Willy Sansen 10-05 071
Important opamp configurations
Willy Sansen
KULeuven, ESAT-MICASLeuven, Belgium
Willy Sansen 10-05 072
Table of contents
• Simple CMOS OTA• CMOS Miller OTA• Symmetrical CMOS OTA• Folded cascode OTA• Other opamps
Willy Sansen 10-05 073
Simple CMOS OTA
12
3
VDD
VSS
vOUT
CL
M1 M2
M3 M4
M5
+ -
Cn2
GBW = gm12π CL
fnd = gm32π Cn2
Cn2= 2CGS3+CDB3+CDB1 ≈ 4CGS3
fnd ≈ ?fT3
4
Differential pairCurrent mirror
Willy Sansen 10-05 074
Simple CMOS OTA : fnd
12
3
vOUT
M1 M2
M3 M4
M5
+ -
Cn2fnd =
gm3
2π Cn2
-90o
0o
Av
f
f
fnd 2fnd
2x
PM = 90o - arctan + arctan ≈ 85oGBWfnd
GBW2 fnd
2x
Willy Sansen 10-05 075
Telescopic CMOS OTA
M1 M2
M3 M4
M9
+ - More gainAt low frequencies
GBW = 2π CL
gm1
vOUT
CL
M5 M6
M7 M8
Gulati, JSSC Dec.98, 2010-2019
Willy Sansen 10-05 076
Cascodes increase gain at low frequencies
GBW = 2π CL
gm1
Av
GBW
f
Cascodes : high gain at low frequencies
No cascodes
104
102
1
Willy Sansen 10-05 077
Table of contents
• Simple CMOS OTA• CMOS Miller OTA• Symmetrical CMOS OTA• Folded cascode OTA• Other opamps
Willy Sansen 10-05 078
Miller CMOS OTA
12
34
VDD
VSS
vOUT
CLM1 M2
M3 M4
M5
M6
M7
+- Cc
GBW = gm12π Cc
fnd = gm62π CL
1
1 +Cc
Cn1
Differential pairCurrent mirrorSecond stage
Willy Sansen 10-05 079
Miller BiCMOS OTA
12
34
VDD
VSS
vOUT
CLM1 M2
M3 M4
M5
M6
M7
+- Cc
M8 M9
56
Node 1:High impedance
Extra nodes 5, 6
gm6NPN > gm6NMOS
fnd ?
Willy Sansen 10-05 0710
Table of contents
• Simple CMOS OTA• CMOS Miller OTA• Symmetrical CMOS OTA• Folded cascode OTA• Other opamps
Willy Sansen 10-05 0711
Symmetrical CMOS OTA
12
345
VDD
VSS
vOUT
CLM1 M2
M3 M4
M8
M6
M7
+-
M5
M9
1 : BB : 1
B ?
Symmetrical :Node 1 = Node 2
Rail-to-rail output swing
Differential pair3 Current mirrors
1 : : 1
Willy Sansen 10-05 0712
Symmetrical CMOS OTA : GBW
12
345
VDD
VSS
vOUT
CLM1 M2
M3 M4
M8
M6
M7
+-
M5
M9
1 : BB : 1
B ?
