bandwidth requirement to time domain
Thanks FvM and rames,
Yes, I think the UGB of the opamp should be determined by the application (especially the distortion) requirement of the filter. If the UGB is too low, at 20MHz the opamp gain is too low to make the closed loop (thst is, the filter) effective, which will lead to high frequency component peaking. So, the opamp UGB design is a kind of iterative process: start with a common sense (like 5 times the filter bandwidth requirement) value, test the circuit, if pass the spec, decrease the opamp UGB (to save power); if not, increase it. Correct me if I'm wrong here.
rames, I think you mis-understood my question.