Suppose I have a circuit I want to power, nominally 6V DC, 500 mA max. The circuit could be modelled as purely resistive (an incandescent bulb, for example).
Let Vx and Vy be two different voltage sources. Each of these sources might oscillate around 6.2V and 0V (just an hypothetical value, meaning "slightly above nominal, but not much").
I would like to drive current only from the highest voltage source at every time, while turning the other off. The switching between them should be transparent to the powered circuit, so as it doesn't get voltage drops.
I have seen similar problems being solved with two diodes, but my proposed circuit could not afford the voltage drop, since most the time the max available voltage would be very near 6V or slightly below it.
I have thought about using an operatioal amplfier, perhaps combined with, say, some Mosfet(s), but I don't know how exactly that would be done, or if this problem has some better solution.
Also I would like a simple solution with "a handful of readily available generic components" instead of some pre-made module.