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ECEN 607 (ESS)
Additional notes on the Flipped Voltage Follower
Courtesy of Prof. Jaime Ramirez‐Angulo
Analog and Mixed‐Signal Center, TAMU
Local Feedback Loops
Basic Idea
T i f f i l t i t
V. Ivanov, J. Zhou and I.M. Filanovsky, "A 100‐dB CMRR CMOS Operational AmplifierWith Single‐Supply Capability," IEEE Transactions on Circuits and Systems II: ExpressBriefs, vol. 54, no. 5, May 2007, pp.397‐401
• To improve performance of simple transistors:
» By adding local feedback loops to:
» Decrease/increase impedance levels at certain nodes.Decrease/increase impedance levels at certain nodes.
» Decrease/increase signal swing at certain nodes.• Drawbacks:
» Feedback always implies stability problems• Advantages:
» Local Feedback is faster and does not require» Local Feedback is faster and does not requirecompensation if properly designed.
» It allows easy Class AB implementation
2
Local Feedback Loops
A possible Implementation
IM2
Vo
I b
XMV i
Iout
2X
o
Vi M1
MV i 1
I b
Flipped Voltage Follower(FVF)Voltage Follower 3
Local Feedback Loops
Flipped Voltage Follower (FVF)
● VX is defined by the value of Viand Ib and follows the
i ti f V
YVDS,sat
variations of Vi
• The circuit is able to source (Iout) currents much larger than
XVTHP + VDS,sat
(Iout) currents much larger than Ib
• Low impedance at node X RX=1/(gm2 gm1 ro1)
VDD,min = |VTHP| + 2VDS,sat 4
Local Feedback Loops
Flipped Voltage Follower (FVF)
• It is able to operate with Low‐Voltage supplyVDD
MIN=|VTP|+2VDS,sat
• It features a low impedance node: RX≈1/(gm2 gm1 ro1)
• And a high impedance nodeg pRi ≈∞
• There are no stability problems [*]i bl id l d i h h l• It is able to provide a load with currents much larger
than the bias current Class‐AB
[*] R.G. Carvajal, J. Ramírez‐Angulo, A. López‐Martin, A. Torralba, J. Galán, A. Carlosena and F. Muñoz,“The Flipped Voltage Follower: A useful cell for low‐voltage low‐power circuit design,” IEEE Trans. onCircuits and Systems‐I, vol 52, no. 7, Julio 2005. 5
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
b ll f f l b hObjective: Design cells for VDD<VDDmin featuring Class‐AB behavior
M 2X
IM1
Y
Iin Vb
II b
6
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
b ll f f l b hObjective: Design cells for VDD<VDDmin featuring Class‐AB behavior
50
75
(uA
)
M 2X
I
M 5M 2
0
25
-5 0 5
I b
I
I out
(
( A)
M1Y
Iin Vb
I
Iout
C t S i
Iin (uA)I bIout=Iin + Ib
Current Sensing
7
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
b ll f f l b hObjective: Design cells for VDD<VDDmin featuring Class‐AB behavior
10.0
12.5
VE
S(u
A)
Ib
X
M3I(D1P)I(D2P)
I(D1) I(D2)2.5
5.0
7.5
DC
TRA
NS
FER
CU
RV
M1 M2V1 V2
X
M1 M2V1 V2
X
MaVa
Cl A Cl AB d
0-400 -200 0 200 400
VIN (mV)
D
iD1 iD2 iD1 iD2Ib
I(D2P)I(D1P)
Class-A differential pair
Class-AB pseudo differential pair
Comparison
8
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
b ll f f l b hObjective: Design cells for VDD<VDDmin featuring Class‐AB behavior
Quiescent Current in Class‐A based SC circuits:
Limits Slew‐RateLimits GBW
Depending on clock freq :Depending on clock freq.:One of them sets IQ
For low freq. SR limits.
