Question: Difference between comparators and op amps?

[Speakerguy]

Inspired by another thread, I was wondering how comparators differed from op-amps in their design and/or behavior. I know opamps can be used as comparators (although with poor performance relative to true comparators), and that I've never seen a comparator used as an op amp.

But if an op amp is designed to be a differential amplifier with as much open loop gain as possible (say infininite), what makes that any different from a comparator? Why does the application of feedback work well to linearize one system (op amp) but not the other (comparator)?

 

[Nigel Goodwin]

For one obvious difference, the comparator usually only has a single ended, open collector, output - an opamp can both source and sink current.

An opamp is also obviously designed to operate in a linear mode, a comparator isn't designed that way, there's no need for it to be so.

 

[Hero999]

The two main differences I can see is that a comparator lacks a compensation capacitor and has an open collector output rather than a push pull emitter follower output,

Both have as much gain as possible.

Using lots of negative feedback on a comparator will probably cause oscillation as it lacks the compensation capacitor.

I would also imagine that op-amps are designed to be more linear and low noise, both of which aren't critical in comparators.

Look at the diagram of the LM392 on the datasheet and compare the differences between the comparator and the op-amp for yourself.

 

[ericgibbs]

hi,
This explains it.

 

[Hero999]

I've read that article before and whilst I agree with it, there are plenty of op-amps that you can reliably use as comparators with no problems. The older op-amps like the :mu:A741 and LM358 seem to be more commonly used as comparators than the newer op-amps like the TL071. I presume it's because the older op-amps are more stable as they're not so highly engineered.

Also not that the article only covers using op-amps as comparators, not the other way round. I presume it's because they just assumed that no one would be silly enough to use a comparator as an op-amp.

 

[kchriste]

I presume it's because they just assumed that no one would be silly enough to use a comparator as an op-amp.

Microchip actually has an ap note that explains how to use the built in comparator on a PIC as a OpAmp. It's tip #12:
https://www.electro-tech-online.com/custompdfs/2007/12/41215C.pdf

 

[Hero999]

Which is handy if you dont want to add an op-amp just to amplify the output from a sensor but it isn't suppoed to be high quality.

What I was saying was that no one would go and buy a comparator IC if they wanted an op-amp but some people buy op-amp ICs and use them as comparators.

 

[ericgibbs]

Which is handy if you dont want to add an op-amp just to amplify the output from a sensor but it isn't suppoed to be high quality.

What I was saying was that no one would go and buy a comparator IC if they wanted an op-amp but some people buy op-amp ICs and use them as comparators.

hi,
I would agree, but sometimes we just happen to have them on the shelf.

The sensor comp/amp works fine for a slowly changing transducer sensor. [IIRC about 20HZ max]

 

[eng1]

I've recently looked for a voltage comparator + op amp in the same package, but it seems that the LM392 is unique (does anyone know other similar, better spec ICs?). I bought a few LM392s and they were not that cheap. I used them in very low frequency applications and they worked fine.
As usual one should consider/evaluate the requirements of the particular application he is designing (frequency response, slew rate, input currents, output voltage swing, etc). National's app note 74 explains how to configure an unused comparator of an LM339 as an operational amplifier, suitable for low frequency applications (for example it can provide a DC reference voltage for the other three comparators in the same package).

 

[kchriste]

What I was saying was that no one would go and buy a comparator IC if they wanted an op-amp but some people buy op-amp ICs and use them as comparators.

I agree with you 100%

 

[Hero999]

I've recently looked for a voltage comparator + op amp in the same package, but it seems that the LM392 is unique (does anyone know other similar, better spec ICs?)

I haven't seen any.

Perhaps someone should design a quality op-amp that can be used as a comparator or a linear amplifier and do both really well. If they made it cheap, robust and stable enough then I don't see why it wouldn't catch on.

 

[Roff]

I've recently looked for a voltage comparator + op amp in the same package, but it seems that the LM392 is unique (does anyone know other similar, better spec ICs?). I bought a few LM392s and they were not that cheap. I used them in very low frequency applications and they worked fine.
As usual one should consider/evaluate the requirements of the particular application he is designing (frequency response, slew rate, input currents, output voltage swing, etc). National's app note 74 explains how to configure an unused comparator of an LM339 as an operational amplifier, suitable for low frequency applications (for example it can provide a DC reference voltage for the other three comparators in the same package).

I found a few by Googling "op amp/comparator combination". Two of them were micropower, and consequently very slow. If you're interested, check out TLV2702 and LTC1541.
The MAX9000 has respectable speed, but is restricted to a 5.5V supply.

 

[eng1]

Thanks a lot, Ron.