Ive used op amps as comparators a number of times because of

I\'ve used op amps as comparators a number of times because of the easy availability (they can be purchased 4 op amps to the IC package) so one was used as a comparator while the other three for other purposes. But there are units made that are actually called comparators. Is there something different about these - not just ordinary op amps?

Solution

The basic function of a high-gain comparator is to determine whether an input voltage is higher or lower than a reference voltage—and to present that decision as one of two voltage levels, established by the output’s limiting values.They are same if you operate the Differential amplifier in Saturation then it performs the function of Comparator.

But there are some basic differences between devices designed as op amps and devices designed to be comparators

DIFFERENCES-

1-OP AMPS uses JFETS.This is because JFETs provide extremely high input impedance, which is desirable for an op-amp. Most op-amp analysis assumes that the bias currents (the currents moving through the inputs) are zero, but this is only true to the extent that the input impedance is infinite. For a comparator, you do want high input impedance to avoid loading the source excessively, but it\'s not so important that input impedance is very high

2-The COMPARATOR has an open collector output. For a comparator, this is good, because it allows it to interface with any number of output voltages through a pull-up resistor, or trivially implement a wired-or bus. You wouldn\'t want this for an op-amp, because usually, you want an op-amp to be as good at sourcing current as sinking it, so that your output waveform is symmetrical.

Amplifiers should be considered for use as comparators in applications where low offset and drift, and low bias current, are needed—combined with low cost. On the other hand, there are many designs where an amplifier could not be considered as a comparator because of its lengthy recovery time from output saturation, its long propagation delay, and the inconvenience of making its output compatible with digital logic. Additionally, dynamic stability is a concern.