FET audio compressor help

Discussion in 'The Projects Forum' started by Veracohr, Aug 11, 2013.

  1. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    Hi, I'm working on an audio compressor circuit for a class and there's one issue I can't see my way around.

    The attached circuit uses a JFET as a voltage-controlled resistor. The problem is that the compression ratio (ΔVout/ΔVin) changes when the threshold is changed. This is because the JFET Vgs-Rds characteristic is non-linear. Rds changes much more quickly with Vgs values near the cutoff voltage than near the fully-on voltage. As a result, the compression ratio is much higher when the control voltage hovers near the threshold than when the control voltage is well past the threshold.

    Most of the compressor circuits using FET's I've found in searching are either very simple and don't have controls for threshold and ratio, or they are just clones of the venerable 1176LN. I haven't seen anything talk about the dependence of the compression ratio on the threshold setting.

    So can anyone suggest a way to compensate for the Vgs-Rds curve?
     
  2. #12

    Expert

    Nov 30, 2010
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  3. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    Thanks, unfortunately that's linearizing a different aspect of the transistor and I've already incorporated that. It involves linearizing the Vds-Ids curve in the ohmic region for a given value of Vgs.

    This linearizing method is talked about in everything I've read about using a JFET as a VCR, but no one seems to ever mention that the Vgs-Rds curve is also highly non-linear.
     
  4. rogs

    Active Member

    Aug 28, 2009
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    You might like to take a look at figure 15 (page 8) of this data sheet:

    http://www.silonex.com/audiohm/pdf/levelcontrol.pdf

    Although it is describing LDRs rather than FETs as the variable resistor in the audio path, it does deal with the non linearity problem you are concerned with. (Opto isolators are of course also non linear).

    Not sure I've ever seen any solution to the non linearity in the FET chararcteristic you are describing, other than incorporating the control circuitry into the feedback loop, to help 'linearise' the operating area.

    Mostly variations on the 1176 type of circuit usually......
     
  5. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    Thanks rogs. I'll have to figure out how to use that concept with an FET.
     
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