@TimFox:
Albeit quite expensive, the CA3130A has been added to my shopping list. You can't have everything, I guess, so let's see how it turns out. Otherwise I will need to reconsider going dual rail. Thanks for that recommendation.
At least it may be helpful in practical understanding some of the other comments in this thread about specs and the limits of it for an opamp.
@blueskull:
I do not need to source 40mA, I would be happy with 20. And probably could even go down to 15mA. But that's about the limit. The 15V are set in stone. The 40 mA output current of the LM324 just gave an extremely confident headroom. My assumption: The larger the headroom, the higher the chances to actually meeting the spec. Sorry, if this lead to any misconception.
As I am looking for a single supply buffer, that works from ground up, I do not need to sink any current and the gain is (ideally) one. Now while I do accept, that there may be cases, where a voltage follower may need to sink current, I did not sink about this scenario. Think.
However, with the on following discussion, as far as I have been able to follow it, it was rather helpful for my understanding, how any why sinking current affects the specs. Especially in term of reading a datasheet. Or at least I hope so. Practice will show
So thanks all again for your time and the explanations
Do you want to source or sink current?
Sourcing current is easy, because the lower output transistor can be off. You also won't be sourcing the full current, at the lowest output voltage, unless you're using it to drive an extremely low load impedance. 15mA, with an output voltage of 5mV, would be a load impedance of just 300mOhm. If it has a class A stage, such as the CA3130A, it will be gauranteed to output 0V, as long as the output stage is not sinking any current, as no current will be flowing through the output stage.
Sinking current is more tricky, because the lower transistor in the output stage needs to have a low enough resistance, to get below the required on voltage. As above, sinking 15mA, with an output voltage of 5mA, would require an on resistance of 300mOhm and no small op-amp IC can do that, because the output transistors will be too small. You might be able to use a ridiculously over-rated power amplifier IC, designed for Amps, but it's likely to have a poorer offset voltage. A discrete output stage, as mentioned above, could be added, but it would be very challenging to stabilse and avoid oscillation.