Feedback AmplifiersFeedback Amplifiers

Subject : Advance Electronics

By Ronak Gadaria 1

IntroductionIntroduction

When the part of output is sampled & fed When the part of output is sampled & fed back to the input of amplifier.back to the input of amplifier.

Input & part of output which is fed back to Input & part of output which is fed back to the inputthe 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 Improve its performance & to make it more ideal.ideal.

+ve Feedback results in oscillators & +ve Feedback results in oscillators & hence not used in amplifiers.hence not used in amplifiers.

By Ronak Gadaria 2

Classification of Classification of AmplifiersAmplifiers

Classification of amplifiers based on the Classification of amplifiers based on the mag. of input & output imp. of an amplifier mag. of input & output imp. of an amplifier relative to the source & load impedance, relative to the source & load impedance, respectivelyrespectively

Voltage Amplifier (VVoltage Amplifier (Voo/V/Vii))Current Amplifier (ICurrent Amplifier (Ioo/I/Iii))Transconduction Amplifier (ITransconduction Amplifier (Ioo/V/Vii))Transresistance Amplifier (VTransresistance Amplifier (Voo/I/Iii))

By Ronak Gadaria 3

Voltage AmplifierVoltage Amplifier

Thevenin’s equivalent circuit of V AThevenin’s equivalent circuit of V ASuch amplifiers will provide a voltage Such amplifiers will provide a voltage

output resistance proportional to voltage output resistance proportional to voltage input.input.

By Ronak Gadaria 4

Current AmplifierCurrent Amplifier

Norton’s equivalent circuit of C ANorton’s equivalent circuit of C AProvides a current output proportional to Provides a current output proportional to

input.input. An ideal C A must have zero input resi. & An ideal C A must have zero input resi. &

infinite output resi.infinite output resi.

By Ronak Gadaria 5

Trans conductance AmplifierTrans conductance Amplifier

Provides a current output proportional to Provides a current output proportional to input voltageinput voltage

Ideally in this amplifier has infinite input Ideally in this amplifier has infinite input resistance & infinite output resistancresistance & infinite output resistancee

By Ronak Gadaria 6

Trans resistance AmplifierTrans resistance Amplifier

Provides a voltage output Provides a voltage output proportional to input Currentproportional to input Current

Ideally in this amplifier has zero Ideally in this amplifier has zero input resistance & zero output input resistance & zero output resistanceresistance

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Block diagram of amplifier Block diagram of amplifier with feed backwith feed back

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Comparator or

mixer network

Sampling

network

Basic Amp. Forward

transfer Gain A

FB n/w reverse Transmission

β

Signal Source

Sampling NetworkSampling Network

Voltage or Current or Node Sampling Loop Sampling

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Mixing NetworkMixing Network

Series Mixing Shunt MixingVf

= β V0

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Transfer Ratio or GainTransfer Ratio or Gain

Ratio of Output signal to input is denoted by A

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Schematic Representation of Schematic Representation of negative Feedback systemnegative Feedback system

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Voltage Amplifier with Voltage Amplifier with voltage series Feedbackvoltage series Feedback

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Trans conductance Amplifier Trans conductance Amplifier with Current series Feedbackwith Current series Feedback

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Current Amplifier with Current Shunt Current Amplifier with Current Shunt FeedbackFeedback

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Transresistance Amplifier Transresistance Amplifier with Voltage Shunt Feedbackwith Voltage Shunt Feedback

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Parameters of –ve Feed Back Parameters of –ve Feed Back AmplifierAmplifier

Transfer Gain with FBLoop Gain or Return GainDe-sensitivity of GainCut off frequency with FB

◦Lower cut off frequency◦Upper cut off frequency

Distortion with FBInput & output Resistance

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Transfer Gain of FeedbackTransfer Gain of Feedback

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Loop Gain or Return RatioLoop 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

De-sensitivity of GainDe-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.

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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.

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Cut off frequencies with Cut off frequencies with FeedbackFeedback

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Lower cut off frequencyLower cut off frequency

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Upper cut off frequencyUpper cut off frequency

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BandwidthBandwidth

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Distortion with FeedbackDistortion 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

Noise & Nonlinear DistortionNoise & 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.

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Input ResistanceInput 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

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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.

Rif is Lesser then that of Ri of without FB

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Input resistance of Voltage series Input resistance of Voltage series FeedbackFeedback

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Input Resistance of current series Input Resistance of current series feedbackfeedback

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Input resistance of current shunt Input resistance of current shunt feedbackfeedback

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Input resistance of Voltage shunt Input resistance of Voltage shunt feedbackfeedback

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Output ResistanceOutput 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

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.

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Output resistance of Voltage series Output resistance of Voltage series feedbackfeedback

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Output Resistance of Voltage shunt Output Resistance of Voltage shunt FBFB

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Output Resistance of Current shunt Output Resistance of Current shunt FBFB

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Output Resistance of Current series Output Resistance of Current series FBFB

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Advantages of Negative Advantages of Negative FeedbackFeedback

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 distortionReduced noise by a factor (1+Aβ)

By Ronak Gadaria 51

Disadvantage of Negative Disadvantage of Negative FeedbackFeedback

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 52

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Method of identifying feedback Method of identifying feedback topology and analysis of a topology and analysis of a

feedback amplifierfeedback amplifierStep: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 network1. 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”

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Step 2 : Find the input circuit1. For voltage sampling make V0 = 0 by shorting the

output2. For current sampling make I0 = 0 by shorting the

outputStep 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.

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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 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 56

Voltage Series FeedbackVoltage Series Feedback

Emitter FollowerVo i.e. Vf i.e.

Voltage across Re increase

• VBE Decrease

• Hence –ve FB

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Input & Output Circuit

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H model of Transistor

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Current Series FeedbackCurrent Series Feedback

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Input and output circuit

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H model of Feed back Amp.H model of Feed back Amp.

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Voltage series Feedback pairVoltage series Feedback pair

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R3 R4

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