G Giustolisi and G Palumbo "A novel 1 V class ABG. Giustolisi and G. Palumbo, A novel 1‐V class‐ABtransconductor for improving speed performance in SCapplications,". ISCAS 2003, vol. 1, 25‐28 May 2003 pp.153‐156
9
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
b ll f f l b hObjective: Design cells for VDD<VDDmin featuring Class‐AB behavior
• Adaptive biasing+LCMFB = Super Class AB
• Improvements: – Slew rate
– Gain‐Bandwidth productGain Bandwidth product
– Current efficiency
• Applicationspp– Low‐voltage low‐power SC circuits
– Buffers for large load caps and testing
10
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
b ll f f l b hObjective: Design cells for VDD<VDDmin featuring Class‐AB behavior
Slew Rate: BIBIAS/CLConventional Class A OTA
M3 M4
IIBIAS
BIAS/ L
GB: Bgm1,2 /(2πCL)Iout
M1 M2
I1 I2
Vin- Vin+ Current Efficiency: B/(B+1)
M5 M6 M7 M8
1:BB:1
Larger B increases all this but also increases power consumption and decreases phase margin 11
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
b ll f f l b hObjective: Design cells for VDD<VDDmin featuring Class‐AB behavior
Super Class‐AB OTA
M3 M4
Adaptive Biasing
Adpatative biasing:↑ Slew rate
LCMFB:
R R
M1 M2
I I
Vin- Vin+Iout
↑↑ Slew rate↑ GBW↑ Current
R1 R2
M5 M6 M7 M8
I1 I2 Efficiency
1:11:1LCMFB
12
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
b ll f f l b hObjective: Design cells for VDD<VDDmin featuring Class‐AB behavior
Local Common‐Mode Feedback (I)
R1 R2
X Y
I1 I2I8
IRI5Quiescent conditions
ZM5 M6 M7 M8
7,6
2β
BTHZYX
IVVVV +===
IB: Quiescent value of I1 and I2Low quiescent power consumption 13
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
b ll f f l b hObjective: Design cells for VDD<VDDmin featuring Class‐AB behavior
Local Common‐Mode Feedback (II)
R1 R2
X Y
I1 I2I8
IRI5
Dynamic conditions
ZM5 M6 M7 M8
2I 2⎞⎛
2
2
1
7,6
dZX
cmTHZ
IRVV
IVV
+=
+=β
2
1
7,6
55 2
22 ⎟
⎟⎠
⎞⎜⎜⎝
⎛+= dcm IRI
Iβ
β
If I1>I2:I I I I
where Icm = (I1 + I2)/2 and Id = I1 ‐ I22
22 d
ZYIRVV −=
Iout=I5‐I8 ≈ I5
14
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
b ll f f l b hObjective: Design cells for VDD<VDDmin featuring Class‐AB behavior
Local Common‐Mode Feedback (III)
R1 R2
X Y
I1 I2I8
IRI5Dynamic conditions
ZM5 M6 M7 M8
2 cmIVV += 2⎞⎛β
21
7,6
dZX
THZ
IRVV
VV
+=
+=β
If I1<I2:
I =I I ≈ I
2
7,6
88 2
22 ⎟
⎟⎠
⎞⎜⎜⎝
⎛−= dcm IRI
Iβ
β
where Icm = (I1 + I2)/2 and Id = I1 ‐ I22
22 d
ZYIRVV −=
Iout=I5‐I8 ≈ ‐I8
15
