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As mentioned above, you're exceeding the common mode range of the op-amp.Ideally a comparator IC should be used, such as the LM393 or LM311, rather than a generic op-amp, which will be designed for linear operation.
I was just experimenting with comparators and found a circuit online. I had an LM741 so I gave it a go. When I used an LM393 all was well but I just wanted to understand what was happening.
I looked on the datasheet for "input common voltage range" but can't find it nor anything that mentions the 2V headroom. I do have difficulty with datasheets though: it says that for my LM741C it says that the supply voltage can be +/-18V but doesn't mention that it only works properly at the higher values.
It also includes an equivalent schematic from which it is possible to derive the input and output voltage ranges without too much trouble.
Thanks all.I was just experimenting with comparators and found a circuit online. I had an LM741 so I gave it a go. When I used an LM393 all was well but I just wanted to understand what was happening.I looked on the datasheet for "input common voltage range" but can't find it nor anything that mentions the 2V headroom. I do have difficulty with datasheets though: it says that for my LM741C it says that the supply voltage can be +/-18V but doesn't mention that it only works properly at the higher values. Oh well, new stuff learned and thanks all.
it says that for my LM741C it says that the supply voltage can be +/-18V but doesn't mention that it only works properly at the higher values.
Quote from: David Hess on May 04, 2018, 05:33:48 pmIt also includes an equivalent schematic from which it is possible to derive the input and output voltage ranges without too much trouble.Really. How is this done?