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    Falstad JFET switch schematic?

    Hey all, I'm having a real hard time designing a simple on / off switch for an AC audio signal using an N-channel JFET. I've got some schematics (which I haven't breadboarded out yet), but in trying to simulate these ideas in Falstad, they never work. The circuit will run for a while and then quit, or occasionally i'll "reverse bias" the JFET, which also seems to be a big no-no in Falstad. Regardless of what I do I can never seem to get the AC signal to be affected.

    Can anybody design a switch that works in Falstad... using a switch to control the gate voltage that will then cut off (pinch off?) the AC signal. It seems like that should be pretty easy, but I'll be darned if I can get anything to work. Lets say switch open, AC signal flows... switch closed, AC signal off.

    thanks in advance.

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  2. #2
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    Re: Falstad JFET switch schematic?

    Quote Originally Posted by maark6000 View Post
    Hey all, I'm having a real hard time designing a simple on / off switch for an AC audio signal using an N-channel JFET. I've got some schematics (which I haven't breadboarded out yet), but in trying to simulate these ideas in Falstad, they never work. The circuit will run for a while and then quit
    Falstad's contains several jfet sample circuits. Look under 'Misc devices' in the menu. Find one that is similar to what you want. See if you can add your changes, and still have it run without errors.

    occasionally i'll "reverse bias" the JFET, which also seems to be a big no-no in Falstad.
    Yes, Falstad's warns about gate current in the wrong direction.
    However the sample jfet circuits apparently do something right, because bias polarity on a jfet normally ranges from plus to minus. Pinch-off requires bias polarity which is outside the supply rails.

    I also have seen Falstad's run for a while, then stop because of an error, in cases where a mosfet bias voltage goes outside its supply rails. (Sometimes, not always.)

    Many times I have seen problems in Falstad's are frequently solved when you add resistors at strategic points.

    I recommend starting with one of the sample circuits. Add a switched source as you prefer. Put a resistor in line with it. Do not leave the jfet gate isolated. Have it connected to something at all times rather than floating. A high-ohm resistor to a supply rail may be sufficient. You'll need to experiment because of the unorthodox bias range of a jfet.



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  3. #3
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    Re: Falstad JFET switch schematic?

    Take this code falstad_jfet.txt or this link

    It's nothing else than Falstad's Circuits -> Misc Devices -> JFETs ->Volume Control, Triangle Source replaced by a Square Wave Source (1-terminal), with voltage limits adapted.

    HTH!



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