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
b ll f f l b hObjective: Design cells for VDD<VDDmin featuring Class‐AB behavior
I
Local Common‐Mode Feedback (IV)
R1 R2
X Y
M M M M
I1 I2I8
IRI5Dynamic conditions
ZM5 M6 M7 M8
General expression for I ≠ I :General expression for I1 ≠ I2:2
2,18,585 2
22 ⎟
⎟⎠
⎞⎜⎜⎝
⎛+±≈−= dcm
out
IRIIII
ββ
Quadratic boosting of differential current Id
7,6 22 ⎟⎠
⎜⎝ β
16
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
b ll f f l b hObjective: Design cells for VDD<VDDmin featuring Class‐AB behavior
Adaptive Biasing+LCMFB
RgkgM3 M4
YXRkg 85
GB product:L
YXmm
CRgkg
GBπ2
,8,52,1=
⇒ Increase ofM1
M2
Adaptive Biasing
Vin- Vin+Iout
YXm Rkg ,8,5
2
221212185 ⎟⎞
⎜⎛ R βββ
Large‐signal output current:⇒ Increase ofR1 R2
X
Z
Y
M5 M6 M7 M8
I1 I2
1 11 1 22,12,1
7,6
2,18,5
42 ⎟⎟⎠
⎜⎜⎝
+±≈ ididout VVIβ
βββ
Phase margin:
( ) ⎤⎡⎟⎞
⎜⎛ GSCGBW 852ºº
1:11:1
( ) ⎥⎦
⎤⎢⎣
⎡−≈⎟
⎟⎠
⎞⎜⎜⎝
⎛−≈
L
GSoomm
YX CC
rrRgkgarctgf
GBWarctgPM 8,522,17,62,18,52,1
º
,
º ||||9090
High SR with low R1,2 ⇒ stability 17
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
Adaptive Biasing of Input Pair: SIM2A
M1A
IB
VBVB
IB
M1B
M2B
Vin Vin+ V V 2
02
22
21
2
2
2,1
2,11
⎟⎞
⎜⎛
><⎟⎟⎠
⎞⎜⎜⎝
⎛+= idBid
B
I
VIIVI
I
β
ββ
(a)
M1M2
I1 I2
M1 M2
I1 I2
M
(b)
Vcm+VB
in- in+ Vin- Vin+
V V
02
2 12,1
2,12 <<⎟
⎟⎠
⎞⎜⎜⎝
⎛−= idBid
B VIIVI
Iβ
β
022
2,1 ><⎟⎞
⎜⎛
+ idB VIIVIIβ
M1M2
Vcm
VB
IB
M1 M2
M2A
M1AVcm
CMS
(c) (d)
I1 I2I1 I2
Vin- Vin+Vin- Vin+
02
22
022
1
2
2,1
2,12
22,1
2,11
<<⎟⎟⎠
⎞⎜⎜⎝
⎛−=
><⎟⎟⎠
⎜⎜⎝
+=
idBidB
idBidB
VIIVII
VIII
ββ
β
M1M2
Vmax
VBI1 I2
Vmax+VB
IB IB
Vin- Vin+
V V
NM2A
M1A M1B
M2B
2
022
2
2
2
2,1
2,11
⎟⎞
⎜⎛
>=⎟⎟⎠
⎞⎜⎜⎝
⎛+= idBid
B
I
VIIVII
β
ββ
Strong inversion: Quadratic boosting of Vid not limited by IB
WTA
(e)
M1M2
I1 I2
(f)
Vin- Vin+ 022 1
2,1
2,12 <=⎟
⎟⎠
⎞⎜⎜⎝
⎛−= idBid
B VIIVIIβ
β
18
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
Adaptive Biasing of Input Pair: WIM2A
M1A
IB
VBVB
IB
M1B
M2B
Vin Vin+ V VT
id
T
id
nUV
BnUV
B eIIeII−
== 21
(a)
M1M2
I1 I2
M1 M2
I1 I2
M2A
(b)
Vcm+VB
in- in+ Vin- Vin+
V V
BB 21
VVM1
M2
Vcm
VB
IB
M1 M2
M2A
M1AVcm
CMS
(c) (d)
I1 I2I1 I2
Vin- Vin+Vin- Vin+
T
id
T
id
nUV
BnUV
B eIIeII 22
21
−
==
M1M2
Vmax
VBI1 I2
Vmax+VB
IB IB
Vin- Vin+
V V
NM2A
M1A M1B
M2B
⎟⎟⎠
⎞⎜⎜⎝
⎛=⎟
⎟⎠
⎞⎜⎜⎝
⎛=
−
BnUV
BBnUV
B IeIIIeII T
id
T
id
,max,max 21
Weak inversion: Exponential boosting of Vid not limited by IB
WTA
(e)
M1M2
I1 I2
(f)
Vin- Vin+ ⎠⎝⎠⎝
19
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
