Please explain how the MOSFET and the diode work.

  1. I have been working on some music electronics, I got this on the web and I don't understand how the clipping circuit with germanium diode work.

    [​IMG]

    The part in question is the enhancement mode MOSFET IRF510 with the 1N34 diode. It is a clipping circuit common in distortion box used in guitar effect. Normal it is just two back to back diode for clipping the signal. I would imagine just two back to back germanium diode will work. What is the MOSFET for, I don't even see how the two MOSFET are turn on!!!

    Please explain.

    Thanks

    Alan
     
  2. jcsd
  3. Those are diode connected mosfet. They work in saturation only.
     
  4. I know about MOSFET rectifier, I read about the MOSFET bridge and understand how it works. Here is the link I read and it should work:

    http://www.thetaeng.com/FETBridge.htm

    I analyzed the circuit and I cannot see how the two MOSFET even get turn on. I understand the body diode play a role in the ordinary rectifier to turn on the FET, but I don't see how this circuit work.

    The reason I put this here because it is published and is used, it must be doing something and I just don't get it.
     
  5. if the diodes were the other way round, the fets would add to the clipping threshold, and probably soften the edges up a bit.

    http://www.generalguitargadgets.com/richardo/distortion/index.html

    that page has some info, it says
    edit: ha ha, on closer look, i think thats all the page says.
     
  6. Thanks for the reply, that's the Jack A. Orman's and Aaron Nelson's Shaka Braddah III design. My question is how does it work. My confusion is how does the circuit conduct. If you look at the link you provided, the left hand side is the enhancement NMOS with the source connects to the anode of the 1N34.

    Let see if top side is positive, the gate of the NMOS is tied to the Drain and when positive, the NMOS is trying to turn on. BUT the diode is connected backward and is OFF. There will be no conduction.

    Now if the bottom is positive and top is negative, the diode want to turn on. BUT the NMOS is up side down where the Source is on the positive end and the Gate and Drain is connected to the negative side. Even if you say the S and D is interchangeable, the gate is tide to the D(which is S if you switch S to D) and it is not going to turn on as it is an enhancement mode FET.

    This is my confusion. It is obvious the circuit works, but how?
     
  7. It seems that you completely forgot about build-in in every MOSFET parasitic diode (body diode). This diode act just like a Zener diode. The Zener action occurs when Vds > Vds_max. And act like a "normal" diode when Vsd > 0.6V.
     
  8. I did not forget the body diode, in fact the body diode is the very thing that make MOSFET as rectifier possible. In fact the body diode is shown in the drawing I posted. It does not make any sense to use the MOSFET just to use the body diode and the MOSFET never get turn on. There got to be a better reason. I designed a lot of MOSFET circuits, just cannot make sense out of this.

    It is not used as Zener as the voltage is much higher, this is a clamping circuit for distortion that use 9V battery, never even touch the zener break down voltage.
     
  9. Well, my scope don't see any difference

    [​IMG]

    It's looks like another audiophile myth.
     

    Attached Files:

  10. I see a difference in your scope, those peaks are clipped asymetrically.

    it should be noted that DIY guitar effects pedals are usually built by random trial and error, by people who haven't studied electronic design.

    http://www.diystompboxes.com/wiki/index.php?title=Simple_mods
     
  11. Yeh, I really think it's those people that really don't know electronics doing something really funny. I did the simulation with LTSpice and see the same thing as your scope trace. The MOSFET is nothing more than a body diode.
     
  12. I think it's due to the Vbe difference. I am busy working on the diode clipping distortion circuit and I can change the duty cycle by introducing offset.
     
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