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))

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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

]

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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!!

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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.

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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!!

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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)

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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)

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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.

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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

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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?