general purpose MOSFET nmos and pmos

Discussion in 'General Electronics Chat' started by reaman4ever, Apr 7, 2008.

  1. reaman4ever

    Thread Starter Member

    Sep 2, 2007
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    0
    Does anyone know what the general purpose mosfet equivelent of the bipolar 2n2222 is? I'm looking for pmos and nmos mosfet, that are balanced (ie when used as an inverter the midpoint on the output curve would be about in the middle of the power supllies?) Also what would be a good choice for a matched PNP equivelent of the 2n2222 NPN?
    thanks
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    The PNP compliment of the 2N2222A is the 2N2907A.

    MOSFETs are all over the map. There isn't really any analog to the venerable & reliable "Quad Duce."

    MOSFETs in inverters are typically used as switches, so complementation is not required.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    For complementing the 2N2222, you could use a 2N2907 or 2N2905.
    Not an exact match, but there are plenty of worse combinations.

    MOSFETs and BJTs are very different 3-legged critters.
    A BJT is a current amplifier; a relatively small current on the base controls a relatively large current on the collector.
    An MOSFET is basically a voltage-controlled switch. When Vgs=0, it's turned off. When (N-ch) Vgs=10, (P-ch) Vgs=-10 it's turned on. You really don't want to run MOSFETS in a partially conducting state.
     
  4. reaman4ever

    Thread Starter Member

    Sep 2, 2007
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    why can't I use mosfets as amplifiers? I understand that bipolars will give higher gain, but can't they (MOSAFETs) be used for both?
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Some MOSFETs can indeed be used as amplifiers. The opening post spoke of power supply inverters. One does not use amplifiers in inverters, one uses switches.

    Are you making an amplifier, or a switching inverter?

    Or a "Class E" amplifier?

    Or something else?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    FETs are used quite a bit in operational amplifiers for the front ends. This is because their input impedance is so incredibly high.

    But you wouldn't want to use a power MOSFET for an audio amplifier.

    I think you're confusing FETs with MOSFETs. They're also different critters, although related. FETs work fine in the linear region, but they're very low gain. In an opamp, a FET input would generally be followed by BJT and/or CMOS amplifier stages.

    MOSFETS are a wonderful creation, but they aren't the end-all, be-all. They certainly are easier to parallel than BJTs due to their inherent positive temperature coefficient, and make a much better switch than a standard BJT does.
     
  7. John Luciani

    Active Member

    Apr 3, 2007
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    For small signal MOSFETs check out Supertex. They have P and N devices
    with matching specs.

    Are you sure need matching specs in your design?

    (* jcl *)
     
  8. pander

    New Member

    Apr 10, 2008
    2
    0
    You probably should have described the differences between enhanced- and depletion-mode MOSFETs there. Granted, most everybody uses enhancement-mode, but depletion mode is hardly deprecated just yet.

    Here's the very quick skinny. JFETs are depletion mode. They conduct until the channel, modified by the gate, pinches off the conduction. MOSFETs can be had in two modes. Depletion works the same way as for JFETs, apply a voltage to the gate and the channel starts shutting down. Enhancement mode means conduction begins with some gate voltage.

    Actually, running MOSFETs in ohmic mode (partially conducting) is becoming very unusual, but it is essentially the only way to conduct an amplifier outside of Class C mode with them.

    One benefit to MOSFETs over BJTs is temperature sensitivity and paralleling. BJTs have a PTC (ergo, they conduct more when they get hotter). That means they can self-destruct very easily under certain conditions. When paralleling them they need to be balanced so their current draw is essentially equalized (sometimes this is far from trivial). MOSFETs can be paralleled with essential immunity and no harm, no foul.
     
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