Editing Operational amplifier (section)

=== Applications without feedback ===
Without feedback, the op amp may be used as a [[voltage comparator]]. Note that a device designed primarily as a comparator may be better if, for instance, speed is important or a wide range of input voltages may be found since such devices can quickly recover from full-on or full-off ''saturated'' states.

A ''voltage level detector'' can be obtained if a reference voltage ''V''<sub>ref</sub> is applied to one of the op amp's inputs. This means that the op amp is set up as a comparator to detect a positive voltage. If the voltage to be sensed, ''E''<sub>i</sub>, is applied to op amp's (+) input, the result is a noninverting positive-level detector: when ''E''<sub>i</sub> is above ''V''<sub>ref</sub>, ''V''<sub>O</sub> equals +''V''<sub>sat</sub>; when ''E''<sub>i</sub> is below ''V''<sub>ref</sub>, ''V''<sub>O</sub> equals −''V''<sub>sat</sub>. If ''E''<sub>i</sub> is applied to the inverting input, the circuit is an inverting positive-level detector: When ''E''<sub>i</sub> is above ''V''<sub>ref</sub>, ''V''<sub>O</sub> equals −''V''<sub>sat</sub>.

A ''zero voltage level detector'' (''E''<sub>i</sub> = 0) can convert, for example, the output of a sine-wave from a function generator into a variable-frequency square wave. If ''E''<sub>i</sub> is a sine wave, triangular wave, or wave of any other shape that is symmetrical around zero, the zero-crossing detector's output will be square. Zero-crossing detection may also be useful in triggering [[TRIAC]]s at the best time to reduce mains interference and current spikes.