When I simulate real-time clock crystal's open-loop with ac, there are two cases as attachment:
1)The BLUE result, it uses a ~15meg resisitor as the feedback for setting inverter's operating point. This result is what I need.
2)The PINK result. it uses several switches in series(a switch is a nmos and a pmos in parallel)  instead of the 15meg resistor for saving area. The result is strange and not reasonable.

Why? I only find case 2) when temperature is 120 degree. The other situations such as SS/0, SS/60, FF/0, FF/60, TT/0 and TT/60 are all case 1).

Thanks!
Trashbox