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Comparator Inputs ? Answered

On the two inputs of a standard comparator op amp (say for example  LM741)  they usually  connect resistors from both the positive and negative voltage rails .Why do they do this and how does this work . This has been on my mind for a really long time . Could someone give me a really detailed explanation maybe with images if possible , cause this would be really helpful in the building of my next project .Most of the tutorial videos out there , i don't understand either. Thank you in advance  

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framistan (author)2014-05-07

I understand your confusion, I had the same confusion many years until I built a couple things using comparators and also using op-amps. The schematic symbol is about the same for both circuits also. The difference is the part number. An Op-amp is just a regular analog amplifier. But they are more like PRE-amps not power amps. They will not drive a speaker. They are useful for amplifying low level audio such as from a microphone... then it is fed to a power-amplifier which drives a speaker. The COMPARATOR has 2 inputs (labeled + and - ) ... and it has ONE output. The output has no actual OUTPUT VOLTAGE. The output will either be GROUNDED or NOT grounded depending on the inputs. If one of the inputs is just SLIGHTLY higher than the other input, then the output is grounded. Just a few millivolts LOWER, then the output un-grounds. If a "PULL-UP" resistor is attached to the output pin and goes to PLUS Vcc. ... then it APPEARS that there is an output because the output resistor provides a voltage when the output of the comparator is UN-grounded. I forget which one + or - has to be higher or lower to cause the switching.

The best way to learn things like this is to just start building things... read other peoples articles... and keep trying. Slowly one piece at a time sinks in to your knowledge until pretty soon you can really understand most of what these electronic components are doing.

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steveastrouk (author)2014-05-06

An LM741 is an opamp, not a comparator. Can you show me a specific
circuit ? If an amplifier is being used on a single supply rail, usually
because its working with AC signals, then resistors like you describe
bias the inputs to mid-rail, and stop the output running away to the supply
rail. If an opamp is used "bipolar", connected to +ve and -ve rails,
then they are not needed.

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