A comparator is a great circuit because it provides a near ideal transition between the analog and digital domains. The comparator looks at two linear input signals and gives a digital output that’s either high or low depending on whether one input is above or below the other. Simple, but widely useful.
If you need this type of circuit in your design, it’s best to use an IC comparator that was designed for such applications. However, most designers know that a standard op amp can be used as a comparator. And in some designs, an extra op amp may be available, thus avoiding any extra cost or need for more space.
Chances are that the resulting op-amp comparator will probably not deliver the desired optimum performance. This amateur design error can result in more debugging and redesign time than planned. The best advice is that if you need a comparator, use an IC comparator to avoid problems and achieve the best possible outcome.
What are the Real Differences Between an Op Amp and Comparator?
The main differences between these two include:
- The built-in phase compensation needed to stabilize an op amp usually makes the device too slow for comparator switching operations.
- The input stages of an op amp are typically protected with diodes or other transistors that often compromise its use as a comparator.
- The output stage of an op amp is designed for linear operation. The output with bipolar power supplies swings positive and negative, and must be conditioned for use in digital circuits.
- A real comparator output stage is designed for saturated operation and matches common digital logic levels. The output is often of the open-collector (drain) type.
- An op amp is usually configured with external input and feedback resistors to set the gain and other characteristics of the circuit. A comparator usually runs open loop; i.e., no feedback.
- Comparators offer shorter delay times and a very high slew rate compared to an op amp.
Despite what may appear as similarities, the two circuits are different animals with different applications.