Active band-reject filter analysis

Discussion in 'General Electronics Chat' started by Let_07, Mar 16, 2017.

  1. Let_07

    Thread Starter New Member

    Mar 16, 2017
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    Hello :)

    Recently I've been trying to analyse an active band-reject filter in order to make some simulations but so far I didn't have success.

    The filter looks like this:

    [​IMG]

    Considering a capacitor of 1uF, how can I calculate the rest of the components having a low-cut frequency of about 20KHz and high-cut frequency of about 30KHz? Or better say central frequency of 25KHz

    Also I would like to have no gain, just to simply cut between those frequencies and be able to "mess" with the Q factor... However I'm really struggling to have the correct formulas. I know it might look easy but I'm not very experienced in this and I would like your help

    Thank you
     
  2. crutschow

    Expert

    Mar 14, 2008
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    It is not particular easy to analyze that circuit.
    Here's a reference on active bandstop filters.
     
  3. Nykolas

    Active Member

    Aug 27, 2013
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    It seems to me this is a band-pass, not band-reject filter. The gain is varied by changing the values of R1/R2. E
     
  4. AnalogKid

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  5. joeyd999

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    The "loop gain" is determined by Q, which is a function of R1 and R2.

    The overall gain is attenuated by voltage divider R3 and R4.

    Here's a spreadsheet I designed for designing 4th order circuits.

    Edit: Sorry, slightly different topology.
     
  6. Let_07

    Thread Starter New Member

    Mar 16, 2017
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    Thank you for the answers. I'll try to analyze some of the files sent.

    Regarding this being a band-pass filter, I did some simulations on LTSpice and obtained the following results:

    ltspice.png

    So it seems to work as a band-reject. Before I was doing some research on internet and bandpass filters look similar except that there are no resistors R3 and R4... (I'm not saying you are wrong, just providing the information I got from my research)

    So, I basically have a band reject filter, I just don't know how to tune it correctly expect from "throwing" random values at the simulation and hope for it to work, which is not really ideal solution :/
     
  7. joeyd999

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    You are correct. That accounts for the difference in topology I noticed.

    Sorry, I don't have a spreadsheet for that.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Perhaps a notch filter would better do what you want.
    Below is the LTspice simulation of a Twin-T active notch filter with adjustable Q.
    The output is shown for U1 Q-control pot settings of 10%, 50% and 95%.

    Note that for good response with high Q values at a notch frequency of 25kHz, the gain-bandwidth of the op amp should be at least 10MHz.

    The calculations can be found here.

    upload_2017-3-16_12-33-51.png
     
    Last edited: Mar 16, 2017
  9. Let_07

    Thread Starter New Member

    Mar 16, 2017
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    Thank you. Actually I was reading about the notch filters but they seem too "narrow" for what I wanted... However I put a Q control very low and it kind of worked in the end...

    Now I just have one problem, its there any way where I can make this calculations on my own? I mean, without using the calculator of the website? There is some transfer function there but still I don't see how I can calculate the values of resistors/capacitors.
     
  10. joeyd999

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    You've intrigued me with this circuit. I've never seen it before and I've searched far and wide trying to find an online example -- to no avail. I've only had time to do about half of the analysis. Maybe this weekend I'll be able to complete it and share my results with you.
     
  11. joeyd999

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    FYI, the top half of the circuit is a standard MFBF. The difference is in the lower half at the non-inverting input. I assume the attenuation network is designed to exactly cancel at the center frequency. Neat design, if it works.
     
  12. Let_07

    Thread Starter New Member

    Mar 16, 2017
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    To be honest I'm looking for some example of this circuit for ages, but for some reason Internet doesn't want to sharei it with me :D I can only find the schematics of the circuit and that's it. I find similar circuits, but not that one.
    That's why I asked for help.

    That would be nice, if somehow you manage to analyze it fully just write me something!
     
  13. crutschow

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    As noted in my simulation, the Twin-T center frequency if 1 / (4πRC).
    What else do you want to calculate?
     
  14. Let_07

    Thread Starter New Member

    Mar 16, 2017
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    I'm just trying to understand from where the calculations come from. About the center frequency I got it. But for example, is there any way to get the value of Q factor? In my case it is important because I need to analyze this Q factor and have the calculations would be nice. I mean, I know how to calculate this factor if I have already values for bandwidth, etc. But not when I have a scheme of the circuit on a paper.

    Also, from what I found everything tells that R1 and R2 are just 2xR3, as well C3 is equal to 2xC1/C2, but I can't find any explanation for this. Is there any?
     
  15. crutschow

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    Here's a reference that shows the calculation of the Q factor.
    It simplifies the equations and makes the response basically symmetrical around the notch frequency.
     
  16. The Electrician

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  17. Bordodynov

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    crutschow.
    Beside I appeared the problems with your symbol POT.sym. There is symbol In my library with such name, but several another.
    Could Not you load Your symbol?
     
  18. crutschow

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    Attached are the two LTspice files I use for the potentiometer.
    The .sub file goes in the sub directory and the .asy file goes in the desired symbol directory.
     
    • pot.asy
      File size:
      710 bytes
      Views:
      2
    • pot.sub
      File size:
      193 bytes
      Views:
      2
    Bordodynov likes this.
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