I want to have a 1 stage analog active filter with a cutoff frequency of 26 kHz. I used the MATLAB analog filter design toolbox to create such a filter. The filter that was provided by MATLAB looks like follows:

Filter designed by MATLAB at 26 kHz cutoff freq

I recreated this filter in LTSpice and did a frequency analysis. It looks like the following:

Filter designed in LTSpice

However, when doing the frequency analysis, the filter gain is already at around -20 dB and at cutoff even lower (as expected).

Frequency analysis in LTSpice

Why does this filter already have such a low gain in the beginning, although I used the exact values that MATLAB gives me?

I changed the op-amp and the gain changed, so I suppose this has something to do with the op-amp. However, shouldnt the op-amp itself have quite a little impact on the frequency response (except the power bandwith product and transition frequency is low)?

Can anyone please explain me why this happens and what I am doing wrong? Would this circuit also behave like this in a real circuit?

  • 4
    \$\begingroup\$ The problem is in the way you supply your op-amp. You need to use a symmetrical supply or add a virtual ground circuit. \$\endgroup\$
    – G36
    Jan 8, 2023 at 15:44
  • \$\begingroup\$ Thank you! I knew that but here I thought it would make sense to supply it with non-symmetrical voltage. I will try it with symmetric voltage \$\endgroup\$
    – Vandrey
    Jan 8, 2023 at 16:50
  • \$\begingroup\$ Thank you! It worked! \$\endgroup\$
    – Vandrey
    Jan 8, 2023 at 16:59


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