Comparators
A comparator is a device that 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 Figure 1.
Figure 1
VOUT = VDD when V1 > V2
VOUT = VSS 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 it can. When the non-inverting input (V1) drops below the inverting input (V2), the op-amp outputs the most negative voltage it can be.
Example
An op-amp comparator can be used as a voltage level detector. Figure 2 shows a level detector that compares a Direct Current (DC) reference voltage (VREF) of 3V to VIN with VDD = 10 V and VSS = -10 V
Figure 2
When Vin exceeds 3V, Vout goes to the 10 V positive rail due to its high gain, open-loop configurations. When it falls below 3V, Vout flips to the -10 V rail.
Figure 3