will have better matching but: only approximate common centroid no pli can be more compact
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S. G. G. S. G. G. S. G. G. S. G. G. S. G. G. S. Will have better matching But: only approximate common centroid no pli can be more compact. HW: suggest a better layout for ratio of 4. REFERENCE CIRCUITS. - PowerPoint PPT PresentationTRANSCRIPT
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Will have better matchingBut: only approximate common centroid
no plican be more compact
HW: suggest a better layout for ratio of 4.
G G G G G GG G G GS S S S S S
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REFERENCE CIRCUITS
A reference circuit is an independent voltage or current source which has a high degree of precision and stability.
• Output voltage/current should be independent of power supply.• Output voltage/current should be independent of temperature.• Output voltage/current should be independent of processing variations.
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I-V curves of ideal references
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Concept of Sensitivity
),,( 321 xxxfy
3
12
11
1
xxf
xxf
xxf
y
3
33
12
22
11
11
1 xx
yx
xf
xx
yx
xf
xx
yx
xf
yy
3
3
2
2
1
1321 xx
Sxx
Sxx
Syy y
xyx
yx
yxiS
Let
Then:
is called the sensitivity of y with respect to xi
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Total percentage change in y = Sensitivity w.r.t. x1 * percentage
change in x1 + Sensitivity w.r.t. x2 * percentage
change in x2 + ……
Goal: Design reference circuits so that the reference’s sensitivities w.r.t.
various variations are minimized.
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Types of commonly used references
• Voltage dividers - passive and active.
• MOS diode reference.
• PN junction diode reference.
• Gate-source threshold reference circuit.
• Base-emitter reference circuit.
• Thermo voltage reference circuit
• Bandgap reference circuit
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Typical variations affecting the references
• Power supply variation (main concern here)
• Load variation (want ro=∞ for I-ref, ro=0 for V-ref)
• Temperature variation (main concern also)
• Processes variation (use good process and layout)
• Interferences and noise (not considered here)
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yTST
1Ty
y 1
For temperature variation, typically use fractional temperature coefficient:
TCF =
rather than sensitivity
=
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Voltage references
Passive Divider Limited accuracy, ~6-bit, or 2%
Large static powerfor small ro
Large area
Power sensitivity =1
Temp coeff depends on material
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Active Dividers
These can be used as “start up” circuits.
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S REF
CC
V
V )/ln(1
sCC RIV
S REF
CC
V
V
PN Junction Voltage References
=
If VCC = 10V, R = 10 k, and IS = 10-15A, then = 0.0362.
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TV
VREF
REF 1
TRR
qVkT
qVk
TVVV
REFREFREF
GOREF
3
)/
exp(3
qkT
VVKT
R
VV GOREFREFCC
For a diode:
Taking ∂/∂T and using: VCC − VREF + kT/q ≈ VCC − VREF:
=
where VGO = 1.205 V is the bandgap voltage of
silicon.If VREF = VBE = 0.6V, TCF of R = 1500 ppm,
then TCF of VREF = -3500 ppm/oC
TCF≈
)exp(
/),exp(
3
t
GOs
tt
s
V
VKTI
qkTVV
vIi
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HW: Calculate S REF
CC
V
V
Calculate TCF
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MOS equivalent of VBE reference:
22
1)(21
)(2
RR
VV
RVV
R
VVVVV
TDDTREF
REFDDTGSREF
GSREF VR
RRV
2
21
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S REF
DD
V
V
S REF
DD
V
V
The sensitivity w.r.t. VDD:
If VDD = 10V, W/L = 10, R = 100k,and using
parameters from Table3.1-2,then VREF = 1.97V and
This is not nearly as good as the VBE reference.
= 0.29
)(1
1
TREFREF
DD
VVRV
V
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o = KT-1.5 ; VT = VT0 - T or VT(T) = VT(To) - (T-To)
For temperature coefficient
T
V
RTR
R
R
VVT
V
R
VV
L
WC
R
VV
T
T
V
RTR
R
R
VV
T
V
T
VVV
L
WCVV
L
WC
T
R
VVVV
L
WC
REFREFDD
REFREFDDoxREFDD
REFREFDD
TREFTREF
oxTREF
ox
REFDDTREF
ox
1
)(2
25.1
1
)(2
22
22
2
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)(2
11
15.121
1
REFDD
REFDD
REF
REF
REFF
VVR
TR
RTRVV
V
T
V
VTC
Solving for ∂VREF/∂T and computer TC:
The book has one example of using this.
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VGS based Current referenceMOS version: use VGS to generate a current and then use negative feed back stabilize i in MOS
Current mirror
Startup
VGS
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Why the start up circuit?There are two possible operating points:
The desired one and
The one that gives I1 = I2 = 0.
At power up, I1 = I2 = 0 without the start up.
RB bias M6 to be on, which turns M2 and M1 on.
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Considering the -effect, (1) is more like:
Then:
Differentiating wrt VDD and assuming constant VDS1 and VGS4 gives the sensitivity of IOUT wrt VDD.
)(1
1
1
1
1
4
3
4
1
2
DSDDP
GSP
DSP
DSP
VV
V
V
V
I
I
1
111
2141
2)(1
))(1()1(
IVVV
RIVVVRI
TDSDDP
DSDDPGSP
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HW: Verify the following sensitivity expression:
HW: Find approximately the temperature coefficient of Iout
1114
1
11
2/)(1)1(
2
IVVVR
IV
I
V
DSDDPGSP
TP
out
DD
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Current mirror
Startup
VGS
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VEB based current reference
VEB=VR
Startup
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A cascoded version to increase ro and reduce sensitivity:
VEB reference
Requires start up
Not shown here
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Come up with a start up circuit for the circuit on the previous slide, using only active resisters without RB. Note that you need to make sure
that at the desired operating point, the connection from the start up circuit should be turned off.
HW:
Analyze the sensitivity of the output I with respect to VDD and temperature.
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A thermal voltage based current reference
I1 = I2, J1 = KJ2,
but J = Jsexp(VEB/Vt)
J1/J2 = K =
exp((VEB1─ VEB2)/Vt)
VEB1─ VEB2 = Vt ln(K)
I = (VEB1─ VEB2)/R
= Vt ln(K)/R Vt = kT/q
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A band gap voltage reference Vout = VEB3 + I*L*R =
VEB3 + (kT/q)*Lln(K)
Vout/T = VEB3/T +
(k/q)*Lln(K)At room temperature,
VEB3/T = ─2.2 mV/oC,
k/q = +0.085 mV/oC.Hence, choosing
appropriate L and K can make
Vout/T=0
When this happens, Vout
= 1.26 V
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