When to use comparators and operational amplifiers

Article By : Radim Smat

Comparators operate in open loop configuration without any negative feedback.

To use operational amplifiers in open loop as comparators is quite common. This especially applies when an op amp is used in the application, giving the user the opportunity to use a dual channel (or quad channel) op amp which can save space in the application. This is possible even if a better alternative is to use comparators that are optimised for this purpose.

The op amp is a device designed to be used with negative feedback. A major concern is to ensure the stability of such a configuration. Other parameters like slew rate and maximum bandwidth are trade-offs with current consumption and the architecture of an op amp.

Comparators, on the other hand, are designed to operate in open loop configuration without any negative feedback. In most cases, they are not internally compensated. The speed (propagation delay) and slew rate (rise and fall time) are maximised. The overall gain is also usually higher. The use of an op amp as a comparator leads to an un-optimised situation, where current consumption versus speed ratio is low. Normally, a comparator cannot be used instead of an op amp. Most probably, the comparator shows instability under negative feedback. Generally speaking, comparators and operational amplifiers cannot substitute each other except for low performance designs.

Concerning the output configuration, there are two main types of comparators: push-pull and open collector (or open drain). Pull-up configuration can be used as a simple voltage level translator. The second advantage of a device with open collector is that more outputs can be connected together. This is useful for wired-OR configuration systems. It is also possible to find comparators with a differential output stage. These devices are less common and mainly used in telecommunication systems as transmission line drivers. For example, the TS3021, TS3011 and TS861 are STMicroelectronics comparators with push-pull output stage while the LMV331, TS7221 and TS331 are open drain configuration.

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