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Question about active low pass filtering

Discussion in 'Audio' started by Chen Ron, Mar 29, 2018.

  1. Chen Ron

    Chen Ron

    1
    0
    Mar 29, 2018
    hello :) trying to analyze a circuit and couldn't understand this part of the circuit, in the description of the circuit it says its an active low pass filter. can anyone help me with analyzing this circuit? THANKS :)
     

    Attached Files:

  2. LvW

    LvW

    604
    143
    Apr 12, 2014
    There are three points to be answered:
    * Is there any negative DC feedback (stable bias point)?
    * What about the gain for low frequencies?
    * What about the gain for rising (that means: high) freqiencies?
     
  3. Audioguru

    Audioguru

    2,616
    591
    Sep 24, 2016
    1) The negative DC feedback is obvious.
    2) The gain for low frequencies is not affected by the 5n6 and 560p capacitors.
    3) "Gain for rising" might be talking about Easter. The mid audio frequencies in this filter are shown to rise only 2dB in my simulation:
     

    Attached Files:

  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,369
    1,903
    Nov 17, 2011
    Ignore the 10μF 100 kΩ RC combination at the input. Its purpose is blocking of DC components only. Then read on e.g. here.
     
  5. Ratch

    Ratch

    1,042
    308
    Mar 10, 2013
    OK, first I will do an analysis of the circuit using LaPlace transforms, and then I will check the results with a CAD program.

    The four simultaneous equations for the frequency analysis are as follows:
    ChenRonA.JPG

    The Bode plot of vout/vin is:

    ChenRonB.JPG
    And, the Bode plot of the CAD program is:

    ChenRonC.JPG
    Notice that the x-axis of the first Bode plot in radians and the x-axis of the CAD program is in hertz. Ask if you have any questions of how the solution was derived.

    Ratch
     
    Arouse1973 likes this.
  6. Audioguru

    Audioguru

    2,616
    591
    Sep 24, 2016
    The Bode plot of Vout/Vin has a peak at 4.3kHz and a slope of 12dB/octave. Why radians?
    The CAD plot has a peak at 7kHz and a slope of 12dB/octave.
    LTspice has a peak at 1.3kHz and a slope at 6dB/octave.
    Which one is correct??
     
  7. Ratch

    Ratch

    1,042
    308
    Mar 10, 2013
    The peak of Vout/Vin has a peak of 44000 radians = 7 kHz. The slope drops 30 dB from 10 kHz to 50 kHz. So, 10 2^x = 50 . x = 2.32 . The slope is 30/2.32 = 12.92 dB/octave. Radians is what my plotting routine does. I have not figured out to make it do Hz yet.

    The peak of the CAD is 7 kHz. The slope drops 30 dB from 10 kHz to 50 kHz. So the figures are the same as above for 12,92 dB/octive

    The peak of Ltspice is 1.3 kHz. I figure the slope to be 5.45 dB/octave.

    I think all presentations are correct. I suspect LTspice did not use an ideal op-amp like I did. I will leave it up to you to check and see. I have great confidence in my calculations for an ideal op-amp.

    Ratch
     
  8. Audioguru

    Audioguru

    2,616
    591
    Sep 24, 2016
    Almost any opamp works fine at 7kHz except a lousy LM324 and LM358. I doubt if LTspice would consider a 50 years old 741 opamp that has trouble above 9kHz.
     
  9. Ratch

    Ratch

    1,042
    308
    Mar 10, 2013
    I would check LTspice and see what parameters they are using. My manual derivation of the Bode plot and the CAD program seem to agree with each other.

    Ratch
     
  10. WHONOES

    WHONOES

    730
    150
    May 20, 2017
    I have tried it out on my trusty simulator using a TL071 and an Ideal opamp getting very similar results. They vary a little from what others have done.
    See attached plot.
     

    Attached Files:

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