4 terminal mosfets

Discussion in 'General Electronics Chat' started by magnet18, Feb 27, 2014.

  1. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    anyone know where to get any mosfets with a floating base terminal?

    I can't seem to find any, the only 4 pin i could find was the STW69N65M5-4, which doesn't seem to truly have a floating base, just 2 sources

    I just wanted to try messing with biasing, and had trouble finding any
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Does MOSFET has a base ?

    Mr. Magnet, are you referring to GATE ?
     
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Only 4 terminal I have is 2N3821, N ch J FET, with gate leakage of .1 ηA, so it couldent have too much tied to the gate.
     
  4. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    I just learned that mosfets are 4 terminal devices, they usually just have the body silicon connected to the source for reference bias

    [​IMG]
    [​IMG]

    I'm trying to find one that doesn't have the body/casing connected to the source

    @bernard, thanks, that MIGHT be what I'm looking for, I can't tell byt the datasheets I found if the body pin is explicitly for the internal silicon, or a way of connecting to the physical casing
    (no datasheets I saw had a diagram, I didn't dig too deep yet though)
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
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    dual gate mosfets were fairley common in rf stuff, lookup the 40673 for an example. they actually used both gates, one for normal input, and the other for controling gain. I have never seen a high power dual gate mosfet.
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    2N3821, bottom view, CW from tab, S, D, G, C for case & measured 0 Ω.
     
  7. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    I found some modern dual gates, like the NTE222
    [​IMG]

    which are interesting, but the 2n3821 is still the only thing I've seen that might be an actual connection to the silicon body.
    I might get some to play with and see, I can't seem to find a good diagram of what the case is connected to
     
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the 2n3821 is a jfet, not a mosfet.
     
  9. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the 2n3821 has the case connected to a lead. if you ground it, it grounds the case, shielding the jfet. it is generaly used for low level rf amplification.
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Are you talking about a mosfet in an "iso-top" case? They have four leads available, and the base is isolated/insulated. But the don't have leads like a regular mosfet, they have screw terminals instead. A standard every day mosfet has four leads but the tab is the other lead/source and has to be insulated.


    Here's an example of the type -
     
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