About the phase margin of triple-op IA

[twteng]

Hi All:
I am designing a triple-op instrumentation amplifier as attached graph.
Do we need to consider the IA's phase margin or just take care of single OPA's phase margin?
From my simulation result, the phase margin of IA will vary with resistors R1&R2(close loop gain)and looks unstable, expecting your comments . thanks a lot.

 

[DarwinNE]

Just my two cents...

Well, what do you mean for "phase margin" of the IA?
From my point of view, the phase margin is something which needs to be considered for stability when there is somewhere a feedback loop. In the discrete IA, of course the three OPAmp's are used with feedback, so they must be stable in the chosen configuration. But once you consider the IA as a whole, you do not apply very often a feedback loop.