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Ebers-moll equation...

Discussion in 'Electronic Basics' started by Ludnough, Sep 4, 2004.

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  1. Ludnough

    Ludnough Guest


    I recently got H&H Art of Electronics, and as I have read in some other
    places, the Ebers-Moll equation more accurately descibes the BJT
    output/input relationship (transconductance) as opposed to the simple
    current amplifier model, using Beta. Well, since Beta is a "bad"
    parameter to use, I have to ask, is there any value to using the current
    amplifier model (other than its simplicity)? Does anyone here use the
    Ebers-Moll eq. for practical design?

    Thanks for shedding some light on this for me.


  2. John Larkin

    John Larkin Guest

    I don't. Most of the time the simple diode-drop-beta model is all you
    need. If you're driving a relay or building a crude voltage regulator,
    transconductance isn't very interesting. When transconductance is an
    issue, simple approximations (like the old 26/Ie thing) are usually
    close enough. Only the guys designing the guts of analog ICs need much
    more. Since transistors vary so much unit-to-unit, precision isn't
    terribly relevant.

    Base and emitter parasitic resistances start to matter at high
    currents, but simple approximations are usually still enough.

    (Flames expected)

  3. If your doing simple switching, Ic=hfe.Ib may be enough. If you are
    doing linear amps, re=1/gm=1/40Ic is absolutely indispensable, with its
    corresponding In practice, you use the full models
    automatically, as sensible people use spice.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
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