Analog Integrated Circuits Fundamental Building Blocks
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Analog Integrated CircuitsFundamental Building Blocks
Faculty of Electronics Telecommunications and Information Technology
Fundamental Building BlocksCurrent mirrors
Information Technology
Gabor CsipkesBases of Electronics Department
Outline
current source biasing – voltage sources
MOS transistor current mirrors
fundamental current mirror fundamental current mirror cascode current mirror low swing cascode current mirror unbalanced and symmetrical Wilson current mirrors
bipolar transistor current mirrors
fundamental current mirror fundamental current mirror with β compensation fundamental current mirror with resistive emitter degeneration
Analog Integrated Circuits – Fundamental building blocks – Current mirrors 2
fundamental current mirror with resistive emitter degeneration cascode current mirror unbalanced and symmetrical Wilson current mirrors
Current mirrors – principles of operation
integrated current sources with transistors need bias voltages → voltage sources
1
mginR outR
Is this a voltage source?
key parameters to consider:
input resistance → must be as small as possible → current input
Current mirror
1: n
Analog Integrated Circuits – Fundamental building blocks – Current mirrors 3
input resistance → must be as small as possible → current input
output resistance → must be as large as possible → current output
minimum required output voltage required input voltage current gain → precision imposed by the application
The fundamental MOS current mirror
Small signal modelNMOS PMOS
2out
out DSout
VR rI
1
1inin
in m
VRI g
min 2o DSatV V1in DSat ThV V V
current gain: 2 2 21GS Th DSout V V VIn
1 outVn
Analog Integrated Circuits – Fundamental building blocks – Current mirrors 4
current gain:
2 2 2
1 1 11GS Th DSout
in GS Th DS
InI V V V
212
Th
DSat
VnV
Δβ – geometry mismatch
ΔVTh – threshold voltage mismatch
1out
in
nV
The MOS cascode current mirrorNMOS PMOS
1 1min 4 2 2o DSat DSat ThV V V V
3 3 1 1in DSat Th DSat ThV V V V V
3BSf V
(!)
2 4 4 4 2 4out DS DS m mb DS DSR r r g g r r
1 3
1 1in
m m
Rg g
Analog Integrated Circuits – Fundamental building blocks – Current mirrors 5
Small signal model
The MOS cascode current mirror
the VGS voltages of M3 and M4 balance the fundamental mirror M1-M2 → n is accurately defined
1 2in out DS DSV V V V
1DSV 2DSV
1 2in out DS DS
Analog Integrated Circuits – Fundamental building blocks – Current mirrors 6
The current gain is very close to unity even when the input-output voltage imbalance ΔV is significant
The MOS low swing cascode current mirror the cascode current mirror is not optimal in terms of Vomin → the gate voltage of M4must be decreased by VTh
4GV1 2
min 2o DSatV V
1 2DS DSV V
3 4
4GV
Analog Integrated Circuits – Fundamental building blocks – Current mirrors 7
1 2DS DS RV V V R
triode region
The MOS low swing cascode current mirror
NMOS PMOS
1R
min 4 2o DSat DSatV V V
1 1in DSat ThV V V
2 4 4 4 2 4out DS DS m mb DS DSR r r g g r r
1in
m
Rg
Analog Integrated Circuits – Fundamental building blocks – Current mirrors 8
Small signal model
The Wilson current mirror
NMOS PMOS
2 3m mg gR
min 3 2 2o DSat DSat ThV V V V
3 3 2 2in DSat Th DSat ThV V V V V
1 3 1 3
2
m m DS DSout
m
g g r rRg
2 3
1 3
m min
m m
g gRg g
Small signal model Vin and Vout create voltage imbalance
between V and V
Analog Integrated Circuits – Fundamental building blocks – Current mirrors 9
modelbetween VDS1 and VDS2
Accuracy issues for the current gain n
The balanced Wilson current mirror
NMOS PMOS
2 3m mg gR
min 3 2 2o DSat DSat ThV V V V
3 3 2 2in DSat Th DSat ThV V V V V
1 3 1 3
2
m m DS DSout
m
g g r rRg
2 3
1 3
m min
m m
g gRg g
Analog Integrated Circuits – Fundamental building blocks – Current mirrors 10
Small signal model
The fundamental bipolar current mirrorNPN PNP
1
1in
m
Rg
min 2o CEV V1in BEV V
2 2 11S CE CEI V V
2out CER r
2 1SI Vn
Analog Integrated Circuits – Fundamental building blocks – Current mirrors 11
current gain:
2 2 1
1 1
2 2 1
1 1
1
11 1
S CE CE
S CE EA
S CE CE
S CE EA
I V VI V V
nI V VI V V
2
1 2S
S
InI
ΔV – input-output voltage imbalance
2
1
1S
S in EA
I VnI V V
The fundamental current mirror with β compensation
NPN PNP
1
1in
m
Rg
min 2o CEV V1 3in BE BEV V V
current gain → β replaced by β (β+1) → n much closer to the ideal value when the
2out CER r
Analog Integrated Circuits – Fundamental building blocks – Current mirrors 12
current gain → β replaced by β (β+1) → n much closer to the ideal value when the input and the output are balanced in voltage
2
1
11 2
S
S
InI
2
1
1S
S in EA
I VnI V V
The degenerated fundamental current mirror
NPN PNP
11
1in
m
R Rg
min 2 2o CE outV V I R 1 1in BE inV V I R
current gain:
2 2 2 2 2out CE m CER r R g r R
1RnR
1 1 2 2BE in BE outV I R V I R
Analog Integrated Circuits – Fundamental building blocks – Current mirrors 13
current gain:2R
remember to adjust the emitter areas of Q1 and Q2proportionally with the current in each branch !!!
n is still affected by the finite β and by ΔV
22 1
1 1 2
S
S
IA R nA I R
1 1 2 2BE in BE outV I R V I R
The bipolar cascode current mirrorNPN PNP
1 3
1 1in
m m
Rg g
min 4 2o CE BEV V V 1 3in BE BEV V V
4 4 2 4
24 2
1
1
m CE CE BEout
mBE CE
m
g r r rRgr rg
Analog Integrated Circuits – Fundamental building blocks – Current mirrors 14
current gain → β influences the accuracy while the fundamental mirror Q1-Q2 is balanced by the cascode transistors Q3-Q4
22
21 4 2
S
S
InI
The bipolar Wilson current mirror
asymmetrical
3 2 3BE m min
r g gR
g g g r
min 4 2o CE BEV V V 1 3in BE BEV V V current gain → β influences the accuracy while the fundamental mirror Q1-Q2 is balanced by the cascode transistors Q3-Q4
balanced
Analog Integrated Circuits – Fundamental building blocks – Current mirrors 15
2 1 3 3in
m m m BE
Rg g g r
11 3 1 3
3
1 12 1
3 3
1
CEm m CE CE
BEout
CE CEm m
BE BE
rg g r rrR
r rg gr r
22
21
24 2
S
S
InI
Bibliography
P.E. Allen, D.R. Holberg, CMOS Analog Circuit Design, Oxford University Press, 2002
B. Razavi, Design of Analog CMOS Integrated Circuits, McGraw-Hill, 2002
D. Johns, K. Martin, Analog Integrated Circuit Design, Wiley, 1996
P.R.Gray, P.J.Hurst, S.H.Lewis, R.G, Meyer, Analysis and Design of Analog Integrated Circuits, Wiley,2009
R.J. Baker, CMOS Circuit Design, Layout and Simulation, 3rd edition, IEEE Press, 2010
Analog Integrated Circuits – Fundamental building blocks – Current mirrors 16
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