I have design a differential buffer which is called fT doubler. I choose the RBS(random bit stream)as my input. When checking output, I find that it can't return to zero(subtract the DC). I don't know how to solve this problem, since I don't know how is this phenomena come out.

Any help will be greatly appreciated!

I'am so sorry to make you feel confused. Here is my schematic and Vin/Vout(Because I can't copy it from the computer I do simulation, So I draw it myself)