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signal chain filtering - filter poles

How does feedback in active filters account for the creation of complex poles?

  • A passive RC circuit has a real pole or zero only, it’s pole or zero is on the real axis of the complex s-plane. Using active feedback, shifts a real pole from the real-axis to two points, one between negative real axis and positive j-axis and one between negative real-axis and negative j-axis (the left plane region) thus generating a complex pole pair. For example, a passive RC, second order real pole, lowpass filter can be transformed by feedback into an active RC, second order, complex pole pair lowpass filter.