Av = gm1BRn4
BW =1
2π Rn4CL
GBW = Bgm1
2π CL
1 : 1 =
if L8 > L6
2VEnL6
VGS1-VT
Willy Sansen 10-05 0713
Symmetrical CMOS OTA : fnd1,2
12
345
VDD
VSS
vOUT
CLM1 M2
M3 M4
M8
M6
M7
+-
M5
M9
1 : BB : 1
GBW = Bgm1
2π CL
Cn2 Cn1
Cn1= (1+B)CGS4+CDB4+CDB2
≈ (3+B)CGS4
fnd = gm42π Cn1
fT43+B
≈
1 : 1
Willy Sansen 10-05 0714
Pole at output of a differential pair
M1 M2
+-
M9
Cn
RL
One single pole : fp = 12π 2RLCn
+ -voRL
M1 M2
+-
M9
2Cn RL
+ -voRL2Cn
Willy Sansen 10-05 0715
Symmetrical CMOS OTA : fnd5
12
345
VDD
VSS
vOUT
CLM1 M2
M3 M4
M8
M6
M7
+-
M5
M9
1 : BB : 1
GBW = Bgm1
2π CLCn5
Cn5 causespole
and zeroat 2 x freq.:≈ 3 …6 o
1 : 1
Willy Sansen 10-05 0716
VGS-VT = 0.5 V; L = 1 µmfT4 = 5 GHzfnd = 0.6 GHzB ≈ 5
Symmetrical CMOS OTA : Design Example
12
345 vOUT
CL = 2 pFM1 M2
M3 M4
M8
M6
M7
+-
M5
M9
1 : BB : 1
GBW = Bgm1
2π CL
fnd ≈fT4
3+B
GBW = 200 MHz
1 : 1
Willy Sansen 10-05 0717
12
345 vOUT
CM1 M2
M3 M4
M8
M6
M7
+-
M5
M9
1 : BB : 1
GBW = Bgm1
2π CL
fnd ≈fT4
3+B
gm4NPN > gm4NMOSCCS4NPN > CDB4NMOS
BiCMOS > CMOS ?
1 : 1
Symmetrical BiCMOS OTA
Willy Sansen 10-05 0718
Symmetrical CMOS OTA with cascodes
12
3 4
5
VDD
VSS
vOUTCLM1 M2
M3 M4
M8
M6
M7
+-
M5
M9
1 : BB : 1
GBW = Bgm1
2π CL
GBW samebut
Av is 100* xhigher !!!
* gmro ≈ 100
+
++
1 : 1
M9M10
M11 M12
Willy Sansen 10-05 0719
Symmetrical Miller CMOS OTA
12
345
VDD
VSS
vOUTCcM1 M2
M3 M4
M8
M6
M7
+-
M5
M9 M12
M10M9
M11
CL
67
8
GBW = Bgm1
2π CcNo zero !
B : 1 1 : B
1 : : 1
Willy Sansen 10-05 0720
Bipolar transistor symmetrical amplifier
CA3080
0.12 MHz1.2 V/µs
I3 = 10 µA
Willy Sansen 10-05 0721
Gain enhancement by current starving
inp inn
outnoutn
2I1
M1
M2M3
K*I1 1:B
kA
VVkA
TGS −=
⋅−−=
1))(1(2 0
31 λ
Yao, ..., JSSC Nov.04, 1809-1818
Willy Sansen 10-05 0722
Table of contents
• Simple CMOS OTA• CMOS Miller OTA• Symmetrical CMOS OTA• Folded cascode OTA• Other opamps
Willy Sansen 10-05 0723
Folded cascode CMOS OTA
VDD
VSS
vOUT
M1 M2
M3 M4
+-
M9 M6
M10 M11
CL
+
M7
M5
M8
6
35
4
2 1
7 Differential pairCascode pair2 Current mirrors
SymmetricalNode 1 = Node 2High speedInput to rail
Willy Sansen 10-05 0724
Folded cascode CMOS OTA : DC
VDD
VSS
vOUT
M1+-
M9
CL
+
3
2 1
10050 50
100 100
5050
M2
5050
DC current flow
M4
M11
Willy Sansen 10-05 0725
Folded cascode CMOS OTA :
VDD
VSS
vOUT
M1 M2
M3 M4
+-
M9 M6
M10 M11
CL+
M7
M5
M8
6
35
4
2 1
7
Av = gm1Rn4
BW = 12π Rn4CL
GBW = gm1
2π CL
Single stage ?
Willy Sansen 10-05 0726
Folded cascode CMOS OTA :
VDD
VSS
vOUT
M1 M2
M3 M4
+-
M9 M6
M10 M11
CL+
M7
M5
M8
6
35
4
2 1
7GBW =
gm12π CL
Cn1= CGS3+CDB1+CDB10 ≈ 3 CGS3
fnd = gm32π Cn1fT33
≈ Hi !
