1. Subject : Advance Electronics
Feedback Amplifiers
Subject : Advance Electronics
By Ronak Gadaria

2. Introduction
When the part of output is sampled & fed back to the input of amplifier.
Input & part of output which is fed back to the input
Positive Feedback (In phase)
Negative Feed back (Output of Phase)
Improve its performance & to make it more ideal.
+ve Feedback results in oscillators & hence not used in amplifiers.
By Ronak Gadaria

3. Classification of Amplifiers
Classification of amplifiers based on the mag. of input & output imp. of an amplifier relative to the source & load impedance, respectively
Voltage Amplifier (Vo/Vi)
Current Amplifier (Io/Ii)
Transconduction Amplifier (Io/Vi)
Transresistance Amplifier (Vo/Ii)
By Ronak Gadaria

4. Voltage Amplifier Thevenin’s equivalent circuit of V A
Such amplifiers will provide a voltage output resistance proportional to voltage input.
By Ronak Gadaria

5. Current Amplifier
Norton’s equivalent circuit of C A
Provides a current output proportional to input.
An ideal C A must have zero input resi. & infinite output resi.
By Ronak Gadaria

6. Trans conductance Amplifier
Provides a current output proportional to input voltage
Ideally in this amplifier has infinite input resistance & infinite output resistance
By Ronak Gadaria

7. Trans resistance Amplifier
Provides a voltage output proportional to input Current
Ideally in this amplifier has zero input resistance & zero output resistance
By Ronak Gadaria

8. Block diagram of amplifier with feed back
Comparator or mixer network
Sampling network
Basic Amp. Forward transfer Gain A
FB n/w reverse Transmission β
Signal Source
By Ronak Gadaria

9. Sampling Network Voltage or Current or Node Sampling Loop Sampling
By Ronak Gadaria

10. Mixing Network
Series Mixing Shunt Mixing
Vf = β V0
By Ronak Gadaria

11. Transfer Ratio or Gain Ratio of Output signal to input is denoted by A
By Ronak Gadaria

12. Schematic Representation of negative Feedback system
By Ronak Gadaria

13. Voltage Amplifier with voltage series Feedback
By Ronak Gadaria

14. Trans conductance Amplifier with Current series Feedback
By Ronak Gadaria

15. Current Amplifier with Current Shunt Feedback
By Ronak Gadaria

16. Transresistance Amplifier with Voltage Shunt Feedback
By Ronak Gadaria

17. By Ronak Gadaria

18. Parameters of –ve Feed Back Amplifier
Transfer Gain with FB
Loop Gain or Return Gain
De-sensitivity of Gain
Cut off frequency with FB
Lower cut off frequency
Upper cut off frequency
Distortion with FB
Input & output Resistance
By Ronak Gadaria

19. Transfer Gain of Feedback

20. Loop Gain or Return Ratio
The gain of the loop is –Aβ
Difference between unity & loop gain is called as return difference. D = 1+Aβ
The amount of the feed back introduced into an amplifier can be expressed on (dB)
For Negative FB, N will be negative.
By Ronak Gadaria

21. De-sensitivity of Gain
Transfer gain is not constant as it depends upon the factors such as operating point, temperature etc.
This lack of stability in amplifiers can be reduced by introducing negative FB.
By Ronak Gadaria

22. By Ronak Gadaria

23. This ratio is called sensitivity of the transfer gain
The reciprocal of the sensitivity is called the De sensitivity. D = 1+Aβ
Thus stability of amplifier increases with increase in de sensitivity.
Aβ >>1 then,
Amplifier Gain is dependent only on gain of the feedback network.
By Ronak Gadaria

24. By Ronak Gadaria

25. Cut off frequencies with Feedback
By Ronak Gadaria

26. Lower cut off frequency
By Ronak Gadaria

27. By Ronak Gadaria

28. By Ronak Gadaria

29. Upper cut off frequency
By Ronak Gadaria

30. By Ronak Gadaria

31. Bandwidth
By Ronak Gadaria

32. Distortion with Feedback
we can say that if the feedback network does not contain reactive elements, the overall gain is not a function of frequency.
Frequency and phase distortion is substantially reduced.
In tuned amplifiers, feedback network is designed such that at tuned frequency β tends 0 and at other frequencies β tends infinity.
By Ronak Gadaria

33. Noise & Nonlinear Distortion
Signal feedback reduces the amount of noise signal and nonlinear distortion.
The factor (1+Aβ) reduces both input noise and resulting nonlinear distortion for considerable improvement.
Thus, noise and nonlinear distortion also reduced by same factor as the gain.
By Ronak Gadaria

