albert comerma (pi) · 2017-07-07 · reference circuits albert comerma (pi)...
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Advanced Analog Building Blocks
Reference Circuits
Albert Comerma (PI)(comerma@physi.uni-heidelberg.de)
Course web
SoSe 2017
Simple ReferencesSupply independent references
Bandgap references
Simple referencesPassive divider and MOS referenceBipolar referenceMOS divider reference
Voltage references
Temperature and power-supply independent references.
I. Passive reference
II. Resistor-MOS reference
III. Bipolar reference
IV. MOS reference
Important parameters:
• Power supply Sensitivity:SVrefVDD
= VDDVref
∂Vref∂VDD
4VrefVref
= SVrefVDD
(4VDDVDD
)• Temperature coeficient:
TC = 1Vref
∂Vref∂T
= 1TSVrefT
(ppm◦C
)c©comerma@physi.uni-heidelberg.de Advanced Analog Building Blocks: Reference Circuits 1 / 13
Simple ReferencesSupply independent references
Bandgap references
Simple referencesPassive divider and MOS referenceBipolar referenceMOS divider reference
Simple references
I. Passive refrence
Vref = VDDR2
R1+R2
SVrefVDD
= VDDVref
∂Vref∂VDD
= 1
TC = 1Vref
∂Vref∂T
= R1R1+R2
(TC(R2)− TC(R1))
(supposed ∂VDD∂T
= 0)If TC(R1) = TC(R2)→ TC = 0, same type of resistors.
II. MOS refrence
Vref = VGS = VTH +√
VDDβR1
(supposed VDD >> Vref )
SVrefVDD≈ VDD
2Vref
√βR1(VDD−Vref )
∂Vref∂T
= ∂VT0∂T− 1
2
√2nVDDβR1
(1R1
∂R1∂T
+ 1β∂β∂T
)TC = VT0
VrefTC(VT0)− 1
2Vref
√2nVDDβR1
(TC(R1)− TC(β))
c©comerma@physi.uni-heidelberg.de Advanced Analog Building Blocks: Reference Circuits 2 / 13
Simple ReferencesSupply independent references
Bandgap references
Simple referencesPassive divider and MOS referenceBipolar referenceMOS divider reference
Simple references
III. Bipolar refrence
Vref ≈ VT ln(
VDD−VrefR1IS
)Vref ≈ VT ln
(VDDR1IS
)(supposed VDD >> Vref )
SVrefVDD
= 1
ln(
VDDR1 IS
)∂Vref∂T
= VrefT− VT
(1IS
∂IS∂T
+ 1R1
∂R1∂T
)TC = Vref
T− VT (TC(IS)− TC(R1))
Numbers start to get difficult to compare;
Normally:
SVrefVDD
∣∣MOS
> SVrefVDD
∣∣BJT
TC∣∣MOS
< TC∣∣BJT
Usually sensitibity studies are performed by means of simulation
c©comerma@physi.uni-heidelberg.de Advanced Analog Building Blocks: Reference Circuits 3 / 13
Simple ReferencesSupply independent references
Bandgap references
Simple referencesPassive divider and MOS referenceBipolar referenceMOS divider reference
Simple references
IV. MOS reference
Assuming same current flow to both transistors;
Vref = VTHN + VDD−VTHN−|VTHP |√β1β2
+1
SVrefVDD
= VDD
VDD−VTHN+
√β1β2−|VTHP|
≈ 1
TC = 1Vref
1√β1β2
+1
[∂(−VTHP )
∂T+ β1
β2
∂VTHN∂T
]
c©comerma@physi.uni-heidelberg.de Advanced Analog Building Blocks: Reference Circuits 4 / 13
Simple ReferencesSupply independent references
Bandgap references
Self biased MOS current sourceSelf biased BJT current sourceThermal Voltage referenced Self-biasing
Self biased MOS
Try to avoid VDD dependent output.
M3 and M4 mirrors make I1 = I2;
IR = VGSM1 = VTHN +√
2Iβ1
Iref ≈ VTHNR
for large β
Caution!!
Two operating points:
A No current:VGM3,4 = VDD
VGM2= 0
B Iref
Startup circuit needed!!
c©comerma@physi.uni-heidelberg.de Advanced Analog Building Blocks: Reference Circuits 5 / 13
Simple ReferencesSupply independent references
Bandgap references
Self biased MOS current sourceSelf biased BJT current sourceThermal Voltage referenced Self-biasing
Startup
Startup needed to set operation pointWhen VGM2
goes low M6 starts to increase current drivingoperation point to the desired current.
c©comerma@physi.uni-heidelberg.de Advanced Analog Building Blocks: Reference Circuits 6 / 13
Simple ReferencesSupply independent references
Bandgap references
Self biased MOS current sourceSelf biased BJT current sourceThermal Voltage referenced Self-biasing
Self biased BJT
I =VD1R
= ISeVD1nVT
Better matching due to cascode mirrors.
