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stability issues of sallen-key active filter

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samiran_dam

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Dear all,

Could anybody explain to me the stability issues (if any) of Sallen-Key Active Filter?. There are two loops - i) frequency-independent negative feedback loop and ii) frequency-dependent positive feedback loop (this loop's feedback-factor's transfer function is bandpass type). Will there be any phase-margin problem in this kind of filter?

Regards
Samiran
 

Of course, some phase margin problems can be expected for each active filter as soon as the designed pole Q is relatively high. For example, consider an equal-component S&K low pass filter with a design value of Qp=10.
In this case, the non-inv. gain is G=3-1/10=+2.9. That means you are very close to the stability limit. If G unintentional reaches a value of 3 the circuit will become unstable.
 

Yes you are right. Could you please refer any paper/docu which may come into help?

Regards
Samiran
 

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