How to design active filters
If you need a special active filter, e.g. for building active loudspeakers or medical electronic equipment, there are many ways you can go for designing this filter. You can use a book like Lancasters Filter Cookbook, "The Art of Electronics" by Horowitz and Hill or the books by Tietze-Schenk or von Wangenheim.
You can also use a filter design software.
We have read many books and have tested lots of programs. Finally we have arrived at the software AktivFilter. This software is easy to use and does a good job when you need an active filter with opamps. They offer a demo edition of this program which can be downloaded from their website for free of charge (freeware). The demo edition of course has some limitations but we have found that it is a very useful tool.
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The software AktivFilter 2.4 Demo Edition can be downloaded for free of charge from the website of SoftwareDidaktik (www.softwaredidaktik.de). The demo edition is able to design highpass filters and lowpass filters of order two. |
Using this demo edition we will show how to design an active filter with this software. We will start, creating a rumble filter, a highpass filter which can be used e.g. at the output of a record player to suppress signals at subsonic frequencies, i.e. below 20 Hz.
This highpass filter should have a flat amplitude response in the passband. This can be reached by a filter with Butterworth characteristic. To develop such a filter start the software AktivFilter, go to New Filter and choose Highpass with Sallen-Key circuit. We want to design a Sallen-Key highpass because it needs fewer parts than a filter with multiple feedback circuit:
A dialog appears which enables us to specify our filter:
The above picture shows all the settings we would like to do: Butterworth characteristic, second order (this is a limit in the demo...), cut-off frequency 20 Hz, impedance level 10 kiloohms, 0 dB gain, E24 resistors, optimized capacitors, sequence of stages for maximum signal level and the opamp LF411 (this small opamp list is another limit in the demo edition).
Let us click on OK and see what happens. The program calculates a while and shows a progress bar while doing this. After that the software displays the amplitude response of the designed filter:
This looks fine! The software has a built-in PSPICE interface, which we can use, for checking our designed circuit with PSPICE. This interface must be configured before using it the first time (a description for doing this can be found at the website of SoftwareDidaktik). We choose PSPICE and Amplitude response in dB from the menu and run the simulation. Here is the result:
As we can see the cut-off frequency is 20.13 Hz and the damping at 10 Hz is -12.383 dB. This is ok. If we need a better suppression at 10 Hz we must design a higher order filter - which cannot be done with the demo edition.
We exit PSPICE and choose Result and Component values.
All the values look fine. So let us save the result by choosing Result and Save.
Here is a screenshot, taken from the saved HTML-File:
