Editing Comparator (section)

==Op-amp voltage comparator==
[[image:Op-Amp Comparator.svg|frame|right|A simple op-amp comparator]]

An [[operational amplifier]] (op-amp) has a well balanced difference input and a very high [[gain (electronics)|gain]]. This parallels the characteristics of comparators and can be substituted in applications with low-performance requirements.<ref>{{cite book |last1=Malmstadt |first1=Howard V. |last2=Enke |first2=Christie G. |last3=Crouch |first3=Stanley R. |title=Electronics and Instrumentation for Scientists |publisher=The Benjamin/Cummings Publishing Co |year=1981 |isbn=978-0-8053-6917-5 |chapter=Chapter 5 |url-access=registration |url=https://archive.org/details/electronicsinstr0000malm}}</ref>

A comparator circuit compares two voltages and outputs either a 1 (the voltage at the plus side) or a 0 (the voltage at the negative side) to indicate which is larger. Comparators are often used, for example, to check whether an input has reached some predetermined value. In most cases a comparator is implemented using a dedicated comparator IC, but op-amps may be used as an alternative. Comparator diagrams and op-amp diagrams use the same symbols.

A simple comparator circuit made using an op-amp without feedback simply heavily amplifies the voltage difference between Vin and VREF and outputs the result as Vout. If Vin is greater than VREF, then voltage at Vout will rise to its positive saturation level; that is, to the voltage at the positive side. If Vin is lower than VREF, then Vout will fall to its negative saturation level, equal to the voltage at the negative side.

In practice, this circuit can be improved by incorporating a [[hysteresis]] voltage range to reduce its sensitivity to noise.

Because of the difference in characteristics of an operational amplifier and comparator, using an [[operational amplifier]] as a comparator presents several disadvantages as compared to using a dedicated comparator.<ref>{{cite web |author=Ron Mancini |url=http://www.edn.com/design/analog/4353925/Designing-with-comparators |title=Designing with comparators |website=EDN |date=March 29, 2001}}</ref>

# Op-amps are designed to operate in the linear mode with negative feedback. Hence, an op-amp typically has a lengthy recovery time from saturation. Almost all op-amps have an internal compensation capacitor which imposes [[slew rate]] limitations for high frequency signals. Consequently, an op-amp makes a sloppy comparator with [[propagation delay]]s that can be as long as tens of microseconds.
# Since op-amps do not have any internal hysteresis, an external hysteresis network is always necessary for slow moving input signals.
# The quiescent current specification of an op-amp is valid only when the feedback is active. Some op-amps show an increased quiescent current when the inputs are not equal.
# A comparator is designed to produce well-limited output voltages that easily interface with digital logic. Compatibility with digital logic must be verified while using an op-amp as a comparator.
# Some multiple-section op-amps may exhibit extreme channel-channel interaction when used as comparators.
# Many op-amps have back to back diodes between their inputs. Op-amp inputs usually follow each other so this is fine. But comparator inputs are not usually the same. The diodes can cause unexpected current through inputs.