Preamplifi ers and Input Signals 189obtaining good linearity lies in the nature of the base voltage/collector current transfer
characteristic, shown in the case of a typical “ NPN ” device (a “ PNP ” device would have a
very similar characteristic, but with negative voltages and currents) in Figure 7.18.
In this, it can be seen that the input/output transfer characteristic is strongly curved in
the region “ X–Y ” and that an input signal applied to the base of such a device, which
is forward biased to operate within this region, would suffer from the very prominent
(second harmonic) waveform distortion shown in Figure 7.19.
The way this type of nonlinearity is infl uenced by the signal output level is shown
in Figure 7.20. It is normally found that the distortion increases as the output signal
increases, and conversely.
There are two major improvements in the performance of such a bipolar amplifi er stage
that can be envisaged from these characteristics. First, because the nonlinearity is due to
the curvature of the input characteristics of the device—the output characteristics, shown
in Figure 7.21 , are linear—the smaller the input signal that is applied to such a stage, the
lower the nonlinearity, so that a higher stage gain will lead to reduced signal distortion
at the same output level. Second, the distortion due to such a stage is very largely second
harmonic in nature.
mACollector current (l)c
0V x
(0.53 V) (0.7 V)y Base voltage (Vb)Figure 7.18 : Typical transfer characteristic of a silicon transistor.