stability of opamp

[bloodynine]

in opamps why is feedback factor =1 considered worst for stability ?

 

[danadakk]

Because thats the wider BW hence greatest phase shift, eg. as BW rolls off its
acting as a LPF and phase growing.

Note also C loading creates phase shift. As OpAmp G drops its output Z rises, that
in turn with Cload creates more phase shift.

Additionally fdbk R and stray C at OpAmp input pin adds more phase shift....

And fast OpAmps, typically two pole, even more phase margin challenges :

Its simple complicated.....

 

[nitishn5]

My Version...

Assuming that the feedback factor is generated by a purely resistive network.

The feedback factor value just "shifts" the Open Loop Gain Plot of the Bode Diagram Up or Down. It does not affect the Phase Plot.
So a Feedback Factor of <1 will have less Open Loop Gain and hence be more stable.


PS Feedback Factor <1 is what we would use for an Opamp with Voltage gain >1.

 

[LvW]

he feedback factor value just "shifts" the Open Loop Gain Plot of the Bode Diagram Up or Down. It does not affect the Phase Plot.
So a Feedback Factor of <1 will have less Open Loop Gain and hence be more stable.

Just to avoid confusion, we should clearly distinguish between
* the opamps "open-loop gai Aol" - that is the gain without any feedback,
and
* the "loop gain Aloop" of a circuit with feedback (a feedback loop) - that is the gain around the complete loop Aloop=Aol*feedback factor.

 

[danadakk]

Gain Phase plots for various G :

beta fdbk factor.

Regards, Dana.