For an Elliptic filter, a circuit with both poles and zeros is required. These circuits are usually referred to as Biquadratic (or Biquad) circuits. Several single opamp Biquad circuits are available in the literature. Here the Biquad based on the State Variable circuit is used. Although expensive in opamps it is easy to control, less sensitive and the easiest to understand.

The circuit for a second order section is shown below. If the Capacitor C6 is added in parallel with R6 then the circuit can be used for a third order section.

Values of R and C and the upper 3dB frequency in Hz can be set by scrollbars (4,5), (6,7) and (38,39,40). These values are used to calculate denormalized component values which are shown in green.

Image below shows component values for a 9th order Elliptic high pass filter implemented as 3 second order and 1 third order section.

r = 20 and q = 43°.

R is set to 100KW, C = 0.1 mF and the 3dB frequency is 2kHz.

The normalized transition BW is ~0.466rad/s, the minimum stop band attenuation is > 100dB and the pass band ripple is 0.177dB.

COPYRIGHT © 2011 Cuthbert Nyack.