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Active low-pass filter with gain

Discussion in 'Electronic Basics' started by Michael, Mar 6, 2008.

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  1. Michael

    Michael Guest

    I'm using an LM741 as an active low-pass filter. Non-inverting input
    with output capacitor feedback between two series resistors. This works
    fine with unity gain (output tied back to inverting input). However, if
    I try to increase the gain using a resistor divider to feedback to the
    inverting input, I stop getting any output (however, if I remove the
    inverting feedback completely, I seem to get an extremely large gain).

    Any ideas on what's happening.

  2. Noway2

    Noway2 Guest

    It is difficult to fully understand your circuit by your description,
    especially the part about "output capacitor feedback between two
    series resistors".

    I would first suggest using a a standard filter topology, suck as a
    Sallen Key, which will reduce the filter design to matching against a
    set of normalized charts and then scaling for frequency and amplitude.

    Another way to look at this is to remove the frequency selective
    components (ie the cap), and concentrate on the amplifier portion
    itself. A non-inverting amplifier configuration has a gain of 1 +
    R_feedback / R_input. Analyze the circuit until you understand why.
    The opamp will TRY to drive the output such that the voltage between
    its input terminals is zero. This creates a 'virtual short' between
    the inputs which will simplify your analysis. Once you have the
    equation (transfer function) describing the amplifier, you can put the
    capacitor back in the circuit and re-analyze. You will want to use
    the LaPlace transform when performing your analysis to convert the
    differential equations caused by the inclusion of the cap to a
    frequency domain for use in filtering.
  3. I think he is describing a Sallen-Key. The input signal goes to
    the non-inverting input of the op-amp, with the input resistor broken
    down into two. So you have to stages of an RC low pass filter, with
    the capacitor of the first stage connected to the output of the op-amp
    rather than ground.

    He's trying to get gain from it by adjusting the gain of the op-amp,
    which isn't going to happen. Likely his "no output" is because the
    signal is swamped by oscillation, though that doesn't fully explain why
    he sees output when he has no negative feedback (which of course would
    give "near infinite" gain.

  4. Michael

    Michael Guest

    You're correct, it looks like a Sallen-Key filter (the R and C values
    came from the output of Microchip's FilterLab). And the output with no
    feedback sounds (and looks) like "near infinite" gain (severe clipping).
    Is there a way to prevent the oscillation problem and get some
    gain from this stage? I have been able to get some gain by taking the
    output to another LM741 stage, but I assume I need to clean things up -
    especially since it's on a breadboard (and not very neat either).
  5. I'm surprised Microchip's FilterLab doesn't include the
    ability to set the gain as you wish. Texas Instruments
    FilterPro does.
  6. Bob Masta

    Bob Masta Guest

    Without looking at the circuit I can't be sure, but note that the gain
    of the filter is often used to determine Q. Your circuit description
    sounds like a 2nd order low-pass, except you haven't mentioned
    a second capacitor to ground. Usually, if you increase gain you
    increase Q, and you will soon get into oscillation... not to mention
    putting a huge peak at the corner frequency.

    Check out Don Lancaster's "Active Filter Cookbook" for some
    reliable circuits. (And note that the 741 is probably not a good
    choice any more, though it should be fine at low frequencies
    if low noise is not important. Consider the LF351 or TL071
    series, for example, if you want drop-in parts with much better

    Best regards,

    Bob Masta

    DAQARTA v3.50
    Data AcQuisition And Real-Time Analysis
    Scope, Spectrum, Spectrogram, FREE Signal Generator
    Science with your sound card!
  7. Michael

    Michael Guest

    Thanks for the link. The program with Sallen-Key selected shows exactly
    what I'm trying to do, so I'm assuming oscillation is preventing the gain.
  8. Michael

    Michael Guest

    Thanks for the book reference. The library has a copy I can check out.
  9. You have mentioned only one capacitor in your filter. The
    minimum filter this program generates uses two. and you
    should also have a small capacitor between each of the
    supply pins and ground, across your opamp, to stabilize it.
  10. Michael

    Michael Guest

    I have the cap on the non-inverting input, but not those on the supply.
    Recently encountered this article which mentioned them:

    Will apply caps and see what results.

    Thanks again.
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