[SOLVED] CMFB loop bandwidth requirement

[Junus2012]

Dear friends,

does the GBW of the CMFB loop should be equal to the differential op-amp GBW ot it has nothing to do with it.

Thank you very much

 

[FvM]

Application wise, you'll be looking for gain dependent closed loop bandwidth rather than gain bandwidth product.

The answer depends. High CMFB bandwidth is desirable if a true differential amplifier is driven by an (partly) unbalanced signal and wanted to balance it, otherwise CMFB performance is not so critical. For GHz amplifiers, it's unlikely to achieve similar CMFB bandwidth.

 

[Junus2012]

Thank you FvM for your reply

Can you please tell me the reason why my vote to help is limited ?

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so I would say the GBW of the closed loop CMFB can be less than or greater or whatever value demands our application, in all cases it has nothing to affect the characteristics of the differential GBW

 

[FvM]

Each member has a limited helpful points budget which is refilled periodically.

 

[vivekroy]

Your CMFB GBW does not need to be high as your differential amplifier GBW, and often its quite difficult to achieve it as your CMFB loop will see more poles than your differential amplifier. If you have large output swings, then because of the "non-linear" behavior of your transistors, the node going up and the node coming down will behave differently. A large CMFB loop GBW will try and reduce that. But its only a minor detail.