Taking feedback from a band-limited location

I am aware of the scenario where an isolation resistor is used to decouple a capacitive load from an opamp. If you then want to take feedback from after the isolation resistor you will be taking feedback from a location that is low pass filtered (because of the capacitive load combined with the isolation resistor). Stability is maintained by placing a small cap between the opamp output and the inverting input to provide a high frequency feedback path.

This is great, but what do you do if you want to take feedback from a location that is bandpass filtered? For instance if you used an opamp to drive a transformer, and you wanted to take the feedback after the transformer to account for the non-linearity of it. I understand this isn't really a perfect example, since transformers can have a fairly wide bandwidth. But still, the situation must come up...

Do you just provide a low frequency and high frequency feedback path directly from the opamp output (i.e. a cap and an inductor). Do the three feedback paths then have to be coupled to one another? What if you want to limit the inductive loading?

Thanks,
Jay

It depends what you want to do! As transformers always have a LF roll-off, it is cute to use the NFB to increase the gain to maintain a flat frequency response. How ever the phase shifts within the transformer can with excessive B/W and gain result in oscillations. You get the same case at the HF end of the band. So proceed with caution and try to measure/calculate the phase shifts at the band edges and sort out your time constants and gain to keep the beast under control.
Frank

cfrequencyhuckey said:

It depends what you want to do! As transformers always have a LF roll-off, it is cute to use the NFB to increase the gain to maintain a flat frequency response. How ever the phase shifts within the transformer can with excessive B/W and gain result in oscillations. You get the same case at the HF end of the band. So proceed with caution and try to measure/calculate the phase shifts at the band edges and sort out your time constants and gain to keep the beast under control.
Frank

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Thanks for the reply Frank,

I was thinking of something simple to start with, audio range only, so 20 to 20k. An xformer shouldn't have much phase shift in that range, so that is one less problem. So I suppose the whole system could be a bandpass filter.

Can you recommend any documentation to look at? I have tried a few setups and haven't had much success. I'll post some more specific problems after I try I couple more setups.