Why do we need a minimum voltage here?

Let's assume we have a circuit like this: (Us should be monopolar but I didn't find something like that). simulate this circuit – Schematic created using CircuitLab

Negative half wave: D2 is active -> output voltage = -0.7V
positive half wave: D1 is active -> output voltage = ?

Well.. I know that we have 5.7V output here but my problem is that I don't get why we need 5.7V at least here by Uin. I would assume that as we have a source here we don't need any other input. So our U in only controls which output gets active.

• Tim, I took the liberty of fixing your schematic. The convention is that higher voltages go to the top of the page (so I re-oriented Us) and GND symbols always point down (to the ground, as it were). – Transistor Sep 22 '18 at 16:13

This looks like an input protection circuit. The diodes prevent the voltage at $$\V_{OUT}\$$ from being higher than $$\V_{DD}+0.7V\$$ or being lower than $$\-0.7V\$$. The resistor is there to limit the current passing through the diodes.
• @TimSch You are not supplying the second source at 5.7V. There is an existing 5V supply for the electronics being protected. Your $V_{IN}$ is some signal coming from the outside world into the protected device. – Elliot Alderson Sep 22 '18 at 16:55