Tuesday, 29 August 2017

voltage - 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:


enter image description here


According to this document:


This circuit is widely used in low Q (< 20) applications. It allows some tuning of the resonant frequency, F 0 , by making R2 variable. Q can be adjusted (with R5) as well, but this also change s F 0 .


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.



enter image description here



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.



enter image description here


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


enter image description here


And this is the respective frequency response:


enter image description here


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



enter image description here


LM324 datasheet.


enter image description here


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.



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:


enter image description here



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


enter image description here


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