Et Dignitas Guangdong Institute of Education ---BTEC electronic Amor Electronics chapter 4 negative...

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Guangdong Institute of Education ---Guangdong Institute of Education ---BTEC electronicBTEC electronic

Electronics chapter 4 negative feedback

Review0、 introduction1、 what is feedback?2、 why we need feedback in our circuit?3、 how many types of feedback?4、 how many types of negative feedback?5、 how to judge all kinds of negative feedback?6、 advantages of negative feedback?7 、 how to analyse the negative feedback circuit?8 、 conclusion----- assignment

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introduction

An audio-frequency amplifier can be designed to have a certain current, voltage or power gain together with particular values of input and output impedance.

The amplifier will add noise and distortion to the signals handled by it. The components employed in the amplifier, both passive (resistors, capacitors, etc.) and active (transistors and f.e.t.s), will vary in value with both time and change in temperature and will have manufacturing tolerances.

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introduction

The gain of the amplifier may therefore vary with

time, with change in ambient temperature, and when

a component has to be replaced by another of the

Same type and(nominal)value.

any fluctuations in the power supply may also cause

the gain of the amplifier to alter.

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feedback

R110kΩ

R210kΩ

R3240Ω

R41kΩ

RL

C1

4.7uF

C2100uF

C3

47uF

VCC

12V

IO1IO1

IO2

IO3

IO4

IO3

0

Q1

2N914

10

9

1

R5

470kΩKey=A

50% VCC

2

Ui

Uo

Ii

Io

1 、 what is feedback?

2 、 For many applications a more or less constant gain iS necessary ， and this can be obtained if negative feedback (n.f.b.) iS applied to the amplifier, at the

expense, however,of a reduction in gain.

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why we need feedback in our circuit?

1、 make our circuit stabilization:Quescent operation point;Stability of gain: current gain, voltage gain;Decrease non-linearity distortionReduces noiseChange input and output impedanceReduces amplitude／ frequency distortion 2、 generate oscillation (oscilloscope)

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Types of feedback

0 、 d.c feedback1 、 a.c. feedback2 、 positive feedback3 、 negative feedback4 、 voltage feedback 5 、 current feedback 6 、 Series feedback7 、 parallel feedback

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chapter 4 negative feedback

4.2 types of negative feedback

4.2.1.voltage-voltage feedback,

4.2.2.voltage-current feedback,

4.2.3.current-current feedback.

4.2.4.current-voltage feedback.

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The negative feedback

An n.f.b. amplifier has a fraction of its output signal feed back into its input terminals in antiphase with the input signal.

four types：voltage-voltage feedback,

voltage-current feedback,

current-current feedback.

current-voltage feedback.




Voltage-Voltage Feedback(Series-voltage Feedback)




S IN OUTV V V

OUT v S v IN OUT v IN v OUTV AV A V V AV AV

(1 )OUT v v INV A AV (1 )

v INOUT

v

AVV

A

( ) (1 )OUT v

v FIN v

V AA

V A

Voltage gain with n.f.b.




EXAMPLE 4.1

A voltage amplifier has a voltage gain of 100 before n.f.b.is applied.Calculate its voltage gain if 3/100 of the output voltage is fed back to the input in antiphase with the input signal.

Solution:

From equation(4.1),

( )

100 10025

(1 3/100 100) 4OUT

v FIN

VA

V




The voltage gain of a transistor amplifier without n.f.b.iS

The effective load resistance RL(eff)seen by the transistor in an emitter follower circuit iS the resultant of the emitter resistor R3 in parallel with the external load resistance RL.The voltage gain of an emitter follower is

i Lv

IN

ARA

R

( ) ( )( )

( ) ( )

/

1

i L eff IN i L effv F

i L eff IN i L eff

IN

AR R ARA

AR R ARR




Voltage-Current Feedback(Series-current Feedback)




When voltage-current feedback is applied to an amplifier the fed-back voltage is proportional to the current-flowing in the load.

OUT OUT F

OUT OUT L

V I R

V I R

F

L

R

R 3i

vIN

ARA

R

3

3( )

34 4

3

1 .

i

IN iv F

i IN i

IN

AR

R ARA

ARR R ARR R

2 2( )

3 32

2

11 .

m mv F

mm

g R g RA

R g Rg RR




EXAMPLE 4.5

An amplifier of the type shown in Fig.4.8b has the following data:R2=4.7 kΩ,R3=1 kΩand gm=5 mS.Calculate the voltage gain of the circuit.

Solution

From equation(4.6) 3 3

( ) 3 3

5 10 4.7 103.92

1 5 10 10v FA




Current-Current Feedback(Shunt-Current Feedback)




1

1 2

OUT FOUT

F F

I RI

R R

1

1 2

F

F F

R

R R

S IN OUTI I I

( )OUT i S i IN OUTI A I A I I

(1 )OUT i i INI A A I (1 )

i INOUT

i

A II

A

( ) (1 )OUT i

i FIN i

I AA

I A




Current-Voltage Feedback （ Shunt-voltage Feedback)




/OUTOUT FI V R

OUT

OUT F

OUTOUT

L

VI R

VIR

L

F

R

R




EXAMPLE 4.6

Calculate the current gain of the circuit shown in Fig.4.12a if the current gain of the transistor is 120.

Solution

The feedback factor is (Ans.)

Therefore

Current gain with feedback

(Ans.)

2

1

3 1

120 40

R

R

( )

120 12030

1 41 12040

i FA




Negative Feedback




4.3 advantages of negative feedback4.3.1. stability of gain4.3.2.amplitude/frequency distortion4.3.3.non-linearity distortion4.3.4.noise4.3.4.input and output impedance4.4 instability in negative feedback amplifiers4.5 operational amplifiers4.5.1.operational amplifier parameters4.5.2.inverting and non-inverting amplifiers4.5.3.frequency compensation

Et Dignitas Guangdong Institute of Education ---BTEC electronic Amor Electronics chapter 4 negative...

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