dark_one,
The Op Amp is usually used with its output in its linear range. However, it can be used as a comparator. There are 3 considerations that may influence your decision:
. 1) Do you need fast switching of the output? If you do, you must select an OP Amp with a fast guaranteed slew rate. Gain-Bandwidth is unimportant if the slew rate is fast enough. An OP Amp will saturate its intermediate gain stages, as well as the output stage. It takes awhile for these stages to come out of saturation once the input voltage approaches the reference voltage. The time to come out of saturation is not a parameter that is usually specified for an OP Amp.
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2) The output voltage will be approximately equal to the supply voltage. Is this acceptable? If not, most comparators have an open collector (or open drain) output. This allows you to connect a pullup resistor to any voltage that is within the maximum voltage rating of the output stage. The max rating of the output stage often is higher than the max supply voltage rating of the rest of the comparator.
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3) If you have an unused section of a multi-OP Amp part, you may want to use it simply because it's available.
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Whether you use an OP Amp or a comparator, it's a good idea to provide some hysteresis (positive feedback) to prevent the the output from staying in its linear range. In fact, many camparators are upen-loop unstable, so it's essential to keep them out of their linear region to avoid oscillation for the condition when the input is very close to the reference.
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Regards,
Kral