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Passive and Active Cu ent Mi o sPassive and Active Current Mirrors (Ch. 5)
김 영 석김 영 석
충북대학교 전자정보대학
2013.3.1. .
Email: [email protected]
전화: 043-261-3137
전자정보대학 김영석 1
5.1 Basic Current MirrorsMOSFET in Saturation = Current Source
= Large Resistor without Excessive Voltage Headroom
Resistive Divider 사용 Current MirrorC W R
문제: un, VTH 편차심함, 온도 함수
IOUT ≈unCox
2WL
(R2
R2 + R1
VDD − VTH )2
문제: un, VTH 편차심함, 온도 함수
해결: 다음 Current Mirror
Simple Current Mirror
IREF,Iout는 공정,온도에 무관
전자정보대학 김영석 2
Current Mirrors (cont.)Amplifier Bias Example
M0: Current Source
M5/M6: Current Breeding => M3,M4 전류를 줄여서Voltage Headroom 증가Voltage Headroom 증가
전자정보대학 김영석 3
5.2 Cascode Current MirrorChannel Length Modulation 고려
VDS1≠VDS2 => ID1 ≠ ID2
이 문제 해결 => Cascode
LWLW
LWLW
If = ,)/()/(
)/()/(
1
2
0
3
REFOUT
yxGSGS
II
VVandVV
=∴
=∴= )()(
03
10
전자정보대학 김영석 4
Cascode Current Mirror (cont.)장점: Rout 증가 2)( osimpleout rR =
단점 V lt H d 문제
2333)(
2)(
)1( oomocascodeout
osimpleout
rrgrR ++=
단점: Voltage Headroom 문제
VVVVVVV THTHTHGSsimpleout Δ=−Δ+=−= )( 2121)min(,
Cascode Voltage Swing 감소
VVVVVVVV THTHGSGSTHNcascodeout Δ+=−+=−= 2)( 3013)min(,
전자정보대학 김영석 5
Cascode Current Mirror (cont.)Low Voltage Cascode Mirror => Voltage Swing 증가
,0/
2)/( 77 VVVVLWC
IVLW THTHGSoxn
D ≈Δ+=≈=Δ=>∞≈μ
21 M1::
22
2
min
감소영향의과낮추어드레인전압의역할 CLMMMMCircuitGenerationV
VVVVVVVSet
b
out,THGSb Δ==>Δ+=Δ+=
2
전자정보대학 김영석 6
Cascode Current Mirror (cont.)Low Voltgae Cascode Using Source Follower
VMVVVV THN Δ+=
)()(2
CurrentLowtodue
VVVVVVVVVV
VMV
THTHNN
THSGS
Δ=Δ+=Δ+=−=
≈
)(2
)(
'
CurrentLowtodue
VVVVVVV
out
THNB
Δ=∴Δ=Δ+−=
2)(
min,
'
전자정보대학 김영석 7
5.3 Active Current MirrorDiff. Pair with Current-Source Load
Node P not Virtual Ground due to Vout through ro2
1ST Approach
)2(||)]1(1[||
)2
( 1 outin
mout
R
Rv
gv
++
=
)]2(||[
)2(||)]1([||
241
24221
24
oomout
v
ooomm
ooout
rrgv
A
rrrgg
rrR
≈=
≈++=
)](||[2 24 oo
inv v
전자정보대학 김영석 8
Diff. Pair with Current-Source Load (cont.)2nd Approach
p
out
in
p
in
outv v
vvv
vv
A ==
/1
1,/1 22
4
21 om
o
meq
meq
eq
in
p
v
rgr
gR
gRR
vv
+≈+
=
)1(/2/1
2244
24
24
omooout
oo
oo
in
p
rgrrvrrrr
vv
+++
=
1
)1()||( :
22
42
22424221
42
4
omout
poo
omopoomp
oo
oout
p
out
rgv
vrr
rgrvrrgv
rrr
vvv
+=
++
=++
=
]||)2[(
/1
422
42
moutp
oop
rrgvv
A
rrv
==
+=
]||)2[(2 42 oo
pinv rr
vvA
전자정보대학 김영석 9
Active Current Mirror (cont.)회로 개선: 버려지는 M1의 소신호 전류 이용
>=>
전자정보대학 김영석 10
5.3.1 Large-Signal AnalysisVin1 small
M1/M3/M4 OFF => No Current
M2/M5 Triode Region => Vout=0
Vi 1 Vi 2Vin1~Vin2
M2/M4 Saturation => High Gain
Vin1 LargeVin1 Large
M1/M3/M4 ON => ID4=0, M4 Triode, Vout=VDD
전자정보대학 김영석 11
5.3.2 Small-Signal AnalysisVx<Vy => Vp not Virtual Ground
1st Approach
Gm
2,1mm
outmv
gGRGA
=•=
2,1mm
outmv
gGRGA
=•=
2,1mm gG =
Rout
12
4
xxx r
VVI +=
2,1mm
outmv
gGRGA
=•=
||
||12
42
43
32,1
ooout
oo
mo
rrR
rrg
r
≈
+
)||( 422,1 oomoutmv rrgRGA =•=
M3/M4 Current Mirror => Factor 2
전자정보대학 김영석 12
Small-Signal Analysis (cont.) 2nd Approach: Diff Pair => Thevenin Eq. CKT
12 xx
x rVV
I +=)()(
)0(
inin
openeq
vv
IVV ==
)||(||
||12
422,1
42
43
32,1
oomoutmv
ooout
oo
mo
rrgRGArrR
rrg
r
=•=≈
+
2,1
2,12,11122
2
)2
()2
(
oeq
inomin
moin
mo
rR
vrggrgr
=
=+=
)2(
1
14
eqout
xoout
VvI
Irv−
=
−=
||13
3
1
t
om
eq
x
v
rg
RI
+=
)||( 4,32,12,1 oomin
out rrgvv
=∴
전자정보대학 김영석 13
5.3.3 Common Mode PropertiesSymmetry => VF=VX
ACM =ΔVout
ΔVin CMACM ≈
−1
2gm3,4
||ro3,4
21 + R
=−1
1 + 2gm1 2 RSS
gm1,2
gm 3 4in ,CM2gm1,2
+ RSSgm1,2 SS gm 3, 4
= gm 3,4 (ro1, 2 || ro3,4 )(1 + 2gm1, 2RSS)CMRR =ADM
A= gm1, 2(ro1,2 || ro3,4 )
gm 3,4 (1 + 2gm1,2 RSS )m 3,4 o1, 2 o3,4 m1, 2 SSACM
gm1, 2( o1,2 || o3,4 )gm1,2
Note: Symmetry라도 CMRR 유한 (Fully Diff. 경우 CMRR=infinity)
전자정보대학 김영석 14
Mismatch 있는 경우의 Common Mode Gaingm Mismatch
전자정보대학 김영석 15