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What do the capacitors in this ducker circuit do?

Discussion in 'Electronic Basics' started by MRW, May 22, 2007.

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  1. MRW

    MRW Guest

    Hello, about this picture:

    I understand that OA1 is a threshold circuit using an opamp precision
    half-wave rectifier and that OA2 is a polarity inverter and a gain
    control circuit combined into an inverting mixer configuration.

    What I don't get is the purpose of capacitors C8 and C5. Can any one
    explain please? Gracias!
  2. Eeyore

    Eeyore Guest

    C5 with R14 is creating a lossy integrator to provide a DC control voltage.

    C8 looks like a bodge to me !

  3. Thanks for high lighting the capacitors, so I didn't have to
    look all over for them. :)

    C5 is easiest. It is connected as negative feedback across
    the feedback resistor, turning the amplifier into a low pass
    filter. Since the capacitor impedance has the same
    magnitude as the 12400 ohm feedback resistor at about 270
    Hz, all frequencies higher than that are progressively
    rolled off.

    C8 is somewhat similarly connected, but in a nonlinear
    situation. It may be there to reduce the overshoot of the
    output as one diode turns off, and the output slews, open
    loop, (going as fast as it can) to the voltage where the
    other diode turns on, reclosing the feedback loop. It has a
    very much higher frequency effect, because it is normally
    across either the impedance of a turned on D2, or across the
    turned on impedance of D1 in series with R2, except for the
    brief moments I mentioned, first. Both those paths are
    pretty low impedance, so the capacitor current is
    significant only when both diodes are turned off.
  4. MRW

    MRW Guest

    Hey Graham, Thanks! What is a bodge?

  5. MRW

    MRW Guest

    Hi John, Thanks! I'm curious about the low pass filter characteristics.
    I'm having a hard time understanding because I'm seeing the output of
    the RMS Detector 4301P us strictly DC. How does the low pass filter
    play in this DC environment? Thanks again!
  6. Eeyore

    Eeyore Guest

    Like kluge/kludge.

    I've never used a cap there myself in a FW rectifier for audio. Maybe there was
    something not quite right with the performance and that fixed it.

  7. The output of the RMS detector must not be pure DC, but
    varying, unidirectional voltage. The low pass filter
    smooths the bumps. Anything that changes faster than the RC
    time constant of the feedback pair is smoothed
    (12400*47n=.00058 seconds), and the faster it changes, the
    more it is smoothed.

    A better question might be:
    If the RMS detector outputs a DC signal, why is it followed
    by a rectifier?
  8. MRW

    MRW Guest

    Hi John, I was looking at the rectifier as circuit that categorizes the
    voltage level. If the output of the RMS detector is above a certain
    value, then the output of the rectifier follows the detector. If the
    RMS detector output is below threshold, then the rectifier stays at 0V.
    Did I look at this correctly?

  9. I haven't studied how the RMS converter works.
    But after some digging, I did find the data sheet:

    The RMS converter does not look very well defined, but it
    does include a low pass filter (time constant = .026*Ct/It).
    But it also shows a bidirectional output current of +- 100 uA.

    So you may be exactly right, that the negative values for
    below threshold levels, are clipped off by the ideal diode.
  10. Ridiculous. I'm not that clever with this stuff, but I recognize high
    frequency roll-off when I see it.
  11. Eeyore

    Eeyore Guest

    That IS NOT high frequiency roll-off.

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