REFERENCE CIRCUITS
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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.
I-V curves of ideal references
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
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.
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
Typical variations affecting the references
• Power supply variation (main concern here)
• Load variation (ro=∞ for I-ref, ro=0 for V-ref)
• Temperature variation (main concern also)
• Processes variation (good process and layout)
• Interferences and noise (not considered here)
yTST
1Ty
y 1
For temperature variation, typically use fractional temperature coefficient:
TCF = =
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
Active Dividers
These can be used as “start up” circuits.
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.
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≈
HW: Calculate S REF
CC
V
V
Calculate TCF
MOS equivalent of VBE reference:
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
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
)(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.
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
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.
Considering the -effect, (1) is more like:
Then:
Differentiating wrt VDD and assuming constant VDS1 and VGS4 gives the sensitivity of IOUT wrt VDD.
HW: Verify the following sensitivity expression:
HW: Show that approximately:
VEB based current reference
VEB=VR
Startup
A cascoded version to increase ro and reduce sensitivity:
VEB reference
Requires start up
Not shown here
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.
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
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 makeVout/T=0
When this happens, Vout
= 1.26 V
General principle of bandgap reference
Generate a negatively PTAT (Proportional To Absolute Temperature) and a positively PTAT voltages and sum them appropriately.
PositiveTemperatureCoefficient
(PTC)
K
NegativeTemperatureCoefficient
(NTC)
XOUT
XP
XN
A Common way of bandgap reference
VBE is negatively PTAT at roughly -2.2 mV/°C at room temperature
Vt (Vt = kT/q) is PTAT that has a temperature coefficient of +0.085 mV/°C at room temperature.
Multiply Vt by a constant K and sum it with the VBE to get
VREF = VBE + KVt
If K is right, temperature coefficient can be zero.
In general, use VBE + VPTAT
How to get Bipolar in CMOS?
A conventional CMOS bandgap reference for a n-well process
VOS represents input offset voltage of the amplifier.Transistors Q1 and Q2 are assumed to have emitter-base areas of AE1 and AE2, respectively.
If VOS is zero, then the voltage across R1 is given as
Bandgap reference still varies a little with temp
Converting a bandgap voltage reference to a current reference
VDD
VREF
R1
R2
R3 R4
Q1 Q2
TR
R
A
A
q
kVVV
E
EBEBEBE
4
3
2
112 ln
111
1
1
1 1 EECIII
222
2
2
2 1 EECIII
BEBEREF
VRR
RR
VV
4
3
2
1
2
1
21
Vref=I3*R3=
)]ln(1
))ln(1
([1
0
1
2
013 T
TkT
qR
mT
T
VV
A
A
q
k
RR
VR o
o
GBEGo
Curvature corrected bandgap circuit
Vref
R3= R4
Q2 Q1
R2
R1
)(2
22
2
122
12
11212
2
122
4431
BEBEBE
RBERBEREF
BEBER
RRRR
VVR
RV
RIVVVV
R
VVI
IIII
:factIn
constbut const,2
:Problem
2
1
T
VV
R
R
TBE
BE
VBE
T
Vref
T
R4= R5
D2 D1
R1
R2
Solution:
Vref
IPTAT↓
R3
IPTAT2
Vref
T
VPTAT
VPTAT2
VBE
? get toHow 2
2
1
1
1
12
32
324
2
ln1
22
PTAT
E
E
BEBEPTAT
PTATPTATBEref
I
A
A
q
kT
R
R
VVI
RIRRR
IVV
Ex:
1. Suppose you have an IPTAT2 source characterized by
IPTAT2 = T2, derive the conditions so that both first
order and second order partial derivative of Vref with respect to T are canceled at a given temperature T0.
2. Suggest a circuit schematic that can be used to generated IPTAT
2 current. You can use some of the circuit elements that we talked about earlier together with current mirrors/amplifiers to construct your circuit. Explain how your circuit work. If you found something in the literature, you can use/modify it but you should state so, give credit, and explain how the circuit works.