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twin T circuit

Discussion in 'Electronic Basics' started by lerameur, Apr 8, 2007.

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  1. lerameur

    lerameur Guest


    I would liek to knwo if someone has the equations for a Twin tee RC
    active filter. I am doing the circuit from analof filter from Les
    Thede. I do the circuit they have in the book , It looks very good on
    my simulation, but when I try ti implement my circuit, I am way off.
    My stop band is wider then his example, Does Twin tee only take
    narrow band ??

  2. ehsjr

    ehsjr Guest

    Lots of web sites. Here's one:

  3. Phil Allison

    Phil Allison Guest

    ** Groper Alert !

    ** Got a low drive impedance and high load impedance ??

    ......... Phil
  4. Phil Allison

    Phil Allison Guest

    "Phil Allison"

    ** For high Q factors - ie a sharp notch use this circuit.

    ........ Phil
  5. john jardine

    john jardine Guest

    Few people will have the expensive Theade book. Best if you could post the
    circuit somewhere, as there are any number of possible TwinT arrangements.
    Would suspect the wide stopband is down to not enough gain from a feedback
    opamp, leading to a low Q notch. Use a pot' for the feedback resistor. Q's >
    say 5 are usually quite critical as to the gain setting resistor value.
    Fractionally too much gain and the whole lot oscillates.
  6. lerameur

    lerameur Guest

    Thanks for the replies.
    I am looking for a circuit with active element not passive.
    The Lm102 looks good but there is not enough tuning.
    My bandstop extend from 1k to 10Khz
    at 2Khz i want it to be at -30dB

    here is a link to my circuit wi th the results, Its almost there, But
    I think this is the maximum i can obtain right ?

    the %7E is a tilt, works either way :)

  7. john jardine

    john jardine Guest

    Circuit seems massive overkill.
    Just one stage could give better results than what you're showing. (also
    need a lot more data points in your sim).
    Is that overwritten "RU4841" gain resistor on the left really "90" ohms and
    not 30k?.
    Why R160 and C90?. Seems no reason for 'em.
  8. Phil Allison

    Phil Allison Guest


    ** Groper Idiot Alert !

    ** Then fucking say so - you moron.

    ** Fucking Say What YOU want in the FIRST POST -


    .......... Phil
  9. lerameur

    lerameur Guest

    actually they are pretty usefull,
    AS for the 90 Ohm resistor, It do not matter to what I chosse, it is
    again. From what I calculated, it did make any sens, I obtained a
    negative value

    I have joined on the web site the steps that I used to obtain my
    circuit (in a pdf).

  10. john jardine

    john jardine Guest

    The TwinT acts like a series resonant LC circuit and rejects only -one-
    particular (designed) frequency. Rolloff slopes dependant on designed Q. You
    will not achieve the 1k-10k bandstop using it.
    You should be designing something like a third order 10kHz low pass followed
    by a third order 1kHz high pass.
  11. lerameur

    lerameur Guest

    Would that create a bandstop or bandpass ??

  12. john jardine

    john jardine Guest

    Oops, yes, bandpass. (my bad!)
    Having reread your wants, it looks you're after a helluva bandstop, needing
    at least 5th order filters plus some kind of (probably inadequate)
    Maybe yes, that 3 section TwinT is looking the way to go.
    Fired up the filter prog and poked around a bit. There's lots of
    arrangements that will do the job nicely but they all use thousands of bits.
    For minimum components and in line with what you've already built, I looked
    at the TwinT arrangements.
    As you calculated, your arrangement needs a negative resistor in the first
    stage, so that's out.
    Using inverting biquads gets rid of the negative element but then there's a
    problem with poor rejection.
    Further poking around turned up a nice 3rd order Elliptic design that will
    nicely do the -30dB at 2kHz (1% ripple).
    Total time steering the prog' was about 15 minutes. Wouldn't have dreamt of
    doing the same manually!.
    If of interest, I could put the circuit diagram on A.B.S.E. or email it.
  13. Component tolerance is a killer with Twin-T circuits ... that's most likely
    why your simulation works and your breadboard does not.
  14. ehsjr

    ehsjr Guest

    The passives determine the frequency in the Twin-T. Mismatched
    passives will minimize the notch depth. The last time I built
    one of these, the math predicted great results, but real world
    components yielded disappointing results. By analogy, you may
    be focussing on improving the transmission in your car, when
    the real problem is that the engine doesn't work.

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