The input characteristics of Fig. 3.7 reveal that for fixed values of collector voltage
(VCB), as the base-to-emitter voltage increases, the emitter current increases in a
manner that closely resembles the diode characteristics. In fact, increasing levels of
VCB have such a small effect on the characteristics that as a first approximation the
change due to changes in VCB can be ignored and the characteristics drawn as shown
in Fig. 3.10a. If we then apply the piecewise-linear approach, the characteristics of
Fig. 3.10b will result. Taking it a step further and ignoring the slope of the curve and
therefore the resistance associated with the forward-biased junction will result in the
characteristics of Fig. 3.10c. For the analysis to follow in this book the equivalent
model of Fig. 3.10c will be employed for all dc analysis of transistor networks. That
is, once a transistor is in the “on” state, the base-to-emitter voltage will be assumed
to be the following: