Cascading two filters - Buffer amplifier?

Discussion in 'Homework Help' started by pianolife, Jan 14, 2013.

  1. pianolife

    Thread Starter New Member

    Nov 15, 2012
    Dear members of AllAboutCircuits, for an Electronics lab report, I need to cascade a high pass filter and a low pass filter, both with the same components:
    - 2 x 100 nF capacitor
    - 2 x 1kΩ resistor.
    I need to answer the question: "Why the effect is not purely the sum of the two previous filters?".

    Shall I say that we need a buffer amplifier?
    Attached there is the bode plot for these two cascaded filters.
    The cutoff frequency is 1592 Hz, and, as expected, the gain is the maximum and the phase shift is the minimum at this frequency.

    How can I explain this?
    Thank you so much

    All the best
  2. JDT

    Well-Known Member

    Feb 12, 2009
    That would be a practical engineering solution but it's not the answer to the question you been given.
  3. pianolife

    Thread Starter New Member

    Nov 15, 2012
    If I am not wrong, this "wrong" behaviour is due to loading effects of the first filter on the second. Is that correct?
    What I wrote in the notes is: "transfer function based on voltage divider, which doesn’t hold up in this case due to current being drawn

    Have I missed anything? I don't get this point.
  4. Veracohr

    Well-Known Member

    Jan 3, 2011
    I believe you're right - loading is the reason.
    pianolife likes this.