Why I get negative voltage? How it can be?
There are several things going on here:
- The trace looks like you have that scope channel set to AC coupling.
- There is nothing pulling Vout up when V1 is high.
- It's hard to get into more detail without knowing the voltage ranges of V1 and V2, but Vout will follow V1 plus the B-E drop when V1 goes low. So if V1 goes negative by more than 700 mV or so, Vout is expected to go low.
- Overall you probably want to replace D1 with a resistor and have a transistor clamp Vout to ground or not. This means replace Q1 with a NPN and add a base resistor.