I implemented a Miller OPAMP. After I have checked it reaches the desired gain and swing, I put it into the simplest form of closed loop with UNITY negative feedback. The schematic is as follows:
As you can see, the loop is simply a direct negative feedback. I expect the overall gain to be 1. So I am expecting the output signal is exactly the same one as the input sine. However, after a period of time, it fails. Really strange as it can follow in the few microseconds, but fail later.
The green curve that is covered and thus invisible is my input; the blue one is the output.
Where goes wrong and how may I fix it?