A comparator is a device that compares two voltages and determines which one is higher. This can be used for indicating if a varying voltage exceeds a certain level with respect to a fixed reference voltage. Comparators are mostly but not always comprised of op-amps. The reason is the very high open loop gain (gain without feedback) of most operational amplifiers. The gain can be as low as ~100,000 to as high as millions. This means the output will change very quickly with only a very slight difference between the input terminals. The power supply voltage limits how far the output can go, which means the output snaps all the way to nearly max + voltage or nearly max – voltage.
As one might imagine, there are specilized comparator chips that are just op-amps designed for use as a comparator. This usually means the output stage is simplified to enable faster switching and the input stage is optimized to provide a lower minimum input voltage. Chips like the LM339 require a pullup resistor to deliver current to the output stage. This usually means that the output is a single transistor with the collector as the output pin. The size of the load resistor may somewhat affect the speed of the output and definitely affect the output current.
Schmitt Triggers are essentially comparators with positive feedback to increase the output response speed. The threshold is fixed however, and only has one input instead of two. These can be used for transforming an analog waveform into a digital one. It will respond to any excursion above the threshold by swinging the output high or low. Typically these come in 14 or 16 pin DIP (dual-inline package) chips with multiple units.
A window comparator is two comparators that have their thresholds set apart from one another with their outputs combined. This means that a for a certain voltage range, the output will be low and outside of that the voltage will be high. Diodes are on the outputs to prevent the op-amp outputs from shorting out into each other and causing damage. Again, this can be used to indicate of an analog signal goes outside of a certain range.
Comparators
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