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Classification of Filters

Depending upon the type of techniques used in signal processing, filters are classified as:
  1. Analog Filters, and
  2. Digital Filters.
Analog filters are designed to process analog signals using analog tech­niques, while digital filters process analog signals using digital techniques.
Depending on the type of elements used in their construction, filters are classified as:
  1. Active Filters, and
  2. Passive Filters.
A passive filter is built with passive components such as resistors, capacitors and inductors. Active filters, on the other hand, make use of transistors or op-amps (providing voltage amplification, and signal isolation or buffering) in addition to resistors and capaci­tors.
Depending upon the type of elements used, the operating frequency range of the filter will be different and accordingly the filters are classified as:
  1. Low Pass Filters,
  2. High Pass Filters,
  3. Bans Pass Filters,
  4. Band Stop Filters, and
  5. All Pass Filters.
  1. Low-Pass Filter It is a circuit that has a constant output (or gain) from zero to a cut-off frequency, fc and attenuation of all frequencies above fc.
Description: Description: 1352.png
Low-pass filter characteristics: (a) Actual (b) Ideal
  1. High-Pass Filter It is a circuit that attenuates all signals of frequency below the cut-off frequency and has a constant output (or gain) above this frequency.
Description: Description: 1401.png
High pass filter characteristics (a) Actual (b) Ideal
  1. Band–Pass Filter It is a circuit that passes a band of frequencies and attenuates all frequencies outside the band.
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Band pass filter characteristics
  1. Band-Rejection/Elimination Filter or Band Stop Filter or Notch Filter It rejects a specified Band of frequencies while passing all other frequencies outside the band.
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Band reject filter characteristics
  1. All–Pass Filter It passes all frequencies equally well, i.e., output and input voltages are equal in magnitude for all frequency; with the phase–shift between the two a function of frequency.
    This filter is also known as a phase-shift filtertime-delay filter, or simply the delay equalizer. One major application of an all-pass filter is the simulation of a lossless transmission line. The magnitude of the output voltage is the same as the input voltage but the output voltage is shifted in phase with respect to the input voltage.
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All pass filters characteristics
The highest frequency up to which the input and output amplitudes remain equal is dependent on the unity-gain bandwidth of the op-amp. At this frequency, however, the phase-shift between the input and output is maximum.

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