05 voltage & current references

EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014

Voltage/Current Reference

Prof. Jader A. De Lima


• a voltage reference is an esential part in ADC´s, DAC´s, linear regulators, etc


• series reference:

• provides load current through a pass transistor between VIN and VREF• requires less power, as load current is provided as it´s needed

• shunt reference:

• regulates VREF by shunting excess current to ground via a parallel transistor• bias current must be equal or greater than load current


• temperature coefficient (TC or tempco)

curvature of the voltage driftFirst-order (linear) TC


Voltage Reference

• avalanche is a rather noisy mechanism• Zener reference: best TC @ high voltages e.g. 6.2V→ high supply voltage is required to achieve low TC from Zener reference

TC + 100ppm/oC

Best TC @ high voltagese.g. 6.2V!!

strong TC -0.3%/oC

TC > 0

TC < 0

• simple circuits


• the bandgap reference concept:

• output voltage for zero TC is close to bandgap voltage of Si (~1.2V)• VCC must be higher than 1.2V → not suitable for sub-1V applications

CTAT

PTAT


• dependence of VBE on temperature (CTAT voltage)


( ) 0.002065T 1.265BE1V T = +−no laboratório (Exp 2)


• PTAT voltage


• Determinar a referência Vout


13 2 1 2

2

2 2 12

3 3 2

ln

ln

BE BE t

BE t

IR I V V

I

R R IV V

R R I

φ

φ

= − =

= ∆ =

2 13 2 3

3 2

lnref BE BE t

R IV V V V

R Iφ= + = +

• problem: maintain I1/I2 independent of T


Since I1 = I2 (assuming R1 = R2)

• bandgap reference with an opamp


Projetar o bandgap abaixo para mínima variação a T = 300oK, admitindo:

R1 = R2IS1= 0.1pAIS2 = 0.4pAI1 = I2 ≈ 1mAβ >> 1Vout ≈ 1.1V∆VBE/∆T = -2mV/oC


Projetar o bandgap abaixo para mínima variação a T = 300oK, admitindo:

R1 = R2IS1= 0.1pAIS2 = 0.4pAI1 = I2 = 1mAβ >> 1∆VBE/∆T = -2mV/oC


para T = T0

R1 = R2 = 555.2 Ohms

para T0 = 300oK


current mirror


• Reference Current Generation

supply dependent!

β >> 1


Robert J. Widlar

• Widlar current source


REFERÊNCIAS:

• Fundamentals of Microelectronics, B. Razavi, John Wiley and Sons, 2006

• Microelectronic Circuits, A. Sedra and K. Smith, Oxford university Press, 5th Edition, 2003

05 voltage & current references

Engineering

Transcript of 05 voltage & current references