Comparators

The comparator is a device which compares two voltages or currents and switches its output to indicate which is larger.
A standard op-amp can be used as a comparator as indicated in the following diagram.

V_OUT = V_DD when V₁ > V₂
V_OUT = V_SS when V₁ < V₂

When the non-inverting input (V₁) is at a higher voltage than the inverting input (V₂), the high gain of the op-amp causes it to output the most positive voltage. When the non-inverting input (V₁) drops below the inverting input (V₂), the op-amp outputs the most negative voltage.

Example:
Op-amp comparator can be used as a voltage level detector. The figure below shows a level detector that compares a DC reference voltage (V_REF) of 3V to V_IN with V_DD = 10V and V_SS = -10V

When V_IN exceeds 3V, V_OUT goes to the 10V positive rail due to its high gain open-loop configurations. When it falls below 3V, V_OUT flips to the -10V rail.