On Sun, 15 Jun 2003 07:52:24 -0000, you wrote:
It seems that this device is not among the existing examples in
LTspice. This device is primarily used for medical instrumentation
and data acquisition. Can someone tell me what it takes to make a
custom model for it? What is the minimum number of parameters from
its data sheet that should be taken into account?
I'm no expert on this, but it would seem that your needs would
depend on which model you can accept. There are three
Ebers-Moll models I'm familiar with, but the help file for
LTSpice only mentions "the Ebers-Moll." I guess it just depends
on which parameters you want to specify.
EM1 has an injection version, a transport version, and a
nonlinear hybrid-pi version. They use the forward beta (Bf),
reverse beta (Br), saturation current (Is, which is usually just
a hypothetical value designed to minimize errors against a
realistic curve of behavior), the nominal temperature (Tnom),
and the energy gap (Eg.) The energy gap is used to modify Is
over temperature.
For these parameters, you just need to look at a curve trace
which shows the collector current vs the collector-emitter
voltage for a fixed base-current drive. Select the Bf value
found on the desired I(B) curve at the point where I(C) and
V(CE) are at the values you intend to operate the transistor.
Use the DC value, not the AC value. Br can just be set to 1, if
you aren't going to operate it that way or else you can try and
find a curve with the emitter and collector leads interchanged
and pick off the right value for Br. Is is more complex. You
can use a numerical method for this, if you capture some
complete curves. But it is often just set to arbitrary values
like 1E-16 or else measured by considering a simple relationship
when the transistor is in the normal, active region and
zero-biased base-collector junction. In this case, you use the
standard Ebers-Moll equation, I(C)=I(S)*(e^((q*V(BE))/(k*T))-1),
and solve for I(S) (Is.) Eg is typically 1.11eV for silicon but
its often obtained by curve-fitting Is. This would probably
take some effort on your part, as the required information isn't
usually present on a data sheet for this. You might also steal
it from some similar transistor or else ignored if you assume
you will operate the transistor at the specified Is value.
EM2 uses EM1's parameters and adds the emitter ohmic resistance
(Re), the collector ohmic resistance (Rc), the base ohmic
resistance (Rb), the zero-bias emitter-base junction capacitance
(CJEO or CJE) and barrier potential and gradient factor, the
zero-bias collector-base junction capacitance (CJCO or CJC) and
barrier potential and gradient factor, the forward transit time
(Tf), the reverse transit time (Tr), and the substrate
capacitance (Csub). I think you'd need a curve tracer,
capacitance bridge, power supply, pulse generator, and a small
signal measurement system at a minimum. I don't think all these
can be fetched from a typical data sheet.
EM3 adds more, and so does Gummel-Poon. It gets really hairy at
this point.
My guess is that a data sheet is good for EM1, and even that is
only without really getting a good bead on Eg unless you
determine it by fiat. But like I said, I've only played around
with this a little bit with just a couple of unmarked
transistors, for the fun of it. I don't have good experience at
this. I'll be reading to see if someone can wisely point out
some good techniques for better modeling from data sheet curves.
Best of luck,
Jon
Mike Engelhardt