A comparator compares two voltages and indicates which is larger by driving its output to one of two extreme levels. To do this, an op-amp is used in open-loop mode, meaning there is no feedback path from the output back to the inverting input — in particular, no negative feedback.

The reason lies in the op-amp's enormous open-loop gain, A_OL, which is typically of the order 10⁵ to 10⁶. The output is A_OL times the tiny difference between the two inputs, so even a difference of a few microvolts is amplified far beyond the supply rails. The output therefore cannot stay in its linear range; it slams up to positive saturation (+V_sat) when the non-inverting input is higher, and down to negative saturation (−V_sat) when the inverting input is higher. The result is a two-level, digital-like output that reports the comparison.

• Closed-loop mode with negative feedback is used to build linear circuits such as amplifiers, where feedback tames the huge gain into a stable, predictable value — the opposite of what a comparator needs.
• Unity gain mode (a buffer/follower) is a specific closed-loop configuration with gain of exactly 1; it reproduces the input, it does not compare.
• Operating in the linear region only is impossible for a comparator, whose whole purpose is to be driven into saturation; it deliberately spends nearly all its time at the output extremes, not in the linear region.

Hence a comparator characteristically runs in open-loop mode.