Comparators

Op amps are normally not used in an open-loop configuration except when being used as comparators and/or oscillators due to their extremely high open-loop gain. When used as a comparator, an op amp compares two voltages or currents and switches its output to indicate which is larger.

A standard op amp used as a comparator is shown in the accompanying image.

Vout = Vdd when V1 > V2
Vout = Vss when V1 < V2

When the non-inverting voltage input (V+) is higher than the inverting input (V-), the high gain of the op amp causes it to output the most positive voltage it can be (positive rail shown as Vdd). When the non-inverting input (V+ ) drops below the inverting input (V- ), the op amp outputs the most negative voltage it can be (negative rail shown as Vss).