PowerPoint Presentationece.tamu.edu/~jose-silva-martinez/courses/ECEN489/Lecture Notes/… · A ar...

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Using NF:The properties of the feedback amplifier are modified in respect with the properties of the basic amplifier:

- the gain is reduced- other properties are improved: the gain sensitivity is reduced the nonlinear distortion is reduced the bandwidth is extended the effect of the noise is reduced the input and output impedances are enhanced

Negative feedback effects on amplifiers

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1 Reduction in gain

araA

1

aA

0ar

If ar >> 1 (e.g. for op amp )a

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2 Reduction in the gain sensitivity (desensitize the gain)

The gain of the basic amplifier depends generally on certain factors (temperature, parameters of active devices, OP, etc.)

What is the relative variation of the feedback gain in comparison with the relative variation of the basic gain?

22 )1(1

)1()1(

arararar

dadA

araA

araara

dadA

111

111

1

araA

1

ada

arAdA

11

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Illustration:1+ar =100

%10ada %1,0

AdA

NF improves the stability of the feedback gain (reduces the sensitivity of the gain against the action of some perturbing factors.

2 Reduction in the gain sensitivity (desensitize the gain)

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3 Reduction in the nonlinear distortion

Generally the gain is linear for small-signal, around the operating point

If the output signal increases, the gain becomes nonlinear

raaA

1

11 1

raaA

2

22 1

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The range of the input signal increases

basic amplifier NF amplifier

];[ 22 ii xx ];[ 22 ss xx

1001 a 502 a 09,0r

1009,01001

1001

A

1,909,0501

502

A

%91

12 A

AA%50

1

12 a

aa

1

12

21

12

11

aaa

raAAA

Illustration

Price: reduction of the gain

Solution: use a preamplifier

Increases the input signal range for linear behaviour

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4 Bandwidth extension

H

j

aja

1)( 0

2

HHf

)1(1

1)(

0

0

0

raj

raa

jA

H

Hr

j

AjA

1)( 0

raaA

0

00 1 )1( 0raff HHr

rjajajA

)(1)()(

Low-pass-type amplifier

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raaA

0

00 1

)1( 0raff HHr

HfB Hrr fB

BraBr )1( 0

Consequence: BaBA r 00 If the high-frequency response is characterized by a single pole

Gain bandwidth product is constant

Introducing NF, de number of times that reduces the gain is equal to the numbers of times that extends the bandwidth – amount of feedback

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Band-pass-type amplifier

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5 Increasing the signal-to-noise ratio

in

s

n

s

on

oso ns

xx

axax

xxns )()(

se

eos x

raaaax

1

ne

on xraa

ax

1

ien

se

ne

se

e

on

oso nsa

xxa

xraa

a

xraa

aa

xxns )(

1

1)(

optional

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6 Enhancement of the input and output impedances

input output

series

parallel

voltage current

arRR i

ir

1 arRR o

or

1

)1( arRR iir )1( arRR oor

voltagecurrent

1 2

3 4