what is the main input for the bandgap?

[wolf001]

Now I need to do AC analasis for a bandgap. But whar is the main input for the bandgap?

 

[pixel]

From my view it is Vcc.
When you put AC generator (with VDC=Vcc, ampl 1V) instead of Vcc you will see output voltage freq. dependance on Vcc noise.

 

[Fom]

Inside bangap you should disconnect feedback, insert DC source vdc=0V vac=1mV and run AC analysis. Input is one terminal of DC source output is another. If gain>1 when phase=0 the bandgap is unstable. For your schematic I suppose to insert the DC source into base Q79. Terminal connected to base will be input, another terminal is output.

 

[kkoko]

Bangap has not a input , but you can make noise analysis to view output noise