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**Subject : Advance Electronics**

Feedback Amplifiers Subject : Advance Electronics By Ronak Gadaria

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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

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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 Voltage Amplifier (Vo/Vi) Current Amplifier (Io/Ii) Transconduction Amplifier (Io/Vi) Transresistance Amplifier (Vo/Ii) By Ronak Gadaria

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**Voltage Amplifier Thevenin’s equivalent circuit of V A**

Such amplifiers will provide a voltage output resistance proportional to voltage input. By Ronak Gadaria

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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

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**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

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**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

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**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

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**Sampling Network Voltage or Current or Node Sampling Loop Sampling**

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

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**Transfer Ratio or Gain Ratio of Output signal to input is denoted by A**

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**Schematic Representation of negative Feedback system**

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**Voltage Amplifier with voltage series Feedback**

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**Trans conductance Amplifier with Current series Feedback**

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**Current Amplifier with Current Shunt Feedback**

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**Transresistance Amplifier with Voltage Shunt Feedback**

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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 By Ronak Gadaria

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**Transfer Gain of Feedback**

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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

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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

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By Ronak Gadaria

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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

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By Ronak Gadaria

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**Cut off frequencies with Feedback**

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**Lower cut off frequency**

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**Upper cut off frequency**

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Bandwidth By Ronak Gadaria

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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

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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

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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

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**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

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**Input resistance of Voltage series Feedback**

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By Ronak Gadaria

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**Input Resistance of current series feedback**

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**Input resistance of current shunt feedback**

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**Input resistance of Voltage shunt feedback**

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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

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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

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**Output resistance of Voltage series feedback**

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**Output Resistance of Voltage shunt FB**

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By Ronak Gadaria

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**Output Resistance of Current shunt FB**

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**Output Resistance of Current series FB**

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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 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

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**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

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By Ronak Gadaria

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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. 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

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**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

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**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

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**Voltage Series Feedback**

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

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Input & Output Circuit By Ronak Gadaria

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H model of Transistor By Ronak Gadaria

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By Ronak Gadaria

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**Current Series Feedback**

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**Input and output circuit**

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H model of Feed back Amp. By Ronak Gadaria

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**Voltage series Feedback pair**

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R3 R4 By Ronak Gadaria

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By Ronak Gadaria

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