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Feedback Amplifiers Subject : Advance Electronics By Ronak Gadaria 1

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

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

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

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

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

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Trans resistance Amplifier Provides a voltage output proportional to input Current Provides a voltage output proportional to input Current Ideally in this amplifier has zero input resistance & zero output resistance Ideally in this amplifier has zero input resistance & zero output resistance By Ronak Gadaria7

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Block diagram of amplifier with feed back By Ronak Gadaria8 Comparator or mixer network Sampling network Basic Amp. Forward transfer Gain A FB n/w reverse Transmission β Signal Source

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Sampling Network Voltage orCurrent or Node SamplingLoop Sampling By Ronak Gadaria 9

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Mixing Network Series MixingShunt Mixing V f = β V 0 By Ronak Gadaria 10

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

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Schematic Representation of negative Feedback system By Ronak Gadaria 12

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Voltage Amplifier with voltage series Feedback By Ronak Gadaria 13

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Trans conductance Amplifier with Current series Feedback By Ronak Gadaria 14

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Current Amplifier with Current Shunt Feedback By Ronak Gadaria 15

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Transresistance Amplifier with Voltage Shunt Feedback By Ronak Gadaria 16

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By Ronak Gadaria 17

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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 18 By Ronak Gadaria

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Transfer Gain of Feedback 19

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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 20

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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 21

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By Ronak Gadaria 22

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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 23

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By Ronak Gadaria 24

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Cut off frequencies with Feedback By Ronak Gadaria 25

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Lower cut off frequency By Ronak Gadaria 26

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By Ronak Gadaria 27

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By Ronak Gadaria 28

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Upper cut off frequency By Ronak Gadaria 29

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By Ronak Gadaria 30

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Bandwidth By Ronak Gadaria 31

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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 32

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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 33

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Input Resistance V f opposes V s, input current I i is less than that of without Feed back. R if is greater then that of R i of without FB By Ronak Gadaria 34

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

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Input resistance of Voltage series Feedback By Ronak Gadaria 36

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By Ronak Gadaria37

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Input Resistance of current series feedback By Ronak Gadaria 38

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Input resistance of current shunt feedback By Ronak Gadaria 39

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Input resistance of Voltage shunt feedback By Ronak Gadaria 40

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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 41

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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 42

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Output resistance of Voltage series feedback By Ronak Gadaria43

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By Ronak Gadaria44

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By Ronak Gadaria45 Output Resistance of Voltage shunt FB

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By Ronak Gadaria46

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Output Resistance of Current shunt FB By Ronak Gadaria 47

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By Ronak Gadaria48

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Output Resistance of Current series FB By Ronak Gadaria 49

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By Ronak Gadaria50

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Advantages of Negative Feedback Input Impedance Increased by factor (1+Aβ) Output Impedance Reduced by factor (1+Aβ) Gain Stability : The transfer gain A f 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 51

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Disadvantage of Negative Feedback All the advantages mentioned above are obtained at the expense of the gain A f 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 52

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By Ronak Gadaria 53

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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. V 0 =0, if feedback signal(x f ) becomes zero then we can say that is Voltage Sampling 2. By shorting the output loop i.e. I 0 = 0, if feedback signal(x f ) 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 54

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Step 2 : Find the input circuit 1. For voltage sampling make V 0 = 0 by shorting the output 2. For current sampling make I 0 = 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 V i = 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 55

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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 X f and X 0 on the circuit and evaluate β = X f /X 0 Step 7: From A and β A f, R if, R of and R 0f By Ronak Gadaria 56

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

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Input & Output Circuit By Ronak Gadaria 58

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H model of Transistor By Ronak Gadaria 59

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By Ronak Gadaria 60

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Current Series Feedback By Ronak Gadaria 61

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Input and output circuit By Ronak Gadaria 62

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H model of Feed back Amp. By Ronak Gadaria 63

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By Ronak Gadaria 64

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Voltage series Feedback pair By Ronak Gadaria 68

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By Ronak Gadaria 71 R3R3 R4R4

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