Distortion & Feedback.pdf

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CHAPTER 6 - AMPLIFIER DISTORTION AND FEEDBACK
Objectives
1. Description of and general causes of common forms of distortion in power
amplifiers
•
Frequency Distortion
•
Phase Distortion
•
Non-Linear Distortion
∗
Harmonic Distortion
∗
Intermodulation Distortion
2. Non-Linear Distortion
•
causes
∗
mismatch of the 2 power transistors
∗
non-linearity of:
IC/IB characteristic
beta/frequency characteristic
IC/VBE characteristic
3. Harmonic Distortion
•
general description
•
calculating total harmonic distortion
•
measurement of harmonic distortion
4. The Use of Negative Feedback to Reduce Distortion
•
Linearisation of transfer characteristic
•
Calculation of feedback ratio
•
Calculation of reduction in distortion after adding feedback
Common Forms of Distortion
Distortion is caused by an amplifier if the shape of the output waveform is not identical
to that of the input waveform.
All amplifiers distort the signal to some extent and one object of good design is to
minimize this. A number of factors are responsible for distortion and distortion is
generally classified under headings which indicate the effect caused.
The principal types of distortion are :
1. Frequency Distortion
Unequal amplification of all frequencies comprising the signal
2. Phase Distortion
Alteration of the relative phases between components in a complex signal
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Section 6
1
3. Non- Linear Distortion (Amplitude Distortion)
The introduction into the signal of components NOT present in the input signal to the
amplifier. This is due to the non-linearity of the amplifier’s “transfer characteristics”.
Harmonic Distortion
Where harmonics (multiples) of each frequency component are generated within
the amplifier. Both
even
and
odd
harmonic distortion are possible.
a)
Odd harmonic distortion
is caused by the fact that the value of beta for the
power transistors is not constant and changes with changes in collector
current.
b)
Even harmonic distortion
can be caused by a mismatch of the value of beta
in the two output transistors which results in the two halves of the output
waveform being unequal.
c)
Even harmonic distortion.can
also be caused by crossover distortion.
The power amplifier stage is likely to produce the majority of non-linear distortion
since it operates over large regions of its transfer characteristics. Selection of optimum
biasing conditions and use of negative feedback are the means for minimising non-
linear distortion.
Causes of Amplitude Distortion in Power Amplifier Stages
The main causes of distortion here are:
a) Mismatch of the 2 power transistors characteristics
b) Non-linearity of :
i)
IC/IB (beta) Characteristic
- Beta is not constant and changes as IC changes .
ß
IC
Figure 1
Beta – current characteristic
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Section 6
2
ii)
Beta/frequency characteristic
- beta is not the same at all frequencies and
reduces as frequency increases. This happens at fB (the frequency where beta is
3dB below midband value of beta) . This can be reduced by using power
transistors that have a higher beta cut-off frequency than that of the power amp as
a whole.
Figure 2
Beta – frequency
characteristic
Bmid
fB
fT
iii)
IC/VBE Characteristic
- is curved particularly at low current levels - when
there is no DC bias provided for the output transistors this causes
crossover
distortion
but even with a permanent bias to overcome crossover distortion the
collector current waveform can be distorted as shown in the following diagram
where the two halves of the waveform are of unequal size. The end result is
harmonic distortion
in the output waveform.
Figure 3
Collector current – base-
emitter voltage
IC
ICq
VBE
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Section 6
3
Using Negative Feedback to Reduce Distortion
Most amplifiers do not have a linear transfer characteristic - their gain changes as the
amplitude of the input changes. With a sine wave input this can cause harmonic
distortion.
Negative feedback aims to
linearise
the transfer characteristic so that all input
amplitudes are amplified to the same degree.
Negative Feedback Analysis with Non-Inverting Differential Amplifiers
Vi
_
Vi - Vf
_
A
Vo
=
A
(
Vi
−
Vf
)
=
AVi
−
A
β
Vo
Vo
A
=
=
A
CL
Vi
1
+
A
β
Vf =
β
Vo
β
R
1
for negative feedback amps
R
1
+
RF
Beta = feedback ratio
=
Example 1
If an amplifier has a feedback resistor equal to 100k and and R1 value of 1k, calculate
the closed loop gain assuming the open loop gain is 100,000.
Notes :
•
•
the addition of feedback has reduced the gain
the feedback
linearises
the transfer characteristic of the amplifier as
follows…….
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Section 6
4
Comparison of Amplifier with and without Feedback
An Amplifier without Feedback Exhibits a Non-Linear Transfer Characteristic
Vo
15
10
5
Non-Linear
Gain @ Vi = 0.4V = 37.5
Gain @ Vi = 0.2V = 25
0.2
0.4
Vi
Figure 5
Transfer plot for amplifier WITHOUT feedback
The preceeding transfer curve shows a
50% increase in gain
when the input changes
from 0.1 volts to 0.4 volts. Obviously serious output distortion will result.
An Amplifier with Feedback Exhibits a More Linear Transfer Characteristic
Suppose we apply feedback to the above amplifier and the feedback ratio is 0.05; (ß =
0.05). See Figure 6.
A
25
•
The gain @ 0.2V
=
=
=
11
1
+
0.05
×
A
1
+
0.05
×
25
37.5
•
The gain @ 0.4V
=
=
13
1
+
0.05
×
37.5
•
What is the percentage increase in gain now?
Vo
5.2
2.2
0.2
0.4
Gain @ Vi = 0.4V = 13
Gain @ Vi = 0.2V = 11.1
Vi
Figure 6
Transfer plot for amplifier shown in Figure 5 but WITH feedback applied
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Section 6
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