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April 5, 2021 / consort3

2nd order Sallen & Key filter with independent frequency and damping tuning

2nd order Sallen & Key filter with independent frequency and damping tuning

The addition of 2 resistors to the equal component simple circuit gives the above. The  Q = 1/(3 – a) so a = 1.268 gives a Bessel filter, a = 1.586  gives a Butterworth filter and a = 2.234 gives a Chebyshev filter. When a reaches 3 the filter is self oscillating. So R4 is 2.7k for a Bessel , 5.6k for  Butterworth, 10k for a critically damped, 12k for a Chebyshev, 15k for a 6db peak filter and 20k for an oscillator. If you are a perfectionist make R5 5.6k and R4 respectively 1.5k, 3.3k and 6.8k for Bessel, Butterworth and Chebyshev filters.

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