cmos analog circuitshome.iitk.ac.in/~baquer/l20_class_ab.pdfclass ab buffer amplifier b. mazhari,...
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CMOS Analog Circuits
C fL20: Class AB amplifier (24.10.2013)
B Mazhari
G-NumberB. Mazhari, IITK56
B. MazhariDept. of EE, IIT Kanpur
Output Stage
+3.3V
v0321
vS
R = 1K-3.3V RL 1K
Low output Resistance; Rail-to-Rail voltage swing
Low distortion; High efficiency
G-NumberB. Mazhari, IITK57
CS Amplifier
VDD = 3.3V 3.3V
RD0
+3
-3
Vbias2
M2
(W/L)P
1kvO vO
2
(W/L)N
VSS = -3.3VvinvS Vbias1
-3.3V
M1
DDV 3DSQI mA33DD
DSQD
VI mAR
3DSQI mA
G-NumberB. Mazhari, IITK58
CD Amplifier
VDD = 3.3 V = 3 3DD
v
VDD = 3.3
m1VBias1
vin RSRL = 1K
vO
RL = 1K
vO
m2
Bias1
vin
VBias2
VSS =-3.3 VSS =-3.3
m2in
3.3 3 3I A3.3 ~ 33100biasI mA 3.3BiasI mA
G-NumberB. Mazhari, IITK59
Class A amplifiers3.3V
VDD
VBias2m
Vbias2
M2VBias1 RL=1k
vO
m2
vS Vbias1 M1
vO
1kvin
VSS
m1
S b as
-3.3V
In both CS and CD amplifiers the transistor remains ON and conductingIn both CS and CD amplifiers, the transistor remains ON and conductingthroughout out the ac cycle. To achieve this the bias current must be larger thanthe ac drain current.
d dI I i di I di i I Ids ds Bias dsI I i ds Biasi I L ds Biasi i I
0.5 0.5L o L DD BiasP v i V I o DDv V
IBias ids
1 Tss SS ds ss BiasP V I dt V I
T dd DD BiasP V I
G-NumberB. Mazhari, IITK60t
0T
sup0.25L
ply
PP
VDD
Amplifiers with negligible stand-by power dissipationIds
vO
VBias2m2 Even when no input is
applied, power is drawnIBias ids
v
VBias1
m1
RL
pp pfrom the supply.
tA ffi i t lifi ill t k f th lvin
VSS An efficient amplifier will take power from the supplyonly when power is to be delivered to the load.
Bi t t b !!Bias current must be zero !!
VBias1
vO
R
Ids 22 max
0
14
Tds L
L ds Li RP i dt R
T
vin
Bias1
V
m1
RL
ti
0
VSS tids
max1 Tss dsV i
max max0 785ds L oi R v
G-NumberB. Mazhari, IITK61
max
0
1 ss dsss SS ds
V iP V I dt
T
max max0.7854
ds L oSS SSV V
vO
How do we reduce distortion? VDD = 3.3
m2VBias2
VBias1
O
m1
RL
vO
RL
m2
vin
I vinVSS IL
IL
tt t
VDD = 3.3
m2VBias2
Each Transistor conducts forvO
RLvin
only half the cycle resulting inClass B operation
G-NumberB. Mazhari, IITK62VSS
m1VBias1
Class B push-pull amplifier
VDD = 3.3
Vm2
VBias2
vO
mRL
V
vin
V
m1VBias1
VSS
During positive cycle, M2 pushes current into the load, while during the
G-NumberB. Mazhari, IITK63
During positive cycle, M2 pushes current into the load, while during thenegative cycle, M1 pull current from the load and hence the name Push-Pullamplifier
VDD = 3.3
0Vm2
700/1Symmetrical nmos and pmos with identical
RLvin
m2
vO700/1
y pparameters and no body effect for nmos.
