F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” Design of CMOS Analog Integrated Circuits Franco Maloberti Current and Voltage Sources
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources”
Design of CMOS AnalogIntegrated Circuits
Franco Maloberti
Current and Voltage Sources
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 24/
Current Mirrors
A current mirror gives a replica (if necessary, attenuated or amplified) of a bias or signal current.
• Simple current mirror
• Wilson current mirror
• Improved Wilson Current Mirror
• Cascode current mirror
• Modified cascode current mirror
• The high compliance current mirror
• The regulated cascode current mirror
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 34/
Simple current mirror:
to simplify the calculation let
( ) ( )1DS2
Th1GS1
1ref V1VVL
W
2
kII λ+−
==
( ) ( )2DS2
Th2GS2
2out V1VVL
W
2
kII λ+−
==
2GS1GS1DS VVV ==
( )2DS
1
2refout V1
LWLW
II λ+
⋅=
2dsout
out rI
1r =
λ=
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 44/
Factors affecting the mirror accuracy:
• Channel length modulation ( )• Threshold offset• Parasitic resistances• Imperfect geometrical matching and current mobility variation• Technological parameter mismatch
if:
It can be improved by increasing the output resistance.
21 L
W
L
W
=
( )( )1DS
2DS
ref
out
V1
V1
I
I
λ+λ+=
λ
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 54/
The accuracy:
• The threshold mismatch is inversely proportional to the overdrive• The threshold of the MOS transistors placed in close proximity are
matches within within few mV. By contrast, for MOS transistors which are hundred of microns apart have threshold voltages that can differ by few tens of mV.
• The last term refers to a mismatch in VGS. It can be derived from resistive voltage drop. This is due to a parasitic resistance in series with the source.
• The metal resistance is in the order of 20 -50 mΩ/ with 10 squares it results in 0.2 - 0.5 Ω. If the current is 10 mA an equivalent offset of 2 -5 mV.
• and are minimized with closed and centroid common structures
• and depends on the lithographic proces
µδµ
ox
ox
C
Cδ
W
WδL
Lδ
2
ThGS
GS2
ThGS
Th22
ox
ox222
out
out
VV
V2
VV
V2
C
C
L
L
W
W
I
I
−
δ+
−
δ+
µδµ+
δ+
δ+
δ=
δ
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 64/
• For a large W a goodstrategy is to have Wnot much larger thanL and to put equaltransistor in parallel
• The common centroidstructures achieves toreduce the parasitic capacitances.
δ+
δ+⋅⋅⋅=
δ 222
out
out
L
L
W
W
n
1
I
I
Iref IOut
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 74/
Wilson Current Mirror
Increases the output resistance.
• RL must be large• There is a systematic error because VDS1 = VGS3 + VDS2
T2g1m3g2mx3s2g rvgvg/ivv −===
( ) ( )
+++=−== T1m
2m
3m3ds
2mout3ds3gs3mx
2m
xx rg1
g
g1r
g
1rrvgi
g
iv
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 84/
ExampleSimulate with Spice the Wilson current mirror. Use the followingtransistors’ sizing (in µ): M1: 50/1 M3: 100/1. Find the difference between reference and generated current as a function of Iref in the range 0 - 200 µ A.
As expected, the output current is less than the reference current. Moreover, the difference is almost linear with the reference current and has a slope around 1%.
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 94/
Improved Wilson Current Mirror
4GS2DS3GS1DS VVVV −+=
4GS3GS2DS1DS VVifVV ==
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 104/
Small signal equivalent analysis:
• The output swing in the Wilson and improved Wilson schemes is limited to
4mL
4mL'T2g1m3g gR1
gRrVgV
+−=
( )4mL1ds'T g1Rrr +=
4mL
4mL'T1m
2m
3m3dsout gR1
gRrg
g
grr
+≅
3,sat1,satn,Th3,sat1GSmin,out VVVVVV ++=+=
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 114/
It increases the output resistance without loop feedback. It has the same resistance of the cascode load.
• The output swing is limited to:
Cascode Current Mirror
( ) 2ds3m3dsoutx2ds3m3dsx2dsx rgrrirg1rirv ≅++=
satTh3sat3Gmin,out3GS4GS1GS3G V2VVVVVVVV +≈+=−+=
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 124/
• The function of the transistor M4 is to shift the voltage VDS1 of the amount enough to bias te gate of M3 without bringing M2 out of saturation.• The use of M4 (matched with M3) allows to get VDS1 = VDS2 (this is paid with the output swing limitation).• The output swing is improved by the use of a level shift Vsat < ∆V < VTh.
