Environmental Engineering Reference
In-Depth Information
Chapter 7
HARMONIC DISTORTION
Semiconductor devices are inherently nonlinear. For example, Bipolar
transistors in forward active region exhibit an exponential relationship
between the collector current and the base-emitter voltage, while in saturated
MOS transistors the drain current approximately depends on the square of
the gate-source voltage. Therefore, circuits made up with transistors or, more
generally, with real active components exhibit a certain amount of
nonlinearity, and this means that the relationship between their input and the
output variables is not so ideally linear as assumed in the previous chapters.
Usually, active devices used for analog signal processing applications are
operated in a quasi-linear region. Thus the linearity assumption is almost
verified especially when signals with small amplitude are processed.
However, designers are asked to evaluate the limits of the linear
approximation or to characterise the effects of nonlinear distortion in circuits
and systems used as linear blocks [S99]
1
. To achieve these targets harmonic
distortion analysis is customary employed.
Consider the open loop amplifier in Fig. 7.1 with its DC
nonlinear
transfer characteristic When nonlinearities are small, that is the
transcharacteristic is characterised by gradual slope changes, the circuit is
said to operate under
low-distortion conditions
2
.
This implies, in other
words, that transistors do not leave the active region, and small-signal
analysis can be used to produce meaningful results. Harmonic distortion in
this case is usually calculated with the series expansion of the nonlinear DC
1
Linear distortion arises in a linear amplifier which has a non constant frequency
response in the frequency domain [S99].
2
For a rigorous definition of the low-distortion condition see [OS93].