Willy Sansen 10-05 0727
Folded cascode CMOS OTA :
VDD
VSS
vOUT
M1 M2
M3 M4
+-
M9 M6
M10 M11
CL+
M7
M5
M8
6
35
4
2 1
7 GBW = gm1
2π CL
Cn5, Cn7 ,Cn6Cause poleand zeroat 2 x freq.:≈ 5 …10 o
Ref Mallya, JSSC Dec 89, 1737-1740
Willy Sansen 10-05 0728
Folded cascode BiCMOS OTA
VDD
VSS
vOUT
M1 M2
M3 M4
+-
M9 M6
M10 M11
CL
+
M83
5
2 1
GBW = gm1
2π CL
fnd =gm3
2π Cn1fT3NPN
3≈
Higher !
M6
M7
M5
M8
6
54
7
Willy Sansen 10-05 0729
Folded cascode OTA: input to Vss rail
VDD
VSS = 0 Volt
vOUTM1 M2
M3 M4+-
M9 M6
M10 M11
CL
+
M7
M5
M8
6
35
4
1
7
2
0.9 V
VT = 0.7 VVGS1-VT = 0.2 VVGS10-VT = 0.5 V
V+ = 1.5 VVGS3-VT = 0.2 V
0.7 V
+
Willy Sansen 10-05 0730
Folded cascode OTA with 2nd stage
VDD
VSS = 0 Volt
M1 M2
M3 M4+-
M9 M6
M10 M11
+
M7
M5
M8
6
35
4
1
7
2
+
vOUTCc
M12
M13
CL
8
No zero !
Ribner, JSSC Dec.84, 919-925
+
Willy Sansen 10-05 0731
Conventional folded cascode OTA
VDD
VSS = 0 Volt
vOUTM1 M2
M3 M4+-
M9 M6
M10 M11
CL
+
M7
M5
M8
6
35
4
1
7
2
+
+
Willy Sansen 10-05 0732
Alternative folded cascode OTA
VDD
VSS = 0 Volt
vOUTM1 M2
M3M4+-
M9
M10 M11
CL
3
5 4
1 2
+
gm1vINd
gm1vINd /2
M6
M7
M5
6
5 4
7
M8Current diff. amp.
Willy Sansen 10-05 0733
Comparison amplifiers
Telescopic 0.25 4 small
Folded casc. 0.5 4 avg.
Miller 2-stage (CL/Cc= 2.5) 1.1 4 max.
Volt. OTA (4 Ts) 0.25 4 avg.
Symmetrical (B= 3) 0.33 16 max.
ITOT dvin,eq2 Swing
GBW = 100 MHz CL = 2 pF VGS-VT = 0.2 V Fully differential
mA 8/3 kT dfgm1
Willy Sansen 10-05 0734
Table of contents
• Simple CMOS OTA• CMOS Miller OTA• Symmetrical CMOS OTA• Folded cascode OTA• Other opamps
Willy Sansen 10-05 0735
Sub-1 Volt OTA
Baschirotto, .. JSSC Dec.97,pp.1979-1986
1 V 80 µW (min: VT+2VDSsat)Fully differential75 dB 30 MHz (0.1 pF)< 100 ns
Level shifting M64 Switches 2n :Only 2nd stage switched off !
Willy Sansen 10-05 0736
LM 4250
GBW = 0.25 MHzSR = 0.2 V/µsI1 = 10 µA
ITOT = 90 µA38 nVRMS/√Hz
Willy Sansen 10-05 0737
Increased input transconductance - 1
vin
gmR =
RS
1 + gmRS
gm
iout
vin
gmR =
-RS
1 - gmRS
gm
iout
Willy Sansen 10-05 0738
Increased input transconductance - 2
vin
gm =1 +
gm1
iout
gm1gm2
M1
M2
vin
gm =1 -
gm1
iout
gm1gm2
M1
M2
-vin
Willy Sansen 10-05 0739
Ref.: Castello, JSSC June 1990, pp. 669-676
Increased input transconductance - 3
1 : 33 : 1
3 gmGBW =2πCL
CL
gm =1 -
gm1gm1gm3
gm ≈ 3 gm1
9 gm12πCL
≈M5,M6 for overdrive !