Caution!!
Startup circuit also needed!!
c©comerma@physi.uni-heidelberg.de Advanced Analog Building Blocks: Reference Circuits 7 / 13
Simple ReferencesSupply independent references
Bandgap references
Self biased MOS current sourceSelf biased BJT current sourceThermal Voltage referenced Self-biasing
Current references
• Mirrors formed by M1 to M8 force samecurrent;
VD1 = VD2 + ID2R
• Emitter of D2 is K times D1;
I = nkln(K)qR
= nVTR
ln(K)
c©comerma@physi.uni-heidelberg.de Advanced Analog Building Blocks: Reference Circuits 8 / 13
Simple ReferencesSupply independent references
Bandgap references
Song Bandgap referenceBrokaw Bandgap referencePure CMOS Bandgap
Bandgap references
• All previous circuts suffer from temperature dependence.
• Some of previous have positive TC while others negative TC.
Bandgap reference
Combine both TCs to achieve a common TC = 0.
BJT 4VBE variation with temperature;
∂(4VBE )∂T = α∂VT
∂T = α ∂∂T
(kTq
)= αk
q = α0.085mV◦C
Proportional to absolute temperature (PTAT)
c©comerma@physi.uni-heidelberg.de Advanced Analog Building Blocks: Reference Circuits 9 / 13
Simple ReferencesSupply independent references
Bandgap references
Song Bandgap referenceBrokaw Bandgap referencePure CMOS Bandgap
Bandgap references
Cancel temperature dependence:
∂Vout∂T = ∂VBE
∂T + α∂VT∂T
∂VBE∂T = −α∂VT
∂T
α = 2mV /◦C
0.085mV /◦C ≈ 23.53
For VT = 26mv@25◦C output isaround 1.2V, close to the silicon bandgap voltage.
c©comerma@physi.uni-heidelberg.de Advanced Analog Building Blocks: Reference Circuits 10 / 13
Simple ReferencesSupply independent references
Bandgap references
Song Bandgap referenceBrokaw Bandgap referencePure CMOS Bandgap
Song Bandgap reference
• Currents copied by mirrors;VBE1 = VBE2 + ID2R
• PTAT current;
IPTAT = I2 =VBE1−VBE2R
= VTRln(
I1I2
IS2IS1
)• If D2 is N times larger than D1;
IPTAT = VTRlnN
• Output voltage;Vref = VBE3 + VT
R2R1lnN = VBE3 + αVT
• If R2 = LR1;Vref = VBE3 + VTLlnN = VBE3 + αVT
c©comerma@physi.uni-heidelberg.de Advanced Analog Building Blocks: Reference Circuits 11 / 13
Simple ReferencesSupply independent references
Bandgap references
Song Bandgap referenceBrokaw Bandgap referencePure CMOS Bandgap
Brokaw Bandgap reference
• Currents copied by mirrors;VBE1 = VBE2 + ID2R
• PTAT current;
I1 = I2 =VBE1−VBE2R2
= VTR2
ln(
I1I2
IS2IS1
)=
VTRlnN
• Output voltage;Vref = VBE1 + R1(I1 + I2) =VBE1 + VT
2R1R2
lnN = VBE1 + αVT
c©comerma@physi.uni-heidelberg.de Advanced Analog Building Blocks: Reference Circuits 12 / 13
Simple ReferencesSupply independent references
Bandgap references
Song Bandgap referenceBrokaw Bandgap referencePure CMOS Bandgap
Pure CMOS Bandgap
CMOS technologies usually have only lateral PNP bipolartransistors available.Usually called sub-bandgap due to lower output voltage.
VEB1 = VEB2 + I2R1
I2 =VEB1−VEB2R1
=
VTR1
ln(
I1I2
IS2IS1
)= VT
R1lnN
I3 = I2 − I4 − I5 =I2 − Vref
R4+R5− Vref R5
R3(R4+R5)
Vref = VEB3 + I3R2
Vref = β(VEB3 + αVT )
With β independent withtemperature and smaller than1.
c©comerma@physi.uni-heidelberg.de Advanced Analog Building Blocks: Reference Circuits 13 / 13
Exercise 1: Elements dependence on temperature
On Ex8 library you can find a cellview named Bipolar where youcan see a linear effect on temperature.On the RdivRef check the performance with temperature variationand with power supply variation;
• What is the TC?
• What is the Sensitivity to power supply?
Exercise 2: Bandgap references
With the Song and Brokaw schematics try to tune the parametersto get an acceptable voltage output dependent on temperature;
• What is the TC?
• What is the Sensitivity to power supply?
Exercise 3: CMOS Bandgap reference
Design a sub-bandgap CMOS reference using ideal amplifier vcvsfrom analogLib.
• What is the TC?
• What is the Sensitivity to power supply?
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