Handout 7
Overview
Feedback:
Part B – Effect of
Feedback on BW
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Slides taken from:
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The Concept of Feedback
Effects of feedback on Gain
Effects of feedback on non linear distortion
Effects of feedback on noise
Effects of feedback on input and output impedance
Types of feedback networks
Design of feedback amplifiers
Effect of Feedback on Bandwidth
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A.R. Hambley, Electronics, © Prentice Hall,
2/e, 2000
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Transient and frequency response
Effect of feedback on pole location
Gain margin and phase margin
Dominant-pole compensation
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Dependency from frequency
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Transient Response in terms of pole location
Until now we assumed that the open-loop gain
A and feedback ratio β were independent of
frequency
It is possible for an improperly designed
feedback amplifier to oscillate
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Transient Response in terms of pole location
Figure 9.30 Example of Complex poles in the s-plane.
Transient Response in terms of pole location
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Figure 9.31 Transient responses associated with various pole locations.
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Effect of Feedback on pole locations
Desirable pole locations
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Feedback has dramatic effect on pole
locations of amplifiers.
This in turn affect the transient response and
frequency response of the amplifier
Amplifiers Taxonomy:
Dominant-Pole Amplifiers
Two-Pole Amplifiers
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Figure 9.32 Desirable pole locations for most feedback amplifiers
are within ±45˚ of the negative real axis.
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Effect of feedback on
Dominant-Pole Amplifiers
Dominant-Pole Amplifiers
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Gain-Bandwidth Product (for a
dominant pole amplifier)
Figure 9.38 Example of Bode plots for dominant-pole feedback amplifier
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Pole location vs. feedback ratio (for
a dominant pole amplifier)
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Figure 9.39 Negative feedback causes the pole of a dominant-pole amplifier
to move to the left along the negative real axis.
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Two pole amplifiers (1)
Two pole amplifiers (2)
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Consider an amplifier having two poles in its
open-loop TF
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The closed loop poles of the TF are the roots of
1+βA(s) = 0
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Amplifiers with 3 or more poles
Figure 9.41 Root locus for a two-pole feedback amplifier.
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Example of amplifier with 3 poles (1)
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An amplifier with 3 or more poles can become
unstable when feedback is employed.
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Typically the open-loop poles of the amplifier
are on the negative real axis, but feedback can
cause them to move into the right half of the splane.
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Figure 9.45 Root locus for Example.
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Example of amplifier with 3 poles (2)
Figure 9.47 Output voltage versus time for the
unstable feedback amplifier of the previous Example.
Gain and Phase Margin
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Gain and Phase Margin
Figure 9.49 Bode plots illustrating gain margin and phase margin.
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Dominant-Pole Compensation
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Figure 9.53 Poles and corresponding magnitude Bode plot\break for a
multistage amplifier.
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Dominant-Pole Compensation
Figure 9.54 Compensation by adding a pole at -2πfc.
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