Willy Sansen 10-05 0740
Transconductor with C�DG compen.
VDS1 = RDID ≈ 0.2 V
IDS1 = β1VDS1(VGS1-VT)
gm1 = β1VDS1 is constant
Ref. Alini, JSSC, Dec.92, pp.1905-1915
Willy Sansen 10-05 0741
Ref. : Wu etal, JSSC Jan.1994, pp.63-66
14 MHz/ 11pF
5.6 MHz/ 100pF
4 V/µs
36 nV/√ Hz
5 V0.4 mA
gm1vin
gm3vin
Willy Sansen 10-05 0742
Problem: unequal gmtot
nMOST on :VINCM > 1.1 V
VINCMVSS = 0 VDD /2 VDD
gm
pMOST on : VINCM < VDD - 1.1 V
nMOSTpMOST
gmtot
1.1 V VDD-1.1 V
Increase gm:gmn+ gmp= ct
gmn gmp
Willy Sansen 10-05 0743
Wu : input rail-to-rail stage
VDD
2
VDD = 5V
0V
VDD
20
20
5
5
5
5
1010
10
10
20
20
20
0
0
20
5V
1.5V1.5V
pMOSTs off !
V+- high :
nMOSTs :IDS1 x 2gm1 x 2
Weak inv.
Willy Sansen 10-05 0744
Wu : output stage : gain
iin iin
RinRin
Rin Rin
iin
iin iin iin
Z
iin = gm1 v+-
vin
vin
vout
RL
vin
iin= Rin
voutvin
= 2gmA1RL
vout
iin= - 2RingmA1RL
Av = 2gm1RingmA1RL
Willy Sansen 10-05 0745
Wu : output quiescent current control
4 µA
VGS2 + VGS4 = VGS9 + VGS10
M4 = 2 M10 & M9 = M10
VGS2 - VT =IDS2
K’p W/L2
4 µA
IDS2
IDS9
W/L2
W/L9= (2 - ) ≈ 911
2
IDS2 ≈ 364 µA since IDS9 ≈ 4 µA
Willy Sansen 10-05 0746
Enhanced full-differential folded-cascode
Ref.Nakamura, JSSC April 1992, pp.563-568
CLCL
gm1GBW =2πCL
i = gm1vin
i/2
i/2+i/2
i
i/2
i/2
ii/2i/2
i/2i/2
Willy Sansen 10-05 0747
Bipolar opamp LM-124
GBW = 0.5 MHzSR = 0.4 V/µs
I1 = 3 µAITOT = 650 µA
68 nVRMS/√Hz
Willy Sansen 10-05 0748
BiFET opamp TL-070
GBW = 3 MHzSR = 13 V/µs
I1 = 100 µAITOT = 1400 µA18 nVRMS/√Hz
pS D G
pn
n
vGS0 vP
iDS
Willy Sansen 10-05 0749
Bipolar 2-stage opamp 741
GBW = 0.8 MHzSR = 0.7 V/µs
I1 = 10 µAITOT = 1100 µA14 nVRMS/√Hz
Willy Sansen 10-05 0750
Two-stage opamp OP-97
trim
GBW = 0.8 MHzSR = 0.1 V/µsITOT = 600 µA25 µV offset30 pA input
+Vos -0 V
Willy Sansen 10-05 0751
Bootstrap for high gain Av2
Ref.De Man JSSC June 77, pp.217-222
M1
M2
M3Rm
Rout
Rout -> x
Rm -> x β3
Av2 ≈ gm1 ro2 x β3
β3
1
Same GBW !
Willy Sansen 10-05 0752
Table of contents
• Simple CMOS OTA• CMOS Miller OTA• Symmetrical CMOS OTA• Folded cascode OTA• Other opamps