1. Feedback (2)
Section

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

3. Feedback Topologies Types Parameters 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 (RF is large in order to return a current)
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 (polarity check) (Calculate the open loop gain) (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 ImpedanceAI

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

51. Rules for Determining the Feedback
If the output of the feedback depends on voltage, open it.
If the output of the feedback depends on current, short it.

52. Voltage-Voltage Example (1)
(R1+R2 is not much larger than RD)

53. Voltage-Voltage Example(1)

54. Voltage-Voltage Example (2)

55. Voltage-Voltage Example (2)

56. Voltage-Current Example (1)

57. Voltage-Current Example (1)

58. Current-Voltage Example (1)

59. Current-Voltage Example (1)

60. Current-Current Example (1)

61. Current-Current Example (1)