extracting spice model params from bjt plot

Papabravo

Joined Feb 24, 2006
17,300
Set up a Monte Carlo experiment where you keep trying combinations of values until you get what you want. It helps if you know which model we are talking about.
 

neonstrobe

Joined May 15, 2009
180
SPICE modelling is quite involved, if not difficult. You cannot extract IS and RB from a plot of the saturation voltages - maybe they would give you some idea of RE and RC though.
Suggest you read up on an explanation of SPICE parameters and bipolar transistors.
For IS you need a Gummel plot (IC (log) versus Vbe, with collector voltage reasonably high, above saturation, maybe 2-10V, although the best plots set Vc=Vb which works for low to medium currents (saturation conditions may affect this more) by eliminating ICB leakage), and RB is notoriously difficult which is best extracted from high frequency measurements to obtain the input impedance rather than resistance. RB also varies with current, which makes it a hard parameter to pin down. It is sometimes possible to match RB from D.C. measurements (log(IB) versus Vbe) but it is usually necessary to iterate and extract RE as well because that affects Vbe too.
 
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neonstrobe

Joined May 15, 2009
180
Ian Getreu's ancient book is excellent. Published when he worked at Tektronics. You might find it somewhere second hand but I think there may be places offering reprints. It's called "Modeling the bipolar transistor", funnily enough.

You might also find details of SPICE model parameters on Berkeley's web site but they seem to make SPICE records harder to find.
 
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