This free op amp filter design program synthesizes both VCVS and MFB low pass, high pass, notch, band pass, and all pass filters. The program synthesizes filters from the Butterworth, Chebyshev, Gauss, Bessel, Papoulis, and Adjustable Gauss Polynomials.
If you are somewhat experienced with analog filter design, then you understand the importance of selecting the appropriate design polynomial for the filter. Having said this, you need to look at the Adjustable Gauss Polynomial which is incorporated into this program.
This program was written by an experienced engineer. As a consequence, the program only uses standard industry component values for the R’s and C’s. As shown below, the user can set the tolerance value for the components, 1%, 5%, etc. If the user selects 1% resistors for example, the program uses base values of: 1, 1.025, 1.05, 1.075, 1.1, 1.125, 1.15, 1.175, 1.2, etc. If the user selects 10% percent values, then the program uses base values of: 1, 1.2, 1.5, 1.8, 2.2, etc.
To design a filter, the program works by having the user set the capacitor to a preferred value, such as .01uF. It then determines the resistor values from that. The program does worst case simulation using the tolerances selected by the user.
The op amp model is rather simple, but adequate for most situations. It uses the op amp’s Gain Bandwidth and output impedance in the simulation.
Circuit files can be saved and screen shots can be printed. For the less experienced filter designer, the program’s help file offers guidelines for each of the filter structures used. A Do’s and Don’ts of Op Amp Filter design, if you will.