First, realize, that we are talking about the non-fedback OPamp, where you give 0.5V to the "+" input and 0V to the "-" input, means where the output has no chance to generate a suited voltage for the "-" input, via the standard feedback, to equal the potential at "+" input. So, the input is all the time overdriven and the ouptut stage is in saturation.

If you had such a feedback and you would give 0.5V to the "+" input and you had installed a gain of ten, for instance, then the OPamp would try to generate +5V at its output and it would succeed in doing this for a broad range of load resistances. So, it wouldn't make any difference whether there's no load at all at the output, whether it's 10kOhm, or 2.76kOhm, all the time +5V is produced.

There's a nice diagram in the datasheet of TL072 which tells you the typical minimum laod resistance needed to fabricate a certain output voltage:




So, in our example the TL072 could produce the +5V down to a load resistance of only 150Ohm (typically). If the load resistance furtherly decreases the output voltage will finally break down.

Take this diagram above to find an answer to your question...


Of course, is a pleasure...

Kai