28 / 30
180
CAPITOLO 6
■
Ampli!catori
100
dB
60
dB
20log G
loop
20log 1/F
0
20log A
0
log
ω
1-G
loop
τ
1
τ
dB
20log G
TOT
Fig. 6.8
Sistema a singolo polo reazionato negativamente: diagramma del modulo della funzione A(
jω
),
del trasferimento reale
G
(
jω
) e del trasferimento ideale 1/
F
0
.
1
1
1
1
1
1
1
0
0 0
0
0 0
0 0
G s
A
s
A F
s
A
A F
s
A F
( )
τ
τ
τ
=
+
−
⋅
+
=
− ⋅
⋅
+
− ⋅
Eq. 6.11
,O SROR GL WDOH I G W VL WURYD DOOD IUHTXHQ]D
f
= 1 –
G
loop
π
lj
6H LO FLUFXLWR q UHD]LR
-
nato positivamente ed
A
·
F
! LO SROR q UHDOH SRVLWLYR HG LO FLUFXLWR q TXLQGL LQVWDELOH 6H
LQYHFH
A
·
F
VL KD XQ DXPHQWR GHO JXDGDJQR GHOO¶DPSOL¿FDWRUH ULVSHWWR DO YDORUH
A
PD XQD ULGX]LRQH GHOOD EDQGD 6H LQYHFH OD UHD]LRQH q QHJDWLYD
A
·
F
VL KD OD ULGX]LRQH
GHOO¶DPSOL¿FD]LRQH PD FRQWHPSRUDQHDPHQWH XQ DXPHQWR GHOOD EDQGD SDVVDQWH LQ UDJLRQH
GHOOD VROLWD TXDQWLWj ±
A
·
F
±
G
loop
Ê IDFLOH YHUL¿FDUH FKH SHU TXHVWR VLVWHPD D VLQ
-
JROR SROR LO SURGRWWR DPSOL¿FD]LRQH EDQGD *DLQ %DQGZLGWK 3URGXFW *%:3 UHVWD FRVWDQ
-
WH TXDOXQTXH VLD LO YDORUH GL
G
loop
H TXLQGL LQ IDVH GL SURJHWWR VL SXz YDULDUH LO JXDGDJQR
GL DQHOOR D VHFRQGD GHOOH QHFHVVLWj ULGXFHQGR SL R PHQR LO JXDGDJQR SHU DPSOLDUH OD EDQGD
$ ULJRUH TXHVWD VHPSOLFH FRQFOXVLRQH YDOH VROR QHO FDVR GL XQ VLVWHPD D VLQJROR SROR ,Q
JHQHUDOH q EHQ SL FRPSOLFDWR GHWHUPLQDUH O¶HIIHWWR GHOOD UHD]LRQH VXOOH VLQJRODULWj GHO FLUFX
-
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OD UHD]LRQH QHJDWLYD DFFDQWR DJOL DOWUL YDQWDJJL LQ WHUPLQL GL VWDELOL]]D]LRQH GHO WUDVIHULPHQWR
H GL DGDWWDPHQWR LPSHGHQ]LDOH KD DQFKH XQ HIIHWWR EHQH¿FR VXOOD ULVSRVWD LQ IUHTXHQ]D GHO
FLUFXLWR LQ TXDQWR WHQGH DG DPSOLDUQH OD EDQGD
/D )LJ ULSRUWD O¶DQGDPHQWR GHO PRGXOR GHO JXDGDJQR GL DQGDWD
A
MȦ
GHO WUDVIHUL
-
mento reale
G
MȦ
H GHO WUDVIHULPHQWR LGHDOH ±
F
SHU LO VLVWHPD D VLQJROR SROR UHD]LRQDWR
QHJDWLYDPHQWH /D GLVWDQ]D WUD OD FXUYD GL
A
MȦ
H OD UHWWD _
F
_ q SURSULR
ORJ _$
MȦlj
_ ± ORJ _
F
_ ORJ _
G
loop
j
Ȧ
Eq. 6.12
H TXLQGL UDSSUHVHQWD TXDQWLWDWLYDPHQWH O¶HQWLWj GHOOD UHD]LRQH /D IUHTXHQ]D D FXL
A
MȦ
DWWUD
-
YHUVD OD UHWWD
F
q OD IUHTXHQ]D DOOD TXDOH _
G
loop
MȦ
_
$ EDVVD IUHTXHQ]D GRYH _
G
loop
j
Ȧ
_ !! LO WUDVIHULPHQWR UHDOH
G j
Ȧ
≅
±
F
$O GL
-
PLQXLUH GL _
G
loop
j
Ȧ
_
G j
Ȧ
VL VFRVWD VHPSUH SL VHQVLELOPHQWH GD ±
F
H SHU
ω
ĺ OD
FXUYD GL _
G j
Ȧ
_ H TXHOOD GL _
A j
Ȧ
_ VL VRYUDSSRQJRQR FRPH FL VL DWWHQGH GDOOD (T SHU
_
G
loop
j
Ȧ
_
→