Mosfet connections

Discussion in 'General Electronics Chat' started by papagaya, Nov 20, 2011.

  1. papagaya

    Thread Starter New Member

    Dec 11, 2010
    5
    0
    Hi. I've a doubt about connecting mosfets. Here is the design http://imageshack.us/photo/my-images/824/mosfet.gif/

    The first option runs OK in the simulation software, where the gates are connected to ground, but with the second option both lights are always lit.

    - Will the second option work in real life?
    - Does the gates need to be always connected to ground?
    - Happens the same with BJT transistors?
     
  2. nigelwright7557

    Senior Member

    May 10, 2008
    487
    71
    Yes the gates always need to go to ground if not at 5 volts as the gate will hold a charge otherwise.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    FET gates draw extremely small amounts of DC leakage current, even at a relatively big gate voltage when the FET is fully switched on. BJT bases on the other hand require definite current to switch them on.

    The FET gate also possesses significant capacitance. Without some current path such as a resistor to discharge this, the FET cannot be relied on to turn off simply by disconnecting the gate from the supply.
     
  4. papagaya

    Thread Starter New Member

    Dec 11, 2010
    5
    0
    wow, that was fast. Putting some resistors then.
    Thanks
     
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