M3 M4 M3 M43 4
Vout
IBIBIBIB
Vout
M2A
M1AM1B
M2B M2A
M1AM1B
M2B
3 4
Super Class AB OTA Topologies
R1
M5
M1
M6 M7 M8
M2R2
( ) (b)
Vin-Vin+
R1
M1 M2R2
Vin- Vin+
M5 M6 M7 M8
OTA Topologies
M
M10
M
M12R3 R4
(a) (b)
AM10
M
M12
MM M
R3 R4M14 M16A
M3 M4M3 M4
M M
IB
M11M9
IB
M2A
-+
Vout
Vin- Vin+M M
M9
IB
M2A
M1A
M11
IB
M13
IB
M15
IB
Vout
Vin- Vin+
R1
M1 M2R2
(c) (d)
M1 M2IB
R1 R2M5 M6 M7 M8M5 M6 M7 M8
20
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
M t R lt (I) Technology:Measurement Results (I) Technology:0.5 μm CMOSDouble PolyV 0 96VVTHP ≈ ‐0.96V VTHN ≈ 0.67V
OTA Fig. (b)
OTA Fig. (d) Meas. conditions:VDD = ‐VSS = 1 VIB = 10 μAC = 80pFOTA Fig. (a)
600 μm
CL= 80pF
21
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
1.5
2
40
OTA Fig. (a)
0
0.5
1
Cur
rent
(mA
)
OTA Fig. (d)20
30
itude
(dB
)
OTA Fig. (a)
OTA Fig. (d)
1 5
-1
-0.5
Out
put C
Class A OTAOTA Fig. (b) 0
10Mag
n O g (d)
OTA Fig. (b)
-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4-2
-1.5
Vid (V)
OTA Fig. (a)
M d d
102 103 104 105 106-10
Freq (Hz)
M d l fMeasured dc output characteristics of the OTAs
Measured open‐loop frequency response of the OTAs
22
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Class‐AB operation
-0.5
0
Vol
tage
(V)
Class A OTA
OTA Fig. (a)
0 0.5 1 1.5 2 2.5-1
V
0 5
0
ge (V
)
Class A OTA
OTA Fig. (d)
0 0.5 1 1.5 2 2.5-1
-0.5
Vol
tag
0
V)
Class A OTA
0 0 5 1 1 5 2 2 5-1
-0.5
Vol
tage
(V
Class A OTA
OTA Fig. (b)
0 0.5 1 1.5 2 2.5Time (us)
Transient response of the OTAs in unity‐gain configuration 23
LFL: Basic CellsLFL: Basic CellsLocal Feedback Loops: Super Class‐AB operation
PARAMETER Class A OTA OTA OTA PARAMETER OTA Fig. 3(a) Fig. 3(b) Fig. 3(d)
Slew Rate (positive) 0.35 V/μs 100 V/μs 92 V/μs 42 V/μs
Slew Rate (negative) -0.4 V/μs -78 V/μs -76 V/μs -80 V/μs
Pos. settling (1%) 3.1 μs 29 ns 66 ns 100 ns Performanceg ( ) μ
Neg. settling (1%) 3.6 μs 57 ns 62 ns 33 ns
GBW 200 kHz 725 kHz 410 kHz 470 kHz
Equ. Input Noise (1 MHz) 223 μVrms 230 μVrms 230 μVrms 252 μVrms
THD @ 100 kH 0 9V 40 dB 56 dB 43 dB 41 dB
Performance parameters of the OTAs
THD @ 100 kHz, 0.9Vpp -40 dB -56 dB -43 dB -41 dB
Dynamic range 63 dB 66 dB 63.7 dB 62.5 dB
DC gain 30 dB 43 dB 37.5 dB 37.5 dB
CMRR 54 dB 68 dB 70 dB 69 dB
PSRR+ (dc) 48 dB 55 dB 50 dB 57 dB
PSRR- (dc) 44 dB 58 dB 53 dB 46 dB
Static power consumption 80 μW 120 μW 120 μW 140 μW
Die area 0 011 mm2 0 024 mm2 0 024 mm2 0 042 mm2Die area 0.011 mm2 0.024 mm2 0.024 mm2 0.042 mm2
