Op-Amp Versus Comparator (EE Tip #128)

Practically every lecture course or textbook on electronics describes how to use an operational amplifier as a comparator. Here we look at the possibility in more detail, and see how it can often be a very poor idea. The idea behind the comparator configuration is simple. An op-amp has a very high open-loop DC gain which means that even a tiny differential input voltage will drive the output to one extreme or the other. If the voltage at the non-inverting (“+”) input is greater than that at the inverting (“–“) input the output goes high; otherwise the output goes low. In other words the two voltages are compared and the output is a binary indication of which of the two is the greater. So the op-amp looks like the perfect device to use as a comparator. But why then do there exist special-purpose comparator devices? Looked at from the outside, op-amps and comparators appear indistinguishable. Besides power connections, they both have “+” and “–” inputs and a single output. Taking a look at the internal circuit diagram, again the two devices appear broadly very similar (although a comparator device with an open-collector or open-drain output does look more obviously different from an op-amp). The big difference, which is not apparent without looking at the circuit more closely, is that the output stages of operational amplifiers are designed for linear operation, with the general aim of amplifying the input signal with as little distortion as possible (assuming that some negative feedback is provided), but in the case of a comparator the output circuit is designed to operate in saturation, that is, to switch between the upper and lower output voltage limits without the provision of external feedback. Comparators often also offer a ground connectio