Electronic – Why does bringing TL072/TL074 non-inverting input low cause output to go high


I was building a circuit using a Texas Instruments TL074 as a comparator, when I noticed some strange behavior. I isolated the chip and tested it with a couple of potentiometers. Basically, when I put two DC voltages on the inputs of one of the op-amps, everything works as expected except for the case where the non-inverting input is taken below about 0.8 V above Vee. Then the output swings high (when I would expect it to be low). Is this behavior common for op-amps, and is it described somewhere on the data sheet?

I don't have a TL071 to test, but I tried the experiment on a TL072 and found the same behavior. I have Vcc at +5V and Vee at -5V.

I know that I'm not supposed to use op-amps as comparators, but that's what I have on hand today.

Best Answer

Try learn something about allowed Common-mode Input Voltage Range. The Common-mode Input Voltage Range tells you the voltages at the inputs that cause them to work properly or to not work.
As for your problem look here

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And here at question "What other features of op amps should the user know about? " http://www.analog.com/library/analogDialogue/Anniversary/6.html http://www.planetanalog.com/document.asp?doc_id=528175