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8 Distortion
Introduction
As no active device is perfectly linear, all practical amplifiers exhibit some amount of
distortion. Excessive distortion, such as clipping or heavy Class A asymmetry, can
readily be seen on an oscilloscope. More modest forms are not discernable by eye and
require appropriate test instruments, namely a low distortion sine source and a distortion
analyzer. It is important to note that distortion signals are complex and thus do not add
coherently. For example, if the measurement system has a residual THD of .1% and the
device under test weighs in at .12%, it cannot be assumed that the device under test
simply produces the difference, or .02% THD. A common technique for reducing
amplifier distortion is through the use of negative feedback. A simple example of this is
the use of an emitter swamping (AKA degeneration) resistor in a common emitter
amplifier. The greater the swamping, the lower the distortion. Of course, there is a
commensurate reduction in gain as well. In order to compare amplifiers of different gain,
distortion measurements are usually made at a specific output voltage level, such as 1
volt. Generally, the nonlinear nature of amplifiers increases with increasing output level
producing more distortion. Thus, high gain amplifiers would be at a disadvantage
compared to low gain amplifiers if the input levels were the same.
Equipment
Sine generator (preferably low distortion, <.1% THD)
Oscilloscope
DC Power supply
Distortion analyzer
1 small signal NPN transistor (2N3904, 2N2222, etc.)
1 10 k
1 4.7 k
1 3.3 k
1 2.2 k
11k
1 820
1 180
2 1 F
1 470 F
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Diagram
Figure 8.1
Procedure
1. Measure the residual distortion of the sine generator at 1 kHz. Remember that you
will not be able to resolve amplifier distortion below this level.
2. Build the circuit of Figure 8.1.
3. Apply an input sine at 1 kHz, sufficient to create an output load signal of 1 volt peak.
Record the input signal level, output level, and resulting gain in Table 8.2.
4. Using the distortion analyzer, measure the output distortion percentage and record it
in Table 8.1.
5. Replace the 1 k emitter resistor with a 180 in series with an 820, using the 470 F
capacitor to bypass just the 820. This should lower both the gain and the distortion
without changing the DC bias.
6. Repeat steps 3 and 4 using Table 8.3.
7. Simulate both circuits using the Distortion Analyzer tool, recording the results in
Table 8.4. Compare the simulations to your measurements. Make sure that you do not
use an ideal or virtual transistor model, but rather, an accurate model of the device
used for the measurements above. Adjust the input levels to those set in Tables 8.1
and 8.2.
Residual THD
Table 8.1
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Input level
Output Level
Gain
THD
Gain
THD
Table 8.2
Input level
Output Level
Table 8.3
THD Sim 1
THD Sim 2
Table 8.4
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