Replacing potentiometer with MOSFET

Discussion in 'General Electronics Chat' started by ogg, Oct 10, 2009.

  1. ogg

    Thread Starter New Member

    Sep 23, 2009
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    0
    Hi,

    I would like to make the resonance in this low pass filter circuit (first image) voltage controlled. The resonance is controlled by the pot in the top right. I'm quite new to this but I understand that the voltage on the MOSFET's gate will affect the amount of current that can pass from drain to sink, so it seems an appropriate way to do this. It also seems a cheaper and simpler option than using an optocoupler.

    I tried replacing the pot it in a simulator but got errors (second image). Is this an issue with the simulator, or will the circuit actually not work?

    Assuming it would work, is it only the gate voltage that will affect the MOSFET's resistance or are there other factors? As I'm looking to replace a pot, I would like to know what the maximum resistance of the MOSFET would be, i.e. when the gate voltage is at maximum. I'm aiming for 100k, but looking at the datasheet for the 2N7000 MOSFET, I don't see it's electrical characteristics expressed like this at all.

    Have I misunderstood it by thinking in these terms, or is it possible to convert from the figures in the datasheet to an equivalent resistance at maximum gate voltage, like the rating on a potentiometer?

    Thanks,

    Ogg
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    896
    You show a Sallen and Key lowpass filter.
    The two resistors that control the frequency are fixed but you are trying to use a Mosfet to control the gain which adjusts the amount of frequency peaking and/or oscillation.

    The resistors have AC across them but a Mosfet works with DC across it which is very different.

    Many years ago a transconductance amplifier IC was used to be a voltage-controlled resistor. Maybe they are not made anymore.
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Greetings ogg,

    Can you describe how you intend to use your active filter with adjustable response? Maybe there are some altenative solutions that can be offered by our members once we better understand your end game.

    hgmjr
     
  4. ogg

    Thread Starter New Member

    Sep 23, 2009
    10
    0
    Ah, like the LM13700? I just found those for sale here at Rapid Electronics.

    I didn't realise that Mosfets were DC only, no wonder it wasn't working.
     
  5. bertus

    Administrator

    Apr 5, 2008
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  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    The CA3080 transconductance amplifier was used in The Good old Days.
    Tou can try using the LM13700 instead.
     
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