Calculate the collector current of Q1 in Fig. 5.18(a). Assume β = 100 and IS = 10−17 A.

Calculate the collector current of Q1 in Fig. 5.18(a). Assume β = 100 and IS =
10−17 A.
Constructing the equivalent circuit shown in Fig. 5.18(b), we note that
VThev = R2
R1 + R2
VCC (5.43)
= 800mV (5.44)
and
RThev = R1||R2 (5.45)
= 54.4 k. (We begin the iteration with an initial guess VBE = 750mV (because we know that the
voltage drop acrossRThev makesVBE less thanVThev), thereby arriving at the base current:
IB = VThev − VBE
RThev
(5.47)
= 0.919 μA. (Figure 5.18 (a) Stage with resistive divider bias, (b) stage with Thevenin equivalent for the
resistive divider and VCC.
Thus, IC = βIB = 91.9 μA and
VBE = VT ln
IC
IS
(5.49)
= 776mV. (5.50)
It follows that IB = 0.441 μA and hence IC = 44.1 μA, still a large fluctuation
with respect to the first value from above. Continuing the iteration, we obtain
VBE = 757 mV, IB = 0.79 μA and IC = 79.0 μA. After many iterations, VBE ≈
766mV and IC = 63 μA.
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