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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.

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reference circuits
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.
concept of sensitivity
Concept of Sensitivity

Let

Then:

is called the sensitivity of y with respect to xi

slide4

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
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
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)
voltage references
Voltage references

Passive Divider

Limited accuracy,

~6-bit, or 2%

Large static power

for small ro

Large area

Power sensitivity =1

Temp coeff depends on material

slide9

Active Dividers

These can be used as “start up” circuits.

slide10

PN Junction Voltage References

=

If VCC = 10V, R = 10 kW, and IS = 10-15A, then

= 0.0362.

slide11

For a diode:

Taking ∂/∂T and using: VCC − VREF + kT/q ≈ VCC − VREF:

TCF≈

=

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

slide12

HW: Calculate

Calculate TCF

slide14

The sensitivity w.r.t. VDD:

If VDD = 10V, W/L = 10, R = 100kW,and using parameters from Table3.1-2,

then VREF = 1.97V and

= 0.29

This is not nearly as good as the VBE reference.

slide15

For temperature coefficient

mo = KT-1.5 ; VT = VT0 - aT or VT(T) = VT(To) - a(T-To)

slide16

Solving for ∂VREF/∂T and computer TC:

The book has one example of using this.

slide17

VGS based Current reference

MOS version: use VGS to generate a current and then use negative feed back stabilize i in MOS

Start

up

Current mirror

VGS

slide18

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.
slide19

Considering the l-effect, (1) is more like:

Then:

Differentiating wrt VDD and assuming constant VDS1 and VGS4 gives the sensitivity of IOUT wrt VDD.

a cascoded version to increase r o and reduce sensitivity
A cascoded version to increase ro and reduce sensitivity:

Requires start up

Not shown here

VEB reference

slide24

HW:

Analyze the sensitivity of the output I with respect to VDD and temperature.

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.

a thermal voltage based current reference
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
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 make

Vout/T=0

When this happens, Vout = 1.26 V

general principle of bandgap reference
General principle of bandgap reference

Generate a negatively PTAT (Proportional To Absolute Temperature) and a positively PTAT voltages and sum them appropriately.

slide29

VBEis 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 Vtby a constant K and sum it with the VBEto get

VREF = VBE+ KVt

If K is right, temperature coefficient can be zero.

slide34

VOSrepresents input offset voltage of the amplifier.

Transistors Q1 and Q2 are assumed to have emitter-base areas of AE1 and AE2, respectively.

If VOSis zero, then the voltage across R1 is given as

slide45

VBE

T

Vref

T

slide46

Solution:

R4= R5

Vref

IPTAT↓

D2

D1

R1

R2

IPTAT2

R3

slide47

Vref

VBE

VPTAT

VPTAT2

T

slide48

Ex:

  • Suppose you have an IPTAT2 source characterized by IPTAT2 = aT2, 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.
  • Suggest a circuit schematic that can be used to generated IPTAT2 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.