albert comerma (pi) · 2017-07-07 · reference circuits albert comerma (pi)...

Advanced Analog Building Blocks

Reference Circuits

Albert Comerma (PI)([email protected])

Course web

SoSe 2017

http://www.kip.uni-heidelberg.de/~shen/Advanced_Analog_Building_Blocks/

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©[email protected] Advanced Analog Building Blocks: Reference Circuits 1 / 13


Bandgap references


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©[email protected] Advanced Analog Building Blocks: Reference Circuits 2 / 13


Bandgap references


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



Bandgap references


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

]



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!!



Bandgap references


Startup

Startup needed to set operation pointWhen VGM2

goes low M6 starts to increase current drivingoperation point to the desired current.



Bandgap references


Self biased BJT

I =VD1R

= ISeVD1nVT

Better matching due to cascode mirrors.

Caution!!

Startup circuit also needed!!



Bandgap references


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)



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)



Bandgap references


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.



Bandgap references


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



Bandgap references


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



Bandgap references


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.


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?


Exercise 3: CMOS Bandgap reference

Design a sub-bandgap CMOS reference using ideal amplifier vcvsfrom analogLib.

• What is the TC?


albert comerma (pi) · 2017-07-07 · reference circuits albert comerma (pi)...

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