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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.

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  • \$\begingroup\$ A schematic or some idea of how you actually did the testing would help, ie where did the potentiometers go. \$\endgroup\$ – F. Bloggs May 16 '16 at 18:28
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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

enter image description here

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

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  • \$\begingroup\$ Rats. You beat me to it. \$\endgroup\$ – WhatRoughBeast May 16 '16 at 18:31
  • \$\begingroup\$ Sorry. It won't happen again :) \$\endgroup\$ – G36 May 16 '16 at 18:34
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You are violating the input common-mode range, so the behavior is not necessarily going to be what you might expect.

The name for this phenomenon is 'phase reversal' and many older amplifiers exhibit such behavior (eg. LM358). It's often not mentioned in the datasheet, but amplifiers which do not do it may mention that as a feature. The limit that you've exceeded is listed as common mode range and is probably something like 3-4V from each supply rail if you want guarantees over temperature. Typically, if you feel lucky it's much better at room temperature, but that's not engineering.

You can still use this op-amp as a comparator (unlike some newer ones that have diodes across the inputs) just take care not to exceed the input common mode range and be prepared to live with sluggish response. Perhaps you can clamp the inputs with diodes or divide them down.

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  • \$\begingroup\$ Thank you Spehro! I wish I could mark both answers as "accepted" but the other guy provided some useful information about "phase inversion"... \$\endgroup\$ – Metamorphic May 16 '16 at 19:12

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