer the network.
• Linear operation with negative feedback (not comparator mode).

Concept / Approach:
The widely used Sallen–Key topology employs a noninverting op-amp buffer/amp. This configuration offers very high input impedance, making the filter less sensitive to source loading, and provides straightforward gain setting without inverting signal polarity. While inverting multiple-feedback filters exist, the most commonly taught beginner implementation for simple HP/LP sections is noninverting (Sallen–Key).

Step-by-Step Solution:
1) Identify common active filter families: Sallen–Key (noninverting) and multiple-feedback (often inverting).2) Note that introductory designs prefer Sallen–Key for simplicity and buffering.3) Conclude that the noninverting configuration is the most commonly employed for basic HP/LP stages.4) Recognize that other configurations may be used for special requirements, but they are not the default choice.

Verification / Alternative check:
Compare input impedance: noninverting stages present high input impedance; inverting stages present input resistance equal to the input resistor, which can load the source more significantly.

Why Other Options Are Wrong:
Comparator: open-loop threshold device; not a linear filter element.Open-loop: an op-amp without feedback saturates; filters require controlled closed-loop gain.Inverting: used in multiple-feedback filters but is not the most common entry-level choice compared to Sallen–Key.

Common Pitfalls:
Assuming all active filters are inverting because of some multiple-feedback designs; many practical HP/LP stages are noninverting buffers with RC networks around them.

Final Answer:
noninverting