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

schematic

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.

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  • \$\begingroup\$ 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). \$\endgroup\$ – Transistor Sep 22 '18 at 16:13
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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.

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  • \$\begingroup\$ I agree, an ESD input protection circuit that nearly all ICs have looks similar to this but then there will also be two diodes at the node between Uin and R1. The function of an ESD protection circuit is to limit (clamp) the voltage Uout to the values mentioned in this answer. \$\endgroup\$ – Bimpelrekkie Sep 22 '18 at 16:06
  • \$\begingroup\$ My problem is that I would expect that the D1 part gets active once we reach 0.7V. Why do we need 5.7V? Or in other words why do we need to supply our second source? This is kind of confusing. \$\endgroup\$ – TimSch Sep 22 '18 at 16:11
  • \$\begingroup\$ The diode is forward biased when the anode is 0.7 V above the cathode. If the cathode is at 5 V (because of Us), this doesn't happen until the anode reaches 5.7 V. \$\endgroup\$ – The Photon Sep 22 '18 at 16:16
  • \$\begingroup\$ @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. \$\endgroup\$ – Elliot Alderson Sep 22 '18 at 16:55

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