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INF4420Reference circuits

Spring 2012 Jrgen Andreas Michaelsen (jorgenam@ifi.uio.no)

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Outline

Reference circuits and bias circuits

Uses of reference circuits and bias cicuits

MOSFET based references

Parasitic diode based references (bandgaps)

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Reference circuits

"Bandgaps". Why?

Every analog and mixed-signalsystem needs a stable reference.

The reference circuit presents

some physical quantity (voltage,current, frequency, other?) Image courtesy ofTexas Instruments

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Biasing an analog circuit

Analog circuits needa current source toset the operating

point.

Circuit performancevery dependent onthe biasing.

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Data converters

Binary word input is dimensionless.

Need to multiply the dimensionless input with adimensioned (physical) reference.

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Voltage supply regulation

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Temperature behaviour

Predictable behaviour across temperature

Constant Proportional (PTAT)

Additionally, insensitive to supply voltage andprocess variations (PVT).

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A very simple reference

Careful layout gives good matching

between resistors

Temperature affects resistorsequally, good TC

Precisely defined Vref as ratio ofVdd

Precise value of Vdd is notknown (bad)

Poor PSRR! (bad)

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MOSFET-R reference

Can be used to generate abias voltage or referencevoltage.

Better PSRR than thevoltage divider.

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MOSFET-R reference

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MOSFET-R reference

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Beta multiplier reference

Deriving Iref from Iout

Less dependence onVdd

Can be used for biasingand reference

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Beta multiplier reference

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Beta multiplier reference

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Beta multiplier reference

As a voltage reference, take Vgs1 to be thereference voltage, Vref.

We know the current, Iref. Use this to find

Vgs1=Vref

We must find the sensitivity of Vref to Vdd andT

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Beta multiplier reference

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Bandgaps

It turns out, we can make references with lesstemperature dependence using bipolartransistors.

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Temperature independence

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Parasitic diodeCTAT and PTAT

Need elements with well defined temperaturebehaviour. We will use diode connected BJTs.

CTAT from Vbe (biased with constant current)

PTAT from "delta Vbe" (biased with ratioed

current or emitter area), result is the thermalvoltage, Vt

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Bipolar transistor diodes

Current sinked by thedevice is affected bytemperature

Vt is the thermalvoltage(proportional to T),26 mV @ RT

Is is also a functionof termperature

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PTAT voltage

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PTAT voltage

To find temperature behaviour, we take thederivative wrt. temperature (assume the firstderivative is constant)

Delta Vbe isproportional toabsolutetemperature.Defined by twophysical constantsand emitter ratio, n.

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CTAT voltage

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CTAT voltage

Silicon bandgap energy as a function oftemperature

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CTAT voltage

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Bandgap circuits

We know how to generate PTAT and CTAT,and how we should combine thesecontributions for temperature independence (I.

e. scale and add to acheive temperatureindependence).

How do we make a circuit that realizes thissystem?

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Bandgap circuits

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Bandgap circuits

Need a circuit that cansense V1 and V2 andadjust the current

sources so that V1=V2

V2 = Vbe2 + Vt ln n

ln n must be 17.2 for V2to be independent of T

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Bandgap circuits

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Bandgap circuits

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Bandgap circuits

This circuit is used for illustration purposes.Working with CMOS, there are a number ofissues with this circuit which we will discuss inthe following slides. We will try to find circuitswhich are more practical and CMOScompatible.

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A more CMOS friendly BJT

Instead of the diode connected npn that wehave used so far, we will use a pnp. This is sothat we can implement the device in a CMOS

process without any special processing.

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BJT in a CMOS process

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CMOS bandgap

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CMOS bandgap

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Low-voltage bandgap

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Low-voltage bandgap

The core circuit is (again) the PTAT currentgenerator. Although the delta Vbe gives rise toa PTAT voltage (dropped accross R1), theabsolute Vbe of Q1 and Q2 is CTAT. Vbe1controls the current through R2 and R3. Theresult is a temperature independent current ifthe currents are scaled correctly.

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Low-voltage bandgap

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Stability

Stability is a concern for any system withfeedback.

Must make sure that we have more negativefeedback than positive.

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Transient response

Transients may capacitively couple to circuitnodes.

Faster opamp

Decoupling (opamp stability)

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Startup circuit

In the discussion so far, we have assumed thecircuits are at the desireable operating point.

We must add circuitry to make sure the circuitis not stuck at a "zero" operating point.Typically a circuit to inject some current if weare at or close to the undesireable operatingpoint. (Power on reset.)

This is very important. Simulator does notneccessarily reveal this problem.

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Curvature correction

In our analysis we have asumed the PTAT andCTAT to be constant. This assumption will leadto a non-linearity of the TC (curvature),approximately parabolic shape.

Possible to design some function to try tomitigate this effect.

Even possible to use Vos constructively(Cabrini, ESSCIRC 2005). Not curriculum.

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Bandgap circuit issues

Collector current variation CMOS compatibility (BJTs) Opamp offset voltage Opamp resistive loading Stability Startup Transient response PSRR Curvature Limited supply voltage Noise Resistor TC

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Online resources

A Paul Brokaw

This is not part of thecurriculum

http://www.archive.org/details/APaulBro1989

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