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