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cant find the tranfer function of 2nd order lpf

Discussion in 'General Electronics Discussion' started by ikhsanrino, Jan 9, 2016.

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  1. ikhsanrino

    ikhsanrino

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    Jan 9, 2016
    Hi guys . I have a a problem to find the transfer function of second order low pass filter. Ive tried it with the pattern of lpf second order filter . and try the TF with matlab but the result is different with the simulation use multisim ( cut off frequency) and also with the cut off equation of 2nd lpf. Anyone can help me to solve this? Thank you for your help , your help means alot for me
     

    Attached Files:

  2. LvW

    LvW

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    Apr 12, 2014
    Your circuit is one of the classical 2nd order structures: Unity-gain Sallen-Key lowpass.
    Google for it - and you will find many, many answers.
     
    Arouse1973 likes this.
  3. ikhsanrino

    ikhsanrino

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    Jan 9, 2016
    thank you fore your answer, i appreciate alot mate, but im still confuse about the resistor on the negietive input, how about the resistor on the ngetive input ? is that a gain or ? it doesnt gives any impact to the circut and transfer function ?
     
  4. LvW

    LvW

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    Apr 12, 2014
    This resistor is not very imporetant - and it does not influence the transfer function.
    The only purpose is as follows: There is always a small dc bias current into the opamp inputs. At the non-inv. input this current produces a small dc voltage which may cause a small dc offset at the output. This dc value can be reduced if we place a resistor with a corresponding value also at the inverting input.
    But - as mentioned - you can use such a ressitor or not. It does not alter the transfer function.
     
    hevans1944 and Arouse1973 like this.
  5. ikhsanrino

    ikhsanrino

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    Jan 9, 2016
    Thank you very much for your explanation mate, it helps me alot. Greeting from indonesia !
     
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    hevans1944 likes this.
  7. Ratch

    Ratch

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    Mar 10, 2013
    R3 is very important to the operation of the circuit because it provides a conduction path to the negative input of the op-amp, and therefore negative feedback. Its value is not important because it will probably be insignificant compared to the input resistance of terminal 2 of the op-amp. Since the input resistance of terminal 2 swamps out the value of R3, the voltage of terminal 2 and 3 will be the output voltage of the op-amp. Finding the transfer function below is a simple matter of finding the solution of a couple of node equations. Notice that R3 does not enter into the transfer function. The derivation is shown below. e1 is the voltage between R1 and R2.

    ikhsanrion.JPG

    Ratch
     
    Last edited: Jan 13, 2016
    Arouse1973 likes this.
  8. LvW

    LvW

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    Apr 12, 2014
    Of course, I mean that the VALUE of this resistor is not very important - and it can be replaced by a simple short. Otherwise, we wouldn`t have a unity-gain amplifier.
     
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