VSS
m1
L
0V
Cross-over distortion
G-NumberB. Mazhari, IITK64
G-NumberB. Mazhari, IITK65
Note the odd harmonics
VDD = 3.3
m20.7V
vO
RL
m2
vin
VSS
m1
L
-0.7V
P 24 WPsby~24 µW 2.14 49.1%@ 2.34.26
mW VmW
max0 785 ov 0.785SSV
For output swing of 3V, efficiency increase to 71.6%
G-NumberB. Mazhari, IITK66
c ease o 6%
VDD = 3.3
m20.8V
vO
RL
m2
vin
VSS
m1
L
-0.8V
P 2 92 WPsby~2.92mW
2.14 43.6%@ 2.24.9
mW VmW
For output swing of 3V, efficiency
G-NumberB. Mazhari, IITK67
yincrease to 68%
VDD = 3.3
0Vm2
700/1
m1
RLvin
0V
vO700/1
VDD = 3.3
VSS
vO
m20.7V
VSS
m1
RLvin
-0.7V
VDD = 3.3
VSS
vO
DD
m20.8V
G-NumberB. Mazhari, IITK68
VSS
m1
RLvin
-0.8V
Distortion can also be reduced by employing negative feedback
3.3V
M2
700/1
3 3V
M11k
vO
vS
-100100 700/1
-3.3V
G-NumberB. Mazhari, IITK69
0.5um nmos and pmos Tr.With body effect
VDD = 3.3
0Vm2
700/1
mRL
vin
vO700/1
VSS
m10V
Not the strong secondharmonic distortion aswell resulting fromunequal swings inpositive and negativep gdirections..
G-NumberB. Mazhari, IITK70
VDD = 3.3
m20.7V
vO
RL
m2
vin
VSS
m1
L
-0.86V
P 0Pstb~0
G-NumberB. Mazhari, IITK71
VDD = 3.3
m21.0V
vO
RL
m2
vin
VSS
m1
L
-1.16V
P 0 6 WPstb ~0.6mW
G-NumberB. Mazhari, IITK72
VDD = 3.3
1.0V
vO
m2
VSS
m1
RLvin
-1.16V
VSS
G-NumberB. Mazhari, IITK73
Class AB Push-Pull amplifier using CS stage
VDD = 3.3
m2VBias2
vO
m2
vO
RLvin
m1
L
VBias1
VSS
G-NumberB. Mazhari, IITK74
Symmetrical nmos and pmos with identical parameters and no body effect fornmos.
VDD = 3.3
2.6V 700/1
m2
vO
vin RL=1k
700/1
m2
VSS
m1-2.6V
G-NumberB. Mazhari, IITK75
G-NumberB. Mazhari, IITK76
G-NumberB. Mazhari, IITK77
VDD = 3.3
VBias2
vO
m2
m1
RL
VBias1
vin
VSS
VDD = 3.3
VBias2
VDD = 3.3
m2VBi 2
vO
m2m2VBias2
vO
m1
RL
VBias1
V
m1VBias1vin
G-NumberB. Mazhari, IITK78
VSS VSS
Need to address biasing
Biasing Of Class AB Output Stage
VBias2
VDD VDD
m2~ 0.7DDV
vO
m2Bias2
vin VBias3
m1
RL
VBias1
vOvin
VBias4
VSS m1
~ 0.7SSV
VSS m2VBias2
vO
G-NumberB. Mazhari, IITK79VSS
m1VBias1vin
VDD VDD m2
VBias3
m2vOvin
as3
VBias4 vin VBias3m1
VSS
vO
VBias4
m1
VSS m2VBias2
m
vO
G-NumberB. Mazhari, IITK80VSS
m1VBias1vin
VDD
VDD m2
DD
m2
vO
vin VBias3
vVBias3
m1
vO
VBias4
vOvin VBias4
VSS m1
VSS
G-NumberB. Mazhari, IITK81
VDD
m6 m14
vO
VBias3
VBias4
m7 m15
VSS
CS i St Class AB Output Stage
G-NumberB. Mazhari, IITK82
CS gain Stage Class AB Output Stage
Class AB Buffer Amplifier
G-NumberB. Mazhari, IITK83
C.W. Lu,” High-Speed Class AB Buffer Amplifiers with AccurateQuiescent Current Control
G-NumberB. Mazhari, IITK84
G-NumberB. Mazhari, IITK85
Class AB CMOS Pseudo-Source Buffer Amplifier
VDD
VDD
M6
M6
A1
_
+
V V
Vb1Vin Vout
A1
+ RL
Vin Vout
Vb2 A2
RL
_
M6A
Vb2
M6A
2
VSS VSS
G-NumberB. Mazhari, IITK86
M5A
VDD
VBias2M6
M1A M2A
Folded cascode LoadVBias2
M1A M2A
VoutVSS
VDD
M1 M2
M6AFolded cascode Load
Vin
M5
Folded cascode Load
VSS
VBias1
(a)
G-NumberB. Mazhari, IITK87
Positive error amplifier
M44 M45
M52
M44 M45
M54Vin-Vin+
Ibn
M42 M43
M55 M56
VBIAS
M40 M41
G-NumberB. Mazhari, IITK88