From a circuit-classification standpoint, a voltage comparator (which switches between two saturation levels based on input polarity) is best described as which type of circuit?

Introduction:
Comparators are ubiquitous building blocks used to make binary decisions from analog signals. Because their output snaps between two levels depending on input sign relative to a threshold, their behavior is inherently nonlinear, unlike linear amplifiers that preserve proportionality and superposition over a range.

Given Data / Assumptions:

• Ideal comparator with very high open-loop gain.
• Operates in saturation (two-level output), not in the op-amp’s linear region with feedback.
• Reference threshold fixed; input is arbitrary analog signal.

Concept / Approach:
Linearity requires superposition: if input x1 produces y1 and x2 produces y2, then a * x1 + b * x2 should produce a * y1 + b * y2. A comparator violates this because small input changes around the threshold cause large discontinuous output changes; the transfer characteristic is a hard nonlinearity (sign function with saturation limits).

Step-by-Step Solution:
Examine transfer: y = +V_sat for v_in > V_ref; y = −V_sat for v_in < V_ref.This mapping is not proportional (gain depends on input region) and is discontinuous at v_in = V_ref.Therefore, the device is nonlinear by definition.Linear behavior would require y = k * x over an operating range with constant k, which a comparator does not provide.

Verification / Alternative check:
Plotting the comparator’s characteristic shows a steep vertical transition at the threshold with flat saturated regions—classic nonlinearity. In contrast, an op-amp with negative feedback configured as an amplifier has a near-linear region with limited gain and bandwidth.

Why Other Options Are Wrong:

• Active filter / resonant network: Frequency-selective linear networks, not decision devices.
• Current source: A source element, not a comparator function.
• Linear circuit: Contradicts the discontinuous transfer and lack of superposition.

Common Pitfalls:

• Assuming all op-amp-based circuits are linear; comparators deliberately avoid linear operation.
• Confusing Schmitt triggers (with hysteresis) for linear amplifiers; they remain nonlinear.

Final Answer:
nonlinear circuit