Op-amp nonlinearity

I bought a TL082CP biFET op-amp from Radio Shack the other day (two channel), and am trying to understand how to use them. I built the following simple voltage follower on a breadboard.

On the breadboard, the circuit works as expected when the pot isn't near the edges; it just gives me Vout=Vin (with some small error, of course). When I turn Vin above 11.4, I get Vout=11.4v. This is expected: the op-amp certainly can't give me more than its supply rail, and sacrifices some voltage while doing so. However, when I turn Vin below 1.4v, Vout jumps from 1.4v to 11.4v.

I wish to understand this (highly undesirable) latter behavior. From everything I've read, I would assume Vout would just bottom out at 1.4v, the minimum voltage it can deliver. I don't understand why it would jump to 11.4v. Worse, this behavior does not show up in simulation; the simulation (do a DC sweep with the parameters already in there) predicts Vout ranging linearly from 0v to 12v. I believe if you give the op-amp a higher range, the simulator bottoms out at 0 and tops out at 12v, but I haven't tried that recently.

My questions:

  • Why do the simulator and the real world disagree here? If this a limitation of the simulator, can you describe the limitation so I can bear it in mind?

  • What triggers this "jump to the ceiling" behavior? Is it that the inputs are too low, or the output has gone too low? How can I avoid this in more complicated circuits, like differential amplifiers?

  • I haven't yet experimented with any other op-amps. Is this unique to BiFET op-amps? I have three LM741's in my parts bin, for example.

  • I've been over the datasheet at http://www.ti.com/lit/ds/slos081g/slos081g.pdf. There is a lot I don't understand. Does anything in this datasheet indicate this problem, and what its parameters are? Why 1.4 volts minimum? Will that be the same for other op-amps?

Thank you.

by krohneew
December 09, 2013

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