How to build 2nd Order Opamp Filters

August 7, 2010 - category: Audio filter


The figures below illustrate using opamps as active 2nd order filters. Three 2nd order filters are shown, low pass, high pass, and bandpass. Each of these filters will attenuate frequencies outside their passband at a rate of 12dB per octave or 1/4 the voltage amplitude for each octave of frequency increase or decrease outside the passband.

  • First order low or high pass cutoff frequency (-3dB point) = 1/(2pi*R*C)
  • 2nd order low or high pass cutoff frequency (-3dB point) = 1/2pi(R1*R2*C1*C2)^.5
  • Example for 200 Hz cutoff frequency - R1=R2=7.95K, C1=C2=0.1uF

Circuit diagram

Circuit diagram

circuit from http://www.bowdenshobbycircuits.info/opamp.htm#2Ofilter.gif

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