Active Filters

This section compares the gain-versus-frequency responses of the basic low-pass, high-pass, and band-pass filters. It also describes and analyzes the three basic filter response characteristics and other filter parameters.

Key Concepts

  • Low-pass filter (LPF)
  • Passive Low-Pass Filters
  • Active Low-Pass Filter (LPF)
  • Active High-Pass Filter (HPF)
  • Active Band-Pass Filter (BPF)
  • Microchip FilterLab® Filter Design Software
  • Butterworth Filter Topology
  • Chebyshev Filter Topology
  • Bessel Filter Topology
  • The critical frequency of a filter is called the cut-off frequency and defines the pass band of the filter at the point where the output reaches -3dB of the maximum.
  • Feedback or bypass circuits are used to determine the frequency of operation, usually a simple R-C network. Each network is called a “pole.”
  • The number of “poles” determines how rapidly the output decreases after hitting the critical frequency. This is called the “roll-off” rate.
  • Butterworth response provides a very flat amplitude response, referred to as a maximally flat response.
  • Chebyshev response provides a rapid roll-off and is characterized by variations or ripples in the pass band.
  • Bessel response exhibits a linear phase characteristic and is used for filtering pulse waveforms due to minimal distortion.
  • Order of a filter is the number of poles it has. (2nd order = 2 poles)
  • A high-pass filter passes frequencies above the cut-off frequency.
  • A band-stop filter is used to reject a range of frequencies.
  • A band-stop filter is also referred to as a notch filter.
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