16.6. Band-Rejection Filter Design

A band-rejection filter is used to suppress a certain frequency rather than a range of frequencies.
Two of the most popular band-rejection filters are the active twin-T and the active Wien-Robinson circuit, both of which are second-order filters.
To generate the transfer function of a second-order band-rejection filter, replace the s term of a first-order low-pass response with the transformation in Eq. (16.18):

ΔΩs+1s

image (16.18)
which gives

A(s)=A0(1+s2)1+ΔΩ·s+s2

image (16.19)
Thus the passband characteristic of the low-pass filter ...

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