what is the role of this two switch?

[wjxcom]

Hi, all: this circuit is a full differential OPA whose CMFB is controlled by switchs and capacitance. but there has a question: what is the role of the switch M19 and M22? I think the role of the switch of M20 and M21 is clear. But why there have M19 and M22? I do not know how to analyse the circuit.

Help me. please!!

 

[VSMVDD]

they are used as an active bias adjust in the gain stage
to limit any overdrive signal

 

[wjxcom]

Hi VSMVDD: can you explain this problem detailed? or can you introduce some data about this problem to me?
Regards!!!

 

[flushrat]

dummy device to absobe charge injection

 

[wjxcom]

Hi, flushrat: your answer remind me that the book writed by Razavi has mentioned this kind of circuit.
thacs flushrat, and thacs VSMVDD.