34. Input Resistance
Vf opposes Vs, input current Ii is less than that of without Feed back.
Rif is greater then that of Ri of without FB
By Ronak Gadaria

35. Rif is Lesser then that of Ri of without FB
The current Is, drawn from the signal source is increased over what it would be if There were no feedback current.
Rif is Lesser then that of Ri of without FB
By Ronak Gadaria

36. Input resistance of Voltage series Feedback
By Ronak Gadaria

37. By Ronak Gadaria

38. Input Resistance of current series feedback
By Ronak Gadaria

39. Input resistance of current shunt feedback
By Ronak Gadaria

40. Input resistance of Voltage shunt feedback
By Ronak Gadaria

41. Output Resistance
The negative feedback which samples the output voltage, regardless of how this output signal is returned to the input, tends to decrease the output resistance
By Ronak Gadaria

42. On the other hand, the negative feedback which samples the output current, regardless of how this signal is returned to the input, tends to increase the output resistance.
By Ronak Gadaria

43. Output resistance of Voltage series feedback
By Ronak Gadaria

44. By Ronak Gadaria

45. Output Resistance of Voltage shunt FB
By Ronak Gadaria

46. By Ronak Gadaria

47. Output Resistance of Current shunt FB
By Ronak Gadaria

48. By Ronak Gadaria

49. Output Resistance of Current series FB
By Ronak Gadaria

50. By Ronak Gadaria

51. Advantages of Negative Feedback
Input Impedance Increased by factor (1+Aβ)
Output Impedance Reduced by factor (1+Aβ)
Gain Stability : The transfer gain Af of amp with feedback can be stabilized against variations of the h-para of transistor
Increased bandwidth by a factor (1+Aβ).
Reduced non linear distortion
Reduced noise by a factor (1+Aβ)
By Ronak Gadaria

52. Disadvantage of Negative Feedback
All the advantages mentioned above are obtained at the expense of the gain Af with Feedback, which is lowered in comparison with the transfer gain A of an amplifier without feedback by a factor (1+Aβ).
A negative feed back amplifier is designed for the particular range of frequency. It may break out in to oscillation at some high or low frequency.
By Ronak Gadaria

53. By Ronak Gadaria

54. Method of identifying feedback topology and analysis of a feedback amplifier
Step:1 Identify Topology (Type of Feedback)
A) To find out the type of feedback (Sampling)
1 By shorting the output i.e. V0=0, if feedback signal(xf) becomes zero then we can say that is “Voltage Sampling”
2. By shorting the output loop i.e. I0 = 0, if feedback signal(xf) becomes zero then we can say that is “Current Sampling”
B) To find the type of mixing network
1. If the feedback signal is subtracted from the externally applied signal as a voltage in the input loop, we can say that it is “Series mixing”
2. If the feedback signal is subtracted from the externally applied signal as a current in the input loop, we can say that it is “Shunt mixing”
By Ronak Gadaria

55. Step 2 : Find the input circuit
1. For voltage sampling make V0 = 0 by shorting the output
2. For current sampling make I0 = 0 by shorting the output
Step 3: Find the output circuit
1. For series mixing make Ii= 0, by opening the input loop.
2. For shunt mixing make Vi = 0, by shorting the input
Step 2 & Step 3 ensure that the feedback is reduced to zero without altering the loading on the basic amplifier.
By Ronak Gadaria

56. Step 6: Indicate Xf and X0 on the circuit and evaluate β = Xf/X0
Step 4 : Optional. Replace each active device by its h-parameter model at low frequency.
Step 5 : Find out open loop gain (gain without feedback), A of the amplifier.
Step 6: Indicate Xf and X0 on the circuit and evaluate β = Xf/X0
Step 7: From A and β Af, Rif, Rof and R’0f
By Ronak Gadaria

57. Voltage Series Feedback
Emitter Follower
Vo i.e. Vf i.e.
Voltage across Re
increase
VBE Decrease
Hence –ve FB
By Ronak Gadaria

58. Input & Output Circuit
By Ronak Gadaria

59. H model of Transistor
By Ronak Gadaria

60. By Ronak Gadaria

61. Current Series Feedback
By Ronak Gadaria

62. Input and output circuit
By Ronak Gadaria

63. H model of Feed back Amp.
By Ronak Gadaria

64. By Ronak Gadaria

65. By Ronak Gadaria

66. By Ronak Gadaria

67. By Ronak Gadaria

68. Voltage series Feedback pair
By Ronak Gadaria

69. By Ronak Gadaria

70. By Ronak Gadaria

71. R3 R4
By Ronak Gadaria

72. By Ronak Gadaria