CHAPTER 2. AMPLIFIER CHARACTERISTICS
8
There are three basic ways how distortion introduced by a given piece of
audio equipmentmay b e discovered [14]:
The rst and most straightforward one is to display the input and output
waveforms simultaneously on a double b eam oscilloscope. Even if it is possi-
ble to superimp ose the twowaveforms, or to subtract the input signal from
an attenuated version of the output one, this works only for a distortion of
more than several per cent (for a distortion with this order of magnitude also
other possibilities exist [15]).
The second technique is sp ectrum analysis. Here, the magnitudes and fre-
quencies of the components of the output signal are measured, and compared
with the input signal (preferably a sine wave). Spectrum analysis shows its
advantages if two or more pure sine wave signals are introduced at the in-
put: the unwanted intermodulation pro ducts b etween these will b e revealed
as well as the existence of any spurious harmonic products. This shows to
which extent the output is distorted, but it is no help when searching the
source of the distortion [16].
The third metho d is that of total harmonic distortion plus hum and noise
measurement. Here, the equipment under test is fed with a single frequency,
low noise signal. This frequency is removed from the system output bya
simple notch lter. What remains is the total harmonic residue, which is,
in essence, the dierence between perfection and reality. If this lter residue
is displayed on the oscilloscope at the same time as the output signal, it is
often relatively easy to discover what the source of the waveform distortion
could be [17].
2.3.1 Nonlinear Distortion
A p erfectly linear system will perfectly reproduce the shap e of any input
waveform without alteration. In practice all systems involve some degree
of nonlinearity, i.e. curvature, and will therefore mo dify a waveform passing
through the system. This curvature is called
transfer characteristic
,or
in-
put/output characteristic
, or, if the system is an amplier, it is also called
amplier response curve
. It should b e as linear as possible, it should not
change with varying load imp edances and it should neither alter with time
nor with temperature (thermal stability). Fig. 2.2 illustrates the b ehaviour
of a p erfectly linear system (the transfer characteristic is a straight line) for
a sinusoidal input. Some typical transfer characteristics of nonlinear systems
are shown in Fig. 2.3. The dierenttypes of distortion they pro duce will b e
discussed later in this section.
A signal passing through a nonlinear system will experience
nonlinear
distortion
, resulting in a change in wave shap e due to distortion products.
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