9.5: Band-stop Filters

Electric Circuits II Alternating Current · LibreTexts

0%

Contents

    Text

    Size
    100%
    Theme
    Typeface
    Line length
    Spacing

    From LibreTexts, under GNU Free Documentation, as the source states it; its equations are written in TeX, which the PDF shows typeset. This page at LibreTexts.

    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. Not surprisingly, it can be made out of a low-pa ssand a high-pass filter, just like the band-pass design, except that this time we connect the two filter sections in parallel with each other instead of in series. (Figure below)

    02261.png

    System level block diagram of a band-stop filter.

    Constructed using two capacitive filter sections, it looks something like (Figure below).

    02123.png

    “Twin-T” band-stop filter.

    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, giving sharp response when the component values are chosen in the following ratios:

    12098.png

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

    12099.webp

    The impressive band-stopping ability of this filter is illustrated by the following SPICE analysis: (Figure below)

    jj.PNG

    22032.png

    Response of “twin-T” band-stop filter.

    Review

    • A band-stop filter works to screen out frequencies that are within a certain range, giving easy passage only to frequencies outside of that range. Also known as band-elimination, band-reject, or notch filters.
    • Band-stop filters can be made by placing a low-pass filter in parallel with a high-pass filter. Commonly, both the low-pass and high-pass filter sections are of the “T” configuration, giving the name “Twin-T” to the band-stop combination.
    • The frequency of maximum attenuation is called the notch frequency.

    This page titled 9.5: Band-stop Filters was last modified on Sat, 20 Mar 2021 03:11:43 GMT and is shared under a GNU Free Documentation License 1.3 license and was authored, remixed, and/or curated by Tony R. Kuphaldt (All About Circuits) via source content that was edited to the style and standards of the LibreTexts platform.