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Feedback (2)

Feedback (2). Section 8.2-8.4. Topics. Feedback topologies Loading Effects Effect of Feedback on Noise. Feedback Topologies. Types Voltage-voltage Voltage-Current Current-Voltage Current-Current Parameters Closed Loop Gain Input Impedance Output Impedance. Summary.

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Feedback (2)

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  1. Feedback (2) Section 8.2-8.4

  2. Topics • Feedback topologies • Loading Effects • Effect of Feedback on Noise

  3. Feedback Topologies • Types • Voltage-voltage • Voltage-Current • Current-Voltage • Current-Current • Parameters • Closed Loop Gain • Input Impedance • Output Impedance

  4. Summary

  5. General Comment • Parallel Connection: Impedance fall by 1+loop gain. • Series Connection: Impedance Rises by 1+loop gain

  6. Voltage-Voltage Feedback Sense Vout in parallel Return Vin in series Alternative name: Return-Sense=Series-Shunt feedback

  7. Ideal A0 Infinite input resistance so it can sense voltage as an ideal voltmeter. Zero output resistance so as to serve as an ideal voltage source.

  8. Example (R1+R2=large so as not to disturb Vout)

  9. Input Resistance Without feedback: With feedback: (non-ideal) (ideal)

  10. Example

  11. Output Resistance (ideal)

  12. Example

  13. Voltage-Voltage Feedback Sense Vout in parallel Return Vin in series

  14. Voltage-Current Feedback Sense Vout in parallel Return current in parallel Alternative name: Return-Sense=Shunt-Shunt feedback K has a dimension of conductance: K=IF/Vout

  15. Example IRF=Vout/RF K=-1/RF (- comes from the The direction of IF) (RF is large in order to return a current) (Open-loop gain) Assumption:RF is large! Or RF>>RD2

  16. Ideal R0 Zero input impedance so that it can Measure currents as an ideal current meter. Zero output resistance so as to behave as an ideal voltage source.

  17. Calculation of Input Impedance (small resistance)

  18. Example (Open loop input-impedance) R0=RD1(-gm2RD2) IRF=Vout/RF K=-1/RF

  19. Calculation of Output Impedance VA=(-IF)RoRout (small resistance) (Current drawn by the feedback network is neglected)

  20. Example Rout=RD2 R0=RD1(-gm2RD2) IRF=Vout/RF K=-1/RF

  21. Current-Voltage Feedback Sense Iout in series Return Vin in series Alternative name: Return-Sense=series-series feedback (K=VF/Iout, hence a dimension of resistance)

  22. Gm Infinite input resistance so it can sense voltage as an ideal voltmeter. Infinite output resistance in order to behave as an ideal current source.

  23. Example (For sensing current) (polarity check) (Calculate the open loop gain)

  24. Calculation of Input Impedance (Vin-VF)/Rin=Iin VF=KIinRinGm

  25. Example Open Loop Input impedance: 1/gm

  26. Calculation of Output Impedance

  27. Example Open Loop Input impedance: 1/gm2

  28. Current-Current Feedback Sense Vout in parallel Return current in parallel Alternative name: Return-Sense=Shunt-Shunt feedback K has a dimension of conductance: K=IF/Vout

  29. Current-Current Feedback Sense Iout in series Return current in parallel Alternative name: Return-Sense=Shunt-series feedback (current gain) K has a dimension of conductance: K=IF/Iout

  30. Ideal Forward Current Amplifier Zero input impedance in order to maximize current transfer. Infinite output impedance in order to behave as a current source.

  31. Polarity of Feedback

  32. Current and Current Feedback RM is small, therefore VP is small. Vp is IoutRM (RF>>1/gm1) RF is large in order for K to behave as a current source.

  33. Calculation of Input Impedance

  34. Example

  35. Calculation of Output Impedance AI

  36. Example

  37. In Summary

  38. Inclusion of I/O Effects

  39. Rules for Breaking the Feedback Network (1)

  40. Rules for Breaking the Feedback Network (2)

  41. Voltage-Voltage Feedback K is driven by a zero source impedance. K sees the infinite input impedance of the forward amplifier.

  42. Voltage-Current Feedback K is driven by a zero source impedance. K sees a zero input impedance of the forward amplifier.

  43. Current-Voltage Feedback K is driven by an infinite source impedance. K sees the infinite input impedance of the forward amplifier.

  44. Current-Current Feedback K is driven by an infinite source impedance. K sees the zero input impedance of the forward amplifier.

  45. Rules for Breaking the Feedback Network • Applicable for both sense and return duplicate. • Open for series connection • Shorted for parallel connection

  46. Calculate the Feedback Factor

  47. Voltage-Voltage Feedback

  48. Voltage-Current Feedback

  49. Current-Voltage Feedback

  50. Current-Current Feedback

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