PSpice simulation. Bandpass-to-Notch

Discussion in 'Homework Help' started by Utah, Apr 19, 2011.

  1. Utah

    Thread Starter New Member

    Apr 19, 2011
    2
    0
    Hello everyone, I'm new here, but I'd like to ask a question if it's not too much trouble.

    As part of a homework assignment I've been instructed to build a bandpass filter with the specifications: f = 1kHz and Q = 6.

    This part went off without a hitch, but the second part of the problem instructs me to us "an external amplifier to create a notch". And this is where I've been stuck for the past three days.

    Here's what I've tried:

    1. Attaching an inverting amplifier to the output of the bandpass filter.
    2. Attaching a differential amplifier to the output of the bandpass filter.
    3. Attaching a summer to the bandpass filter (wherein I attempt to subtract the input voltage from the bandpass output.)

    None of these work and seem to just reproduce the bandpass. Below is my current schematic:

    [​IMG]

    Uploaded with ImageShack.us

    The resistor values of the summer are just random numbers I've plugged in. Here's the resulting output:

    [​IMG]

    Uploaded with ImageShack.us

    In all honesty, I don't know what's going on. In fact this is the solution that my professor proposed, but it doesn't work.

    Can someone help? It would be greatly appreciated.
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Are the suitcases in your schematic supposed to be opamps?

    You are on the correct track.
    At the peak of the bandpass filter the input signal and output of the bandpass filter have opposite phase and will cancel when summed.
    But the output of the bandpass filter is much higher than the input signal so the value of R6 must be increased.
    Simply replace R4 and R6 with one potentiometer and adjust it for a good notch.
     
  3. Utah

    Thread Starter New Member

    Apr 19, 2011
    2
    0
    Thank you!

    Yes, I'm using the suitcases as op-amp models.

    It didn't quite work, all that happens is that I can shift the signal up and down, not invert it like I want to. I tried your way again, and then set up a difference amplifier schematic and tried that out. Didn't work.

    I'm convinced at this point that there's just a defect in my software. Thanks so much anyways!
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Doesn't the multiple-feedback bandpass filter invert the signal at its peak?
     
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