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will this work?

Discussion in 'Electronic Basics' started by tempus fugit, Jan 6, 2007.

  1. tempus fugit

    tempus fugit Guest

    Hey all;

    I'm trying to design a circuit that will ground a signal when the control
    voltage is applied, like this:




    signal

    o
    |
    |
    |
    o
    |
    ||-+
    ||<- N-channel MOSFET
    control voltage o--------------|--------o-||-+
    .-. |
    | | o
    | | 10K |
    '-' |
    | o
    | |
    === ===
    GND GND



    So, when I apply a logic hi (5v) to the gate, the signal should be muted
    because it is grounded. This part I'm pretty sure I got OK. Now, when I
    remove the control signal, will the signal be un-muted? I would think so,
    but I have tried this with a J111, and I need to actually apply a negative
    voltage to the gate to get it to turn off. Is it different in the case of a
    MOSFET?

    Thanks
     
  2. Eeyore

    Eeyore Guest

    Yes.

    A jfet is the correct one to use. And yes a J111 will need negative volts on the
    gate to turn off. I suggest that if you only have postive volts available then
    you use a p-channel jfet like one from the J175 family. Choose according to your
    available cut-off voltage.

    Graham
     
  3. tempus fugit

    tempus fugit Guest

    Thanks again for the reply Graham (one of many of my posts of late!). Why
    would a JFET be better to use? I have successfully done this with a 2n2222,
    but it doesn't quite kill the whole signal (you can still hear it a bit). I
    was looking at a MOSFET like BS270 because it has a lower Ron than a BJT,
    and of course for the low turn on current.
     
  4. John  Larkin

    John Larkin Guest


    The mosfet will work, 0 or +5 on the gate for off/on, but be aware
    that in the fet OFF state, the D-S substrate diode will conduct if the
    drain tries to swing any more than a few tenths of a volt negative.
    Jfets don't have this problem.

    You could use a CMOS analog switch, 74HC4066 or something. Make a tee
    switch for really good on/off ratios.

    Opto solid-state relays can be interesting, too.

    John
     
  5. Eeyore

    Eeyore Guest

    You *can* do it with a bipolar device but as soon as the signal amplitude
    exceeds ~ 400mV the transistor will conduct in the reverse direction. Also, as
    you've noticed, it doesn't clamp the signal totally. Neither will a jfet totally
    since it'll simply become a small resistance. The very best technique is to have
    a series and shunt fet. I'd choose a series fet alone for best results but it
    requires being a bit more 'clever' with driving the gate since there's signal on
    it.

    I've never used a mosfet for this application and since no pro-audio company I
    know does, I suspect there may be a good reason for that but I've never
    considered the reason why. Asymmetric characteristics strikes me as possible.

    Graham
     
  6. tempus fugit

    tempus fugit Guest


    Hmmm.....

    Your point is well taken Graham. Maybe I'll have to try one and see if there
    are any problems. The signal is not actually going to pass through the
    MOSFET. I plan to connect both output (from guitar) and input (to amp) to
    the drain. SO when the MOSFET is off, there will be a direct connection from
    in to out, but when it is on, both will be grounded.

    Thanks again
     
  7. tempus fugit

    tempus fugit Guest

    Just had a thought....


    What type of material is used in an analog switch (like
    http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2072)? I have been messing
    around with one of these, and it works nicely. I had originally planned to
    use this device, but the way I am switching has changed, so I can go to a
    simpler (and smaller) device.

    I'm just wondering if it uses MOSFETs for switching.
     
  8. tempus fugit

    tempus fugit Guest

    Thanks for the reply John. I wonder what the output voltage of a guitar
    pickup is.....
    So a MOSFET should be fine for switching DC if the voltage is positive then?

    THanks
     
  9. You might also add a second N-channel MOSFET in series with
    the first, but with its source and drain reversed. You
    connect the gates together. This puts a second body diode
    in series with the first that prevents either from
    conducting when the gate is low. When the gate is high,
    both channels have low resistance. The limitation on
    negative off state signal voltage is then that which begins
    to turn the reversed device on with zero gate voltage and
    negative source voltage. But even with a logic level
    MOSFET the turn on threshold is normally above a volt. A
    big improvement on the .3 or .4 volts it takes to turn the
    body diode on. And that turn on threshold is added to the
    body diode turn on voltage before significant current passes.
     
  10. niftydog

    niftydog Guest

    Typically around 1V peaks as an absolute maximum for passive guitars,
    larger for actives. Average voltage is much lower, maybe 0.1V - hard to
    say because it depends on a lot of factors.

    nifty
     
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