Ebers-moll equation...

[Ludnough]

Hi,

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.

Regards,

L

 

[John Larkin]

On Fri, 03 Sep 2004 18:16:16 -0500, Ludnough <me@here.com> wrote:

Hi,

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?

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)

John

 

[Kevin Aylward]

Ludnough wrote:

Hi,

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.

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 ri=hfe.re. In practice, you use the full models
automatically, as sensible people use spice.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.