Negative Feedback Amplifiers

Guided by : Prof. S. S. Patil

Presentation by: Vinay Jasud (52) Vinod Arote (54)

TOPICS TO BE COVERED

Introduction Concept of Feedback Feedback Topologies General Characteristics of Negative Feedback

Amplifiers Analysis Of Feedback Amplifiers Voltage Series Feedback Voltage Shunt Feedback Current Series Feedback Current Shunt Feedback

INTRODUCTION

Feedback (Definition) It is the process in which part of

the output signal is returned back to the input.

Types of Feedback 1.Positive Feedback

2. Negative Feedback

Difference Between Positive &Negative Feedback

Parameter Positive Feedback

Negative Feedback

Overall Phase Shift

0 or 3600 1800

Input &Output Voltage, noise

Increases due to

Feedback

Decreases due to

FeedbackFeedback Signal & Input Signal

In phase Out of phase

Stability Becomes poor

Becomes better

Applications Oscillators Amplifiers

CONCEPT OF FEEDBACK

ConceptOf Feedback

Amplifiers Without Feedback

Amplifiers With Feedback

Amplifiers Without Feedback

Amplifier“A”Vi Vo

Amplifiers With Feedback

Amplifier“A”

Mixer

Feedback Network

Vi Vo

FeedbackSignal

Vs Vo

Vf

Sampling Network

𝜷 Feedback factor

Transfer characteristics with feedback (Af ) Stabilization of Gain Effect on input resistance Effect on output resistance Effect on Bandwidth Effect on nonlinear distortion Effect on noise

GENERAL CHARACTERISTICS OF NEGATIVE FEEDBACK AMPLIFIERS

TRANSFER CHARACTERISTICS WITH FEEDBACK (AF )

A = Transfer gain without feedback

A = Transfer gain with feedback𝜷= 𝑿𝑿

f

f

o

𝑨= 𝑨𝟏 + 𝜷𝑨

Transfer gain with feedback is given by

Where X = Feedback SignalX = output signal

f

o

STABILIZATION OF GAIN Fractional Change in amplification with

feedback is given by

Factors affecting stability are aging, temperature changes, replacement of component.

𝑑𝐴𝐴 = 𝑑𝐴𝐴 𝐴1+ 𝛽𝐴

f

f

SENSITIVITY AND DESENSITIVITY

Sesitivity = 11+ βA

Desensitivity =1+ βA

EFFECT ON INPUT RESISTANCE AND OUTPUT RESISTANCE

Type of feedback

Effect on input resistance

Effect on output resistance

Voltage series Increases Decreases

Current series Increases Increases

Voltage shunt Decreases Decreases

Current shunt Decreases Increases

ANALYSIS OF FEEDBACK AMPLIFIERS

Step 1: Identify the topology. Step 2: Draw the basic amplifier circuit

without feedback. Step 3: Use Thevenin’s theorem for voltage

source and Norton’s theorem for current sou.rce.

Step 4: Replace each device by it’s h-parameter.

Step 5: Indicate Xo and Xf and evaluate . Step 6: Calculate open loop gain A. Step 7: Calculate D, Af, Rif and Rof.

𝜷

VOLTAGE SERIES FEEDBACK

Transistor emitter follower circuit

Make V0=0 If Vf=0 then voltage sampling.

Feedback voltage Vf is in series with the input voltage Vs .Therefore series mixing.

voltage sampling + series mixing =Voltage series feedback

Approximate h-parameter equivalent circuit

ParametersFeedback Factor( ) (Vf / Vo)Open loop gain (Av) (hfeIbRe /Vs)

Sensitivity (D) 1+Av

Voltage gain with feedback (Avf)

(hfe Re /Rs+hie+hfeRe )

Input resistance with feedback (Rif )

hie +hfe Re

Output resistance with feedback (R’of)

Re /D

𝜷

𝜷

CURRENT SERIES FEEDBACK

CE transistor amplifier with unbypassed Re

Set output current Io=0 if Vf=0 then current sampling.

Feedback voltage Vf is in series with the input voltage Vs .Therefore series mixing.

Therefore current series feedback.

h-parameter equivalent circuit

Parameters Feedback Factor( )Open loop gain (GM) (-hfeIb / Vs)

Sensitivity (D) 1+ GM

Transconductance gain (GMF)

-1 /Re

Voltage gain with feedback (AVF)

-RL/Re

Input resistance with feedback (Rin )

Rs +hie +(1+hfe )Re

Output resistance with feedback (R’of)

RL

𝜷 -Re

𝜷

CURRENT SHUNT FEEDBACK

Current shunt feedback

Set output current Io=0 if Vf=0 then current sampling.

Feedback signal is in parallel . Hence it is current shunt feedback.

h-parameter equivalent circuit

ParametersFeedback Factor( ) Re /(Re +Rf)Open loop gain (AI) -Ic2/Is

Sensitivity (D) 1+AI

Voltage gain with feedback (Avf)

AIFRC2/Rs

Input resistance with feedback (Rin )

Ri/D

Output resistance with feedback (R’of)

Rc2

Current gain with feedback (AIF)

AI / D

𝜷

𝜷

VOLTAGE SHUNT FEEDBACK

Voltage shunt feedback

Make V0=0 If Vf=0 then voltage sampling.

Is= Ii +If .It is series mixing.

Feedback is voltage shunt feedback.

h-parameter equivalent circuit

ParametersFeedback Factor( ) -1/RF

Open loop gain (RM) RcIo/IsSensitivity (D) 1+RM

Transresistance with feedback(RMF)

RM/D

Voltage gain with feedback (Avf)

RMF/Rs

Input resistance with feedback (Rin )

(Rs ll RF ll hie /D)

Output resistance with feedback (R’of)

(RF ll RL)/D

𝜷

𝜷

Questions ???

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