Butterworth Bandreject Filter

Discussion in 'The Projects Forum' started by Darknight7, May 21, 2012.

  1. Darknight7

    Thread Starter New Member

    May 21, 2012
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    The question;
    I have been trying design a second-order Butterworth band reject filter
    The voltage gain is |Av|=2
    and
    low cut off freq=10kHz
    high cut off freq=20kHz

    I used the f3dB equtions also decibel voltage gain table and transfer function equations; after that by using pspice I simulated the circuit and ac sweep analysis however I could not obtain the gain and I have a problem for output. How can I fix the error and obtain the gain? Should I add sth...
     
  2. crutschow

    Expert

    Mar 14, 2008
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    3,243
    Why do you expect a gain of two? All your amps have a gain of one. If you change the feedback resistor in the summing amp circuit from 50kΩ to 100kΩ, that will give you a gain of two.

    Is there another problem besides that?
     
  3. Darknight7

    Thread Starter New Member

    May 21, 2012
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    We did it thanks however; it is the same output and than we started to calculate again;
    we will share the results but have you any different idea?
     
  4. Darknight7

    Thread Starter New Member

    May 21, 2012
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    Is there any differeneces for the output graph of filter which I uploaded. I thought that graph is unexpected because of the right side is lower.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Yes, the right side of the graph should have a flat gain. It may not be due to the limited frequency response of the old 741 op amp. Try a more modern op amp with a higher gain-bandwidth.

    Note: ±20V is too high a supply voltage for a 741 (and most op amps). It should be no more than ±15V. Check the data sheets.
     
  6. Darknight7

    Thread Starter New Member

    May 21, 2012
    20
    0
    Thank you.
    I changed the values from 10V to 20V. However I can not use any otjer opamp because this is the pspice student version So I uploaded the graphs on the same figure; The High pass low pass and the output graphs you can see the differences can you make any comment. By the way thank you for help me you are the one who helped me in this topic! Thanks a lot
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,041
    3,243
    Why did you change from 10V to 20V?:confused: The maximum should be 15V.

    The curves look about right, with a second-order rolloff of 12dB/octave for each filter, but I measured a -3dB point of about 11kHz for the LP section and 26kHz for the HP section so your filter values are a little off. (Note that filter curves are normally plotted on a logarithmic or dB scale to make them easier to read).

    If you want a higher rejection value you will need to go to higher order filters or perhaps a notch type filter.
     
  8. Darknight7

    Thread Starter New Member

    May 21, 2012
    20
    0
    thank you again...
     
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