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I never see open collector opamps while I frequently see open collector comparators. What is reason for preferring open collector output in comparators?

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(Inside of LM193.)

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4 Answers 4

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The difference is that the comparator output is a logic output that can only be on or off, and the opamp output is analog.

An open collector output is simple and flexible but relatively slow compared to other logic outputs. Since most comparators are slow, it is OK to use open collector outputs for them.

Fast comparators may have other types of outputs. For example, the extremely fast ADCMP580 has ECL outputs. Some people use it to generate fast edges.

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The main reason for using open-collector is so that several comparators can have their output connected together in a wired OR gate. All the open collectors can be tied together to a single resistor without any conflicts between comparators. This would not have been easy to do when your comparator has also the capability to source current at its output.

This wired OR gate is useful for detecting ranges of voltage, for example, when you want your output to be low when the signal is below a value or above another value.

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I don't think that's the "main" reason; there are many advantages. It's easy to use an open-collector output in an RC timing circuit with separately-controllable "on" and "off" delays. The comparator's output can easy be interfaced to higher- or lower-voltage logic. Any current switched by the comparator can come from the digital supply (thus keeping the analog supply clean). Etc. –  supercat Aug 21 at 17:27
    
I know there are many other advantages. But out of all, I believe the wired logic is the most important. Perhaps the simplest answer should have been "a solution is needed where the ON state pulls down and the OFF is high impedance so it does not interfere with the circuit at all when OFF" –  ceteras Aug 22 at 8:02
    
That would be a good way to put it. The fact that the output is floating in one of its states is often important, even if the pin is being shared with something other than another comparator output. That having been said, it's hardly uncommon to want a comparator with a true totem-pole output which can sit in either state without drawing much current, but they don't seem very common. An op amp may work, but a true comparator would be better. –  supercat Aug 22 at 19:28

An opamp is driving an analogue voltage. To be useful, it needs to control the voltage swing completely.

A comparator is a binary switch. Its output is effectively in only two states. It can work correctly with an external resistor. It doesn't need to drive the signal high when used with an external pull-up resistor.

An NPN transistor can pull down when it is on, so it could pull down on a voltage divider, making the device much more flexible than if it set the output swing internally.

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I always thought of comparators as the simplest A-D converter. You may want a different supply voltage on the analog side (maybe +/-15V or something) and the output going to a different digital voltage. (+5 V perhaps.) The open collector lets you easily adjust the output voltage reference.

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I think this is the best answer. In the saintly days of yore, when such things were being established as 'standards', logic would usually operate from 5V (maybe 10-15V if it was slow CMOS), and analog generally ran from +/-12 or +/-15VDC. So an open-collector output allowed the part to be used in the most applications. 'Tis a pullup, and nothing more. –  Spehro Pefhany Aug 22 at 15:40

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