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I have basic knowledge about op amps and am trying to build a non-inverting amplifier. I build the circuit below using \$V_{CC} = 12V\$ and a salvaged dual-package op amp (BA15218N) from an old radio, in order to test if they work correctly (desoldering them was not my best work).

enter image description here

But I noticed that \$V_{out}=V_{in}\$ was not always true. With \$V_{in}\$ dropping below about \$1.4V\$, \$V_{out}\$ quickly increases until it reaches nearly \$V_{CC}\$ potential at \$V_{in} \approx 1.2V\$.

I found out, that operating these amplifiers near their supply voltage is generally not a good idea/possible, but nowhere was this extreme behaviour mentioned in any tutorial or datasheet. I also read the datasheet of this op amp multiple times to find the voltage range of the input and output pins, but to the best of my knowledge they seem to be missing.

Questions

  1. Is any op amp working as intended when \$V_{in} \approx 1.2V\$, \$V_{out} \approx 12V\$ with gain \$G=1\$, or is my unit defective?
  2. How can one determine the stable operating range of a specific op amp from the datasheet?

Thanks for your support.

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  • \$\begingroup\$ A number of op-amps claim they don't have phase reversal (the sort of behaviour you describe) which implies some op-amps do have it. However, that normally happens when inputs go outside the rail voltages, rather than simply approach the edge of the typical common mode voltage. It sounds like you shouldn't use those op-amps,whether they are good examples of the 15218 design, or partially dud. A long period unsoldering then could cause damage. \$\endgroup\$ – Neil_UK Feb 26 '17 at 5:42
  • \$\begingroup\$ thanks, the keyword phase reversal is nice to know and would have made explaining this a lot easier. \$\endgroup\$ – Araeos Feb 27 '17 at 5:45
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On page 3 it says that the common mode input voltage range is +/- 12 volt on +/- 15 volt rails. This means that if you approach either rail within 3 volts you have a problem. The typical value Might be 1 volt. Output voltage range is also limited (same page).

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1) There are opamps intended to work close to their power supplies voltages, named rail-to-rail opamps since the supplies are also called rails.

2) The datasheet for your opamp seems to be missing this important fact. As an example of a better datasheet, please take a look at the well-known 741

The output voltage swing shows that even for a small load of 10K, and for a symmetric supply of +-15V, the output voltage swing can be as far as 3V from the rails.

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