The next step is the inclusion of the fmite base, collector,
and emitter resistances in the model. The emitter resistance is
determined by stimulating the base with a current in strong
saturation and measuring the collector-emitter voltage. The
collector current is kept small (typically less than 1 microAmp)
and the applied base current is swept up to lOrnA. The inverse
of the gradient of characteristics can then be used to obtain
emitter resistance. The measurement of the collector resistance
is similar to the emitter resistance. Here, current is applied to
the base and collector, and the collector-emitter voltage is
measured. The base current vs. base-collector voltage
characteristics for two collector current values is measured and
the collector resistance is determined from the ratio of
collector-emitter voltage difference to collector current
difference. Figure 4 is a screenshot showing the output
characteristics and extracted SPICE parameters; the Early voltage and collector resistance for a NPN BJT.