Antonio J. López Martín, Sushmita Baswa, Jaime Ramírez Angulo, R.G. Carvajal, “Low‐Voltage Power‐EfficientSuper Class AB CMOS OTA Cells with Very High Slew Rate and Power Efficiency,” IEEE Journal of Solid‐StateCircuits, vol. 40, no. 5, pp. 1068‐1077, Mayo de 2005
24
LFL: Basic Cells
Local Feedback Loops: Super Class‐AB operation
Proposed Circuit Topology
NonLinear Active Load25
LFL: Basic Cells
Local Feedback Loops: Super Class‐AB operation
Class‐AB Input Stage
02
22
2
2
2,1
2,11
⎞⎛
><⎟⎟⎠
⎞⎜⎜⎝
⎛+= idBid
B VIIVI
Iβ
β
02
2 1
2
2,1
2,12 <<⎟
⎟⎠
⎞⎜⎜⎝
⎛−= idBid
B VIIVI
Iβ
β
Local Feedback26
LFL: Basic Cells
Local Feedback Loops: Super Class‐AB operation
Class AB current mirror
25
50
75
I out
(uA
)
1st d OTA0
25
-5 0 5
I b
Iin
I
(uA)
1st proposed OTA
27
LFL: Basic Cells
Local Feedback Loops: Super Class‐AB operation
Non‐linear current mirror
2nd proposed OTA28
LFL: Basic Cells
Local Feedback Loops: Super Class‐AB operation
Non‐linear source degenerated current mirror
Iin Iout
M
M10
VbM8
M7b
3rd proposed OTA29
LFL: Basic Cells
Local Feedback Loops: Super Class‐AB operation
VDD=±1VI =10μAIB=10μACL=80pF
AMI 0.5 μm CMOS Technology Frequency Response30
LFL: Basic Cells
Local Feedback Loops: Super Class‐AB operation
OTA based on FVF OTA based on FVFCSOTA based on FVF OTA based on FVFCS
0.8
0.9
1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
OTA deg. de fuente Class A OTA
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20
0.1
us
31
LFL: Basic Cells
d f f h f b d
Local Feedback Loops: Super Class‐AB operation
PARAMETER OTAclase-A OTA 1 OTA 2 OTA 3
Measured performance of the fabricated OTAs
Slew Rate (positive) 0.35 V/μs 10 V/μs 20 V/μs 14 V/μs
Slew Rate (negative) -0.4 V/μs -15 V/μs -54 V/μs -28 V/μs
GBW 200 kHz 3.27 MHz 3.46 MHz 2.3 MHz
Phase margin 90º 56º 58º 71º
Slew Rate (positive) 0.35 V/μs 10 V/μs 20 V/μs 14 V/μs
Slew Rate (negative) -0.4 V/μs -15 V/μs -54 V/μs -28 V/μs
GBW 200 kHz 3.27 MHz 3.46 MHz 2.3 MHz
THD @ 100 kHz, 0.9Vpp -40 dB -37 dB -41 dB -40 dB
DC gain 30 dB 37 dB 39 dB 41 dB
CMRR 54 dB 85 dB 70 dB 75 dB
PSRR+ (dc) 48 dB 41 dB 37 dB 56 dBPSRR+ (dc) 48 dB 41 dB 37 dB 56 dB
PSRR- (dc) 44 dB 52 dB 70 dB 51 dB
Static power consumption 80 μW 220 μW 140 μW 140 μW
Die area 0.011 mm2 0.054 mm2 0.054 mm2 0.054 mm2
Static power consumption 80 μW 220 μW 140 μW 140 μW
32
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