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Hfe = forward Beta?

Discussion in 'CAD' started by Paul Burridge, Dec 15, 2003.

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  1. Hi gang,

    I'm puzzled by the Fairchild data sheet for the 2N3904 transistor
    (BJT). It states that Hfe is no better than 300 maximum under any
    conditions, yet in the Spice model they supply on the same sheet is
    shown the parameter for forward beta as being 416.4!
    Two questions arise:

    I thought FB and Hfe were essentially the same. Is that not the case?

    and...

    Why give such a *precise* figure for forward beta when it's such a
    notoriously variable quantity not just dependent on circuit conditions
    but varying considerably between devices within the same production
    batch??

    If anyone can shed any light on this seemingly impenetrable conundrum,
    then I'll be mightily impressed.
    p.
     
  2. Fred Bloggs

    Fred Bloggs Guest

    The SPICE forward beta parameter pertains to the basic static Ebers-Moll
    model of the intrinsic transistor. The actual device consists of this
    intrinsic part and extrinsic parasitics resulting from the necessities
    of the semiconductor engineering. The intrinsic model combined with
    other SPICE parameters serves to model the end result with fairly good
    accuracy.
     
  3. Jim Thompson

    Jim Thompson Guest

    Run a simulation sweeping IB for a fixed VCE, then display
    IC(Q)/IB(Q).

    Then comment out every parameter except IS and BF and re-run the
    simulation.

    Note the difference ;-)

    [The main culprits are IKF (current crowding at the emitter edge) and
    ISE (leaky base region).]

    ...Jim Thompson
     
  4. Nope, Hfe is a hybrid parameter of a 2-port network - H21=I2/I1
    Beta is *delta*Ic/*delta*Ib

    So you should see that it depends on the operating point (current bias)
    of the transistor. And beta is for common emitter configurations, where
    a hybrid parameter is for any ol network.
    Convenience for techs not encumbered by the nuances of the Ebers-Moll
    model? A crude figure of merit to contrast devices?


    --
    Scott

    **********************************

    DIY Piezo-Gyro, PCB Drill Bot & More Soon!

    http://home.comcast.net/~scottxs/

    **********************************
     
  5. I read in sci.electronics.design that Scott Stephens
    This is bit hair-splitting. The symbols are a bit off-beam; FB could be
    mistaken for f-beta, which is something entirely different. There are
    four h-parameter series - hfe is the *small signal* common-emitter
    forward current gain, and hFE is the large signal value. These 'signals'
    are a.c., and there is a corresponding pair of d.c. values, Hfe (at
    small Ic) and HFE (at large Ic) (which are rarely quoted).

    All of the h-parameters AND FB depend on the operating point.

    h21 is the small signal forward current gain of ANY 2-port. hfe isn't.
     
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