Band-Stop Filters

Also called band-elimination, band-reject or notch filters, this kind of filter passes all frequencies above and below a particular range set by the component values. It can be made out of a low-pass and a high-pass filter, just like the bandpass design, except that this time we connect the two filter sections in parallel with each other instead of in series.

FIGURE

Constructed using two capacitive filter sections, it looks something like as shown in the figure below.

FIGURE

The low-pass filter section is comprised of R₁, R₂ and C₁ in a "T" configuration. The high-pass filter section is comprised of C₂, C₃ and R₃ in a "T" configuration as well. Together, this arrangement is commonly known as a "Twin-T" filter. Component value ratios for the "Twin-T" band-stop filter:

Given these component ratios, the frequency of maximum rejection (the "notch frequency") can be calculated as follows:

FIGURE