# Circuit Theory/Filters

!Low pass filter !High
pass filter !Band Pass
filter !Band Stop
filter !Comb
filter Some terms used
to describe and classify linear filters:

## Frequency Response

-   -   Low-pass filter -- low
        frequencies are passed, high frequencies are attenuated.
    -   High-pass filter -- high
        frequencies are passed, low frequencies are attenuated.
    -   Band-pass filter -- only
        frequencies in a frequency band are passed.
    -   Band-stop filter or band-reject
        filter -- only frequencies in a frequency band are attenuated.
    -   Notch filter -- rejects just one
        specific frequency - an extreme band-stop filter.
    -   Comb filter -- has multiple
        regularly spaced narrow passbands giving the bandform the
        appearance of a comb.
    -   All-pass filter -- all
        frequencies are passed, but the phase of the output is modified.

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-   Cutoff frequency is the frequency
    beyond which the filter will not pass signals. It is usually
    measured at a specific attenuation such as 3dB.
-   Roll-off is the rate at which attenuation
    increases beyond the cut-off frequency.
-   Transition band, the (usually
    narrow) band of frequencies between a passband and stopband.
-   Ripple#Frequency_domain "wikilink") is the
    variation of the filter\'s insertion
    loss in the passband.
-   The order of a filter is the degree of the approximating
    polynomial and in passive
    filters corresponds to the number of elements required to build it.
    Increasing order increases roll-off and brings the filter closer to
    the ideal response.

