A comparator is a device with two input terminals, inverting and noninverting, and an output that usually swings from rail to rail. So is an op amp.A comparator has low offset, high gain, and high common-mode rejection. So does an op amp.So what is the difference? A comparator has a logic output that indicates which of the two inputs is at a higher potential. If its output is TTL or CMOS compatible (and many comparators are), it is always intended to be at one rail or the other—or making a rapid transition between the two.