1. Feedback loop gain, stability
Introduction
Feedback loop gain, stability 1 1
October 9, 2007

2. HW and Quiz HW #7 due Oct. 15 8.18,8.45,8.63,8.70 Midterm II Oct 29 2

3. Introduction Physical systems incorporate some form of feedback
Theory of negative feedback developed by Electrical Engineers.
Feedback theory used in modeling of systems.
Negative or Positive Feedback 3 3
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4. Today
Stability
Loop Eqn 4 4
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5. Sampling & Mixing Here, we look at feedback ckts
voltage-sampling series-mixing (series-shunt) topology 5 5
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6. current-sampling shunt-mixing (shunt-series) topology6 6
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7. current-sampling series-mixing (series-series) topology7 7
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8. voltage-sampling shunt-mixing (shunt-shunt) topology.8 8
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9. Series-shunt feedback amplifier:
Ideal ckt
Equivalent ckt. 9 9
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10. feedback amplifier of Rof  Vt/I.
Measuring the output resistance of the voltage-sampling series-mixing (series-shunt) topology
feedback amplifier of Rof  Vt/I. 10 10
March 22, 2007

11. Block diagram of a practical series-shunt feedback amplifier.
Derivation of the A circuit and  circuit for the series-shunt feedback amplifier.
Block diagram of a practical series-shunt feedback amplifier.
The circuit in (a) with the feedback network represented by its h parameters.
The circuit in (b) after neglecting h21. 11 11
March 22, 2007

12. Summary of the rules for finding the A circuit and  for the voltage-sampling series-mixing12 12
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13. Example 8.1 Amplifier shown, find simplified feedback models13 13
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14. 14 14
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15. Series-series feedback amplifier
(a) ideal structure;
(b) equivalent circuit. 15 15
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16. Ideal structure for the shunt-shunt feedback amplifier.16 16
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17. Shunt-shunt feedback amplifier.17 17
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18. Finding the A circuit and  for the voltage- sampling shunt- mixing (shunt- shunt) case.18 18
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19. Shunt-series feedback amplifier.
Ideal structure 19 19
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20. Shunt-series feedback
Block diagram 20 20
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21. Finding the A circuit and  for the current-sampling shunt-mixing (shunt-series) case.21 21
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22. Example 8.4 22 22
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23. Example 8.4. 23 23
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24. Amplifier instability
A(jώ180) =999
β(jώ180) = 24 24
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25. Bode plot for the loop gain A illustrating the definitions of the gain and phase margins.25 25
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26. Stability analysis using Bode plot of |A|.26 26
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27. Frequency compensation for  = 10-2.
The response labeled A’ is obtained by introducing an additional pole at fD.
The A” response is obtained by moving the original low-frequency pole to f’D. 27 27
March 22, 2007