Modified Cascode Current Mirror
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 134/
The geometrical dimensions of M1, M6, M4, M5 determine, with Vov,4, ∆V.
5ox
55
4ox
445GS4GS WC
LI2
WC
LI2VVV
µ−
µ=−=∆
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 144/
• All transistors operate in saturation
I = I1 = I2 = I3 = I4 (current scaling ispossible)
β1 = β2 = β3 = βVov1 = Vov2 = Vov3 = Vov
I4 = β4 (Vov4)2 ; I2 = β (Vov)2 ; I3 = β (Vov)2 ;Vov = ∆V
VDS2 = Vov
VGS3 = VTH + Vov
VGS4 = VTH + 2Vov Vov4 = 2Vov Therefore β = 4 β4
High - Compliance Current Mirror
4321 L
W4
L
W
L
W
L
W
=
=
=
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 154/
• Current gain:
Systematic error due to VDS1 ≠ VDS2
Systematic error due to body effect on M3
• Output swing: Voutmin = 2VDSsat
• Output impedance: rout = rout4 gm2 rout2
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 164/
M1 = M2 ; M3 = M4 Therefore Iout = Iref
To properly work, we must impose VDSsatM4 > VTh1
High - Compliance Current Mirror (II)
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 174/
• Output impedance:Rout = (gm3rout3) rout2 (gm4rout4)
• Output swing:Voutmin = VGS4 + VDSsat3 ≈ VTh + 2Vsat
Regulated - Cascode Current Mirror(possible implementation)
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 184/
Most of the analog blocksrequire a reference current.The circuits in the figureare supply dependent:
The accuracy of a generated current is:
• The global inaccuracy is around 32%• The dependence on the bias becomes critical when supply lines are affected by spur signals since the disturbance are transformed in current spurs.
Current Reference
( ) 2
L
L2
1GSDD
1GSDD2
fRe
fRe
R
R
VV
VV
I
δ+
−−δ=
δ
L
1GSDDfRe R
VVI
−=
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 194/
• Self biased (or bootstrapped) current reference
Two operating points. It is necessary to use a start-up circuitry
( )2ThGS1
21 VVL
W
2
kII −
==
43 L
W
L
W
=
Th1
11GS12 V
W
L
k
I2VRIRI +
===
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 204/
• Self biased low current reference generator
• Assume M3 = M4 ideal mirror I3 = I4• Assume VTh1 = VTh2
For k = 10, VR ≈ 60 mV if it is requiredI = 1µA R = 60 kΩ
1kW
Lk
W
L
W
L
W
L
2143
>
=
⋅
=
22ox
22
1ox
11 RIWC
LI2
WC
LI2 +µ
=µ
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 214/
• VT-based reference generatorFor p-well technology, a verticle npn transistor with the collector tied to VDD is available (the complementary for n-well)
both VT and R have positive temperature coefficients
4321 W
L
W
L;
W
L
W
L
=
=
BA21 VV;III ===
SS
1T2BE
SS
1T1BE nAI
IlnVV;
AI
IlnVV ==
1
SS
SS
1T2BE1BE1 I
nAI
AI
IlnVVVRI =−=
( )nlnR
VI T=
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 224/
• VBE multiplier
• Threshold voltage difference
• Band gap
Voltage References
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 234/
• VBE-multiplier:
• VBE has a negative temperature coefficient
521 W
L
n
1
W
L
W
L
=
=
43 W
L
W
L
=
BA VV =
R
VI 1BE=
BEDDDDout nVVnRIVV −=−=
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 244/
• Threshold voltage difference
M1 and M2 have different threshold (enhancement and depletion or enhancement and enhancement)
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 254/
• Band gap voltage reference:
In order to have a reference voltage with zero temperature coefficient (in a defined temperature range) it is necessary to:
• VBE has a negative temperature coefficient (-2.2 mV/°C at 300 K)
• VT has a positive temperature coefficient (+0.086 mV/°C at 300 K)m ≈ 25.6
TBEref mVVV +=
F. Maloberti : Design of CMOS Analog Integrated Circuits - “Current and Voltages Sources” 264/
For a p-well process:
231S
2S
2
1TBE IR
I
I
I
IlnVV ==∆
2Q
1Q
2S
1S
2
1
2
1
A
A
I
I;
LWLW
I
I =
=
=∆−−=−= BE3
21BE222ref V
R
RVIRVV
+−=1Q
2Q
2
1
3
2TBE A
A
LWLW
lnR
RVV