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Instantaneous "on" FET

Discussion in 'Electronic Design' started by Stuart Garet, May 6, 2007.

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  1. Stuart Garet

    Stuart Garet Guest

    I want to use an FET as the first stage in an op amp darlington
    follower. The ones I have on hand require a threshold signal voltage
    to begin conducting. Hence, I am loosing swing at the output. Can
    anyone recommend an FET that turns on from 0V upward, or as close as

    Stu Garet
  2. MooseFET

    MooseFET Guest makes some.
  3. Charles

    Charles Guest

    Have you investigated JFETs and depletion mode MOSFETs?
  4. D from BC

    D from BC Guest

    Some way ..somehow there's has to be a ~3V difference between the gate
    and the source for a mosfet to begin to conduct.
    Example...Vs is ~-3V and Vg= 0V..... or Vs=0V and Vg=3V's not a mosfet..
    D from BC
  5. D from BC

    D from BC Guest

    Damn new shit coming out all the time! :p
    So I checked it out...
    First glance ...the PITA alarms went off..ding ding ding..
    I'm guessing it's an expensive min quantity PITA part..
    I'll be surprised if the OP will be jumping for joy...'s been an hopefully I'll remember the IC
    for some quirky design someday..
    "Yeah..I think it was some 0 Vgs chip that ran off 5V..."
    D from BC
  6. John Larkin

    John Larkin Guest

    Supertex makes depletion-mode mosfets, which can be very handy. They
    make nice current limiters.


    Best regards,
    Spehro Pefhany
  8. D from BC

    D from BC Guest

    The OP wrote "turns on from 0V and upward"..
    I interpreted that as an enhancement mode mosfet.
    D from BC
  9. colin

    colin Guest

    well its not an enhancement mode mosfet anyway,
    but you Can get depletion mode mosfets,
    theyr just not as common,
    at least not in power types anyway,
    theyve been available in small signal devices for some time,
    usualy combination of enhancement/depletion,
    particularly the dual gate devices I like to use.

    some low voltage logic level power mosfets are getting with lower and lower

    the vgth spread of course will still be an issue.

    the jfet type obviously cant be enhancement mode at all.
    so jfet could be used to compensate for the folowing stages voltage drop,
    assuming you have supply headroom.

    Colin =^.^=
  10. bitch, bitch, bitch ;-)

    Small quantities in stock at Mouser, they show reasonable lead times
    for additional quantity. Prices don't look too bad to me-- between
    1.50 and 2.00 in qty 100 for the quad array. Not 3 cents, of course.
    Often the special the part has 'de'parted by the time you need it.

    Best regards,
    Spehro Pefhany
  11. Winfield

    Winfield Guest

    Stu, we're confused, what do you mean by "op amp darlington follower"
    -- an op amp voltage follower doesn't need or involve a Darlington,
    which is a bipolar transistor configuration. As for FET input stages,
    generally we use JFETs. These are depletion type devices, which can
    be operated at their full drain current, called Idss, where Vgs = zero
    volts (it's enhancement-mode MOSFETs that need a volt or two to get
    going). So your question is unclear on that account as well.

    Finally, you can get pretty good JFET opamps, which in all but a few
    special cases are fine. Some special cases would be where you need
    noise levels below 2.5nV/rt-Hz, where you need to work from low supply
    voltages, or where the $8 to $15 cost of low-noise JFET opamps is
    considered to be too high. Tell us more about your project.
  12. Joerg

    Joerg Guest

    Like ye olde SD5400. Hard to obtain these days and for most of my
    designs now prohibitively expensive. And it's always been so good to me,
    it is (was?) a true analog engineer's delight. <sigh>
  13. Joerg

    Joerg Guest

    $8-$15? Ouch! Which one would that be? At that cost I'd go discrete.
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