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1/11
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
4/11
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)
5/11
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
8/11
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
11/11
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