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I am trying to design a bandpass filter that would have cutoff frequencies at 150Hz and 500Hz. The main purpose of this will be filtering notes played on a piano (or generated pure tones) that are outside this range.

I have been trying a lot of architectures on Multisim but unfortunately all of them seem to be giving frequency responses in highly negative dB ranges (-100s), and some don't even behave in the way I expect them to.

I also noticed that changing the op-amps in my designs changed the behavior of my filters quite drastically. For example using LM741 and LMC6464 caused very different frequency responses. Also, in some cases connecting the negative rail to negative voltage made a significant difference compared to connecting it to ground. Unfortunately, since I don't have a way to get negative voltage (or at least not one that I'm aware of) I have to use ground.

I would appreciate any tips on building this filter and also any insight you can share into why I get this sort of behavior (negative dB freq response, completely different response with different op-amps). Even though I'm mainly interested in building an active filter, I'm open to any suggestions and designs in building this filter. Thank you very much for your time and help.

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  • \$\begingroup\$ One reason why you might be getting different results is due to different common-mode input voltaga ranges of the op-amps. First I advice you to choose na appropriate op-amp (operating voltages, common-mode input voltage range, bandwidth, offset voltage, ...). Also if you're going to have bunch of active filters, I would recommend to go for split power supply as it makes things much easier between stages. \$\endgroup\$ – Golaž Aug 11 '15 at 20:09
  • \$\begingroup\$ I think this question would be vastly improved if you posted a proposed schematic showing the input signal source - this would help us advise you most effectively. \$\endgroup\$ – stefandz Aug 11 '15 at 20:22
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There are several different active bandpass topologies available. The most common are Sallen-Key and Multi-Feedback. Show us your first attempt - so we can discuss on a realistic basis. If you are a newcomer in filter design I urgently recommend to use on of the filter design programs (online or downloadable). But note that you need a clear specification (filter order, mid-frequency, bandwidth resp. Q-value). And - if possible - use split supply only!

Selecting a suitable opamp should not be a problem in your case because the operational frequency range is sufficiently low (by the way: Don`t care about common-mode input voltage ranges). Most probably you got "strange" results (negative dB values) because the opamps were not powered correctly (single supply).

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