multiple feedback band pass filter question pls help me, thanks in advance

Discussion in 'The Projects Forum' started by stupidmonkies, May 26, 2009.

  1. stupidmonkies

    Thread Starter New Member

    Dec 10, 2008
    This is a circuit i get in a schematic. I search all over the internet and i get the result is this is similar to a multiple feedback bandpass filter but there is an additional capacitor between non-inverting input and inverting input and connect to ground. Theoretically when high frequency, capacitor have low impedance and so it will short this two input right? what will happen if we short it? If it's not to short the two input, what will it function as? I really scratching my head to figure it out but i just cant get it, hope someone can help me to figure it out, thanks in advance.
    Last edited: May 26, 2009
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    The 27pF value is very small compared with the main frequency determining components for the bandpass region. It won't have any influence on the designed pass band values. It's probably there to improve high frequency behavior or to minimize the likelihood of unstable circuit conditions arising.
  3. DickCappels


    Aug 21, 2008
    Yep. Besides that, its shunted by the feedback. Apparently, its only purpose is to reduce loop gain (and increase noise) at high frequencies.
  4. bertus


    Apr 5, 2008

    Texas made a nice application notes on filters.(see attached PDF)

  5. stupidmonkies

    Thread Starter New Member

    Dec 10, 2008
    I see, now understand already, thanks a lot. But i have another question, what if i add another capacitor (with value of 1uF) before the resistor at Vin? It would apparently became an high pass multiple feedback circuit right? Then is the 27pF capacitor still function as the same? or it became a 3 pole filter? but as i knew, 3 pole filter using only one op-amp will give us unstable output that went into oscillation right?
    Last edited: May 26, 2009