Electronic – Simulating and Building a Multiple Feedback Band-Pass Filter


I have a distorted signal, and only want to allow frequencies between 95kHz and 105kHz. The input voltage is at 300mV peak to peak.

Thus, I need a Pass Band of 10kHz and a Central Frequency at 100kHz.

I was reading through various analogue electronics books to find out some commonly used variations and topologies and decided to go for the Multiple feedback filter.

I will be using this circuitry:

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According to this document:

This circuit is widely used in low Q (<
20) applications. It allows
some tuning of the resonant frequency, F
, by making R2 variable.
Q can be adjusted (with R5) as well, but this also
s F

I then proceeded by follwoing the equations on that same document, or else on the book, Op Amp Applications Handbook.

My calculations and working is listed below:

First, we must determine the centre frequency, bandwidth, and Q.

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The Q is too high to use separate high- and low-pass filters, but
sufficiently low so that a multiple feedback type may be used.

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Before actually building this circuit I want to be able to simulate it. Here is my circuit implemented on Proteus software.

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And this is the respective frequency response:

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It may not be clearly visible, I apologize, but the center frequency is only at 63kHz.

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LM324 datasheet.

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At 100kHz the max output voltage swing is only 1V peak to peak, and thus I am keeping my gain levels, low. (AV = 2)

I followed the instructions, but clearly I am doing something wrong.

How can I get an actual center frequency of 100kHz, and what am I doing wrong?

Any tips and/or suggestions would be appreciated.

Best Answer

Your using the wrong simulator or the wrong opamp. Check the opamp bandwidth and make sure its sufficient (in the simulator, not just on a datasheet). I got 100kHz three ways in LT spice:

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The first circuit is using a 1 pole ideal opamp (no loss, no railing, and nearly infinite bandwidth)

The second uses an ideal opamp, but has parasitics (the caps have ESR and I added a small amount of inductance to simulate real world inductance)

The third uses an OP27

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