The ebook FEEE  Fundamentals of Electrical Engineering and Electronics is based on material originally written by T.R. Kuphaldt and various coauthors. For more information please read the copyright pages. 
Home AC Filters Bandstop filters  
See also: Bandpass filters, Resonant filters  
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Bandstop filtersAlso called bandelimination, bandreject, or notch filters, this kind of filter passes all frequencies above and below a particular range set by the component values. Not surprisingly, it can be made out of a lowpass and a highpass 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. Constructed using two capacitive filter sections, it looks something like this: The lowpass filter section is comprised of R_{1}, R_{2}, and C_{1} in a "T" configuration. The highpass filter section is comprised of C_{2}, C_{3}, and R_{3} in a "T' configuration as well. Together, this arrangement is commonly known as a "TwinT" filter, giving sharp response when the component values are chosen in the following ratios: Given these component ratios, the frequency of maximum rejection (the "notch frequency") can be calculated as follows: The impressive bandstopping ability of this filter is illustrated by the following SPICE analysis:
twint bandstop filter v1 1 0 ac 1 sin r1 1 2 200 c1 2 0 2u r2 2 3 200 c2 1 4 1u r3 4 0 100 c3 4 3 1u rload 3 0 1k .ac lin 20 200 1.5k .plot ac v(3) .end


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