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I have this simple non inverting setup made with a TL082IP Op-Amp and a gain set to 2 (R1 = R2). Since I will only have positive voltage 0V to 5V to amplify, I use for simplicity a single 12V/0V supply voltage (V2).

schematic

simulate this circuit – Schematic created using CircuitLab

If I vary the V+ voltage source between 0V and 5V (V1), I would expect Vout varying between 0 and 10V, which is what the simulation give me.

But in practice, everything works fine between 5V and ~0.8V, then between 0.8V and 0V the Op-Amp saturates at +Vsat ~12V.

What have I missed ? I don't really need precision at low voltage, but at least I would like that low voltage at V+ give me low voltage at Vout, how could I achieve this ?

Thank you !

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Look at this extract from the TL082 data sheet and learn: -

enter image description here

The input signal is invalid if it gets close to the negative rail (within 3 volts). The negative rail is 0 volts in your circuit hence anything below +3 volts is likely to be problematic.

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  • \$\begingroup\$ Thanks ! I understand the concept but I'll have two questions : 1/ How do you deduce the 3V you mention from the data you show ? 2/ Is it possible to suppose before testing it if an Op-Amp will just "clip" Vin at a low level (which would be ok for me) or saturate at +Vsat ? \$\endgroup\$ – Remy F May 14 '18 at 14:59
  • \$\begingroup\$ In the red box ther supplies are stated as being +/- 15 volts and the lower typical range is quoted at -12 volts. Tthis means there is a 3 volt difference and it is fine to rejig this value and assume a negative potential of actually zero volts but then the -12 volts becomes + 3 volts. \$\endgroup\$ – Andy aka May 14 '18 at 15:08
  • \$\begingroup\$ I'm not sure I can digest what part 2 of your question means. \$\endgroup\$ – Andy aka May 14 '18 at 15:09
  • \$\begingroup\$ Ok that's obvious when I read you... For the part 2, I agree it's hard to explain in a comment, english being not my mother tongue... Let's say that just for example if you look at this link edn.com/design/analog/4368301/… I have here something like the Fig.6 behaviour whereas Fig.4 would suit better for my project. So is there a way to guess the non-linear behaviour without testing it, or not ? And anyway thank you, I'll accept your answer. \$\endgroup\$ – Remy F May 14 '18 at 22:07
  • \$\begingroup\$ The bottom line that engineers take is to try and design circuits that remain well constrained within power rails and if this looks like an impossibility rail to rail devices have to be chosen. Many opamps work to within 2 Volta of the power rails ie the TL082 is a tad poor in that respect so I guess, it’s down to recognition and experience. After all, you quickly learn to change into 2nd gear and 3rd gear when driving a car. \$\endgroup\$ – Andy aka May 14 '18 at 23:27
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This OpAmp seems not to be Rail-to-Rail, meaning that the output signal does not take values from 12V-0V (VCC to GND) but will saturate when nearing those values.

If you want to have to full 0V to 10V range, you should either select an OpAmp with Rail-to-Rail output or instead of GND on the negative supply, feed it with some negative voltage so the output can swing down to 0V

You can find more information on Rail to Rail OpAmps in this article.

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