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Bizzare op amp buffer behavior - too much gain
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ChristofferB:
Hi! I'm messing around with an instrumentation amp board i'm designing, and I'm using an op amp to buffer the Vbias set by dividers.

The problem is, the op amp outputs WAY too high a voltage. It's set up as a unity gain buffer, but outputs 3,something volts with a 0.5v input.

I've tried different single op amps i had lying around (to rule out bad parts, and since the op amp choice wasn't that critical), I've tried an LM741, CA3100, and TL071, the latter throwing my voltage all the way up to V+! (24V).

The only logically thing being able to cause this over multiple op amps is that the feedback loop is open, or high impedance, but it ohms out ok. I've checked extensively for wiring faults and solder bridges and such, don't think that's the problem.

It shouldn't be a complex circuit at all, and I'm out of ideas to try. Help.

See attached schematic.

Thanks in advance!
SiliconWizard:
None of those opamps are rail-to-rail. There is no way they can output 0.5V with a 24V single supply AFAIK.
ChristofferB:
Damn, didn't consider that. But now I've bumped the input up to 2.5V and the output is still above 3V+. Wouldn't that be within range?

Thanks for your insight. That's definitely something I completely forgot to look into.
ChristofferB:
I've replaced the op amp with an AA battery as 1.5V DC reference and everything works beautifully. I'll order some pin-compatible rail-to-rail op amps.

What a blunder. But thanks for the quick help.
SiliconWizard:

--- Quote from: ChristofferB on January 23, 2019, 11:23:40 pm ---Damn, didn't consider that. But now I've bumped the input up to 2.5V and the output is still above 3V+. Wouldn't that be within range?

Thanks for your insight. That's definitely something I completely forgot to look into.

--- End quote ---

The TL071 as well as the 741 are both given with as much as 3V dropout from either rail (worst case, +/-12V max swing with a +/-15V supply), and a similar if not worse operating common mode input voltage range.

On top of that, the TL071 is known to suffer from phase reversal in non-inverting configurations when the input is too close to the negative rail (which is ground for you), which is exactly why the output went up to the positive rail.
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