1. Part B – Effect of Feedback on BW
Slides taken from:
A.R. Hambley, Electronics, © Prentice Hall, 2/e, 2000

2. Overview 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
Transient and frequency response
Effect of feedback on pole location
Gain margin and phase margin
Dominant-pole compensation

3. Dependency from frequency
Until now we assumed that the open-loop gain A and feedback ratio b were independent of frequency
It is possible for an improperly designed feedback amplifier to oscillate

4. Transient Response in terms of pole location

5. Figure 9.30 Example of Complex poles in the s-plane.
Transient Response in terms of pole location
Figure Example of Complex poles in the s-plane.

6. Transient Response in terms of pole location
Figure Transient responses associated with various pole locations.

7. Desirable pole locations
Figure Desirable pole locations for most feedback amplifiers
are within ±45˚ of the negative real axis.

8. Effect of Feedback on pole locations
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
Amplifiers with three or more poles

9. Dominant-Pole Amplifiers

10. Figure 9.38 Example of Bode plots for dominant-pole feedback amplifier
Effect of feedback on Dominant-Pole Amplifiers
Figure Example of Bode plots for dominant-pole feedback amplifier

11. Gain-Bandwidth Product (for a dominant pole amplifier)

12. to move to the left along the negative real axis.
Pole location vs. feedback ratio (for a dominant pole amplifier)
Figure Negative feedback causes the pole of a dominant-pole amplifier
to move to the left along the negative real axis.

13. Two pole amplifiers (1)
Consider an amplifier having two poles in its open-loop TF
The closed loop poles of the TF are the roots of 1+bA(s) = 0

14. Figure 9.41 Root locus for a two-pole feedback amplifier.
Two pole amplifiers (2)
Figure Root locus for a two-pole feedback amplifier.

15. Amplifiers with 3 or more poles
An amplifier with 3 or more poles can become unstable when feedback is employed.
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 s-plane.

16. Figure 9.45 Root locus for Example.
Example of amplifier with 3 poles (1)
Figure Root locus for Example.

17. Example of amplifier with 3 poles (2)
Figure Output voltage versus time for the
unstable feedback amplifier of the previous Example.

18. Gain and Phase Margin

19. Figure 9.49 Bode plots illustrating gain margin and phase margin.
Gain and Phase Margin
Figure Bode plots illustrating gain margin and phase margin.

20. Figure 9.53 Poles and corresponding magnitude Bode plot\break for a
Dominant-Pole Compensation
Figure Poles and corresponding magnitude Bode plot\break for a
multistage amplifier.

21. Figure 9.54 Compensation by adding a pole at -2pfc.
Dominant-Pole Compensation
Figure Compensation by adding a pole at -2pfc.