286 Chapter 9
is a grossly nonlinear amplifying device, even if some input (positive in the case of an
NPN device) DC bias voltage has been chosen so that the transistor operates on a part of
the curve away from the nonconducting initial region.
9.2 Control of Operating Bias .....................................................................................
There are three basic ways of providing a DC quiescent voltage bias to a BJT, which is
shown in Figure 9.4. In the fi rst of these methods, shown in Figure 9.4(a) , an arrangement
that is fortunately seldom used, the method adopted is simply to connect an input resistor,
R 1 , between the base of the transistor and some suitable voltage source. This voltage can
then be adjusted so that the collector current of the transistor is of the right order to place
the collector potential near its desired operating voltage. The snag with this scheme is that
transistors vary quite a lot from one to another of nominally the same type, so this would
require to be set anew for each individual device. Also, if the operating temperature
changes, the current gain of the device (which is temperature sensitive) will be altered
and, with it, the collector current ofQ 1 and its working potential. The arrangement shown
in Figure 9.4(b) is somewhat preferable in that a high current gain transistor, or one
working at a higher temperature, will pass more current, and this will lower the collector
voltage of Q 1 , which will, in turn, reduce the bias current fl owing through R 1. However,
this also provides NFB and will limit the stage gain to a value somewhat less than R1/Zin.
cbaIa/Ic543210 0.5 1.0Vb (volts)Vg (volts)
Figure 9.3 : Comparative characteristics of valve, germanium, and silicon based BJTs.