# Sallen Key Circuit for High Pass Bessel Filter.

Cuthbert Nyack
The applet on this page uses 2nd and 3rd order Sallen Key circuits to construct HP Bessel Filters with orders 2 to 8.
Even order filters 2, 4, 6, 8 can be constructed from 1, 2, 3, 4 second order sections. Odd order filters 5, 7 can be constructed from 1 third order and 1, 2 second order sections.

The applet calculates normalized and denormalized components and illustrates the magnitude and phase error introduced when any of the components are out by ± 20%.

The Circuits used are shown above.

Normalized circuit components are calculated by first calculating LP normalized values then replacing each resistor with resistance R ohms with a capacitor = 1/R Farads and each capacitor of C Farads with a resistor of 1/C ohms.
In the applet below, the filter shown is determined by Fn which is set by scrollbar (0). Fn = 1 to 7 shows filters with orders 2 to 8.
The normalized component values are shown by the green text on the applet.
The denormalized component values are calculated from the normalized values, the 3dB frequency set by scrollbars(38, 39 and 40) and the impedance scaling factor set by scrollbars(41 and 42). Denormalized component values are shown by the yellow text on the applet.
Scrollbars 11 to 30 are used to show the effect that inaccuracies in the component values can have on the circuit transfer function. The green text at the top show which component are affected by which scrollbar. eg R1 is changed by scrollbar (11) and C1 by scrollbar(21). The % change from the correct value is shown for each component.
Scrollbars (45 to 48) are used to change the gain of the error and T/F phase and magnitude.

The Image below shows a 8th order filter (Fn = 7) implemented with 3 sections and 3 opamps(8 resistors and 8 capacitors). Normalized components are shown in green at the top and denormalized components are shown in yellow at the bottom. In this case, the 3dB frequency is 200Hz and the Impedance scaling factor is 6.6666E7.
Pink and Cyan plots show the magnitude and phase error introduced when the capacitor C2 in section 3 is 20% less than its correct value. 20% change is shown in the normalized components at the top but does not affect the denormalized components at the bottom.
Numerical values of the errors can be seen at the bottom by using scrollbar 43 to move the white line.