unit 1 - voltage and current references1.pdf

7/27/2019 Unit 1 - Voltage and Current References1.pdf

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Voltage and Current References

Biswajit Behera

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Voltage & Current References

An Ideal reference should be independent of power supply, processand temperature variations.

The term Reference is used when the current or voltage values have

more precision and stability than ordinarily found in a source.

A high performance voltage reference can be used to implement a

high performance current reference and vice versa.

Voltage and current references used to bias the current sources and

sinks, differential amplifiers, operational amplifiers etc..

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To reduce powerdissipation, the resistors

must be made very large,

which requires large area

on die.

Not practicable.

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Used in simple current mirrorto bias the M2 transistor

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Calculate Vref?

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Calculate Sensitivity of Vref to VDD ?

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Calculate Temperature Coefficient of Vref ?

How to make Vrefas more than VGS ?

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How to achieve Vrefas more than VGS ?

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Small layout because both are transistors

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Calculate Vref?

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Calculate Sensitivity of Vrefto VDD ?

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Calculate TC (Vref) ?

How to achieve zero TC (Vref) ?

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Condition for zero TC(Vref)

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Solution :

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Solution (Contd.):

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This is also called Dual Reference voltage divider

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Solution :

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Application ofDual reference voltage divider

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Current Source Self Biasing

Main drawback of References based on voltage divider are

very sensitive to supply voltage, process and temperature.

Three important types of current source self biasing are :

1. Threshold voltage referenced self biasing

2. Diode referenced self biasing

3. Thermal voltage referenced self biasing

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1. Threshold voltage referenced self biasing

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Ideally the Reference current is independent of power supply.

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This circuit requires a start up circuit, for flawless functionality. Why ?

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Requirement ofStart-Up circuit

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Threshold Referenced Self biasing with

start-up circuit

When I1 = I2 = 0 then

VGS1 = I2R = 0 and VDS1=VGS2 = 0.

Now M5 transistor provides current

to flow through drain of M1. so I1 increases

and so I2.

VG2 increases

VGS5 decreases

When I1 reaches point A,

M5 turns off as VGS5 < Vt5.

The start up circuit will be idle until I1 (=I2)

Becomes again zero.

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2. Diode Referenced Self Biasing

VBE referenced self biasing in CMOS

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CMOS circuits rely on using well transistors, which are vertical bipolar

transistors, that use wells as their bases and the substrates as their

collectors.

In an n-well process, the vertical bipolar transistors are PNP type with

their collectors connected to ground.

In a p-well process, the vertical bipolar transistors are NPN transistors,

with their collectors connected to the positive power supply

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The parasitic transistor is connected as a

diode ( base and collector are tied

together), which reduces the effectiveresistance in series with the diode and the

leakage to substrate.

The cascode mirrors made with M1

through M8 force the same current, I, toflow through D1 and R.

Assuming matched devices, VGS1 = VGS2

then

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Advantage : compared to threshold referenced self biasing circuit, this has

better matching, from wafer to wafer and on the same die, of the diodevoltage(VBE) over threshold voltage.

This circuit also depends on temperature.

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3. Thermal Voltage Referenced Self Biasing

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Disadvantage :

1. Since the voltage drop across resistor R is very small, Mismatches

in the gate source voltages of M1 and M2 are made large variations

in reference current I.

2. Susceptibility to externally coupled noise across R.

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Advantage : The reference current of this circuit has less dependence on temperature.

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Disadvantage of above three circuits

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Bandgap Voltage References

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Negative Temperature Coefficient (TC)

The forward voltage of a PN junction ( VBE of a diode

connected BJT) exhibits a negative TC.

For a diode connected BJT,

We know

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Positive Temperature Coefficient (TC)

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Practical circuit to add

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l ll h h l b l


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But CMOS circuits rely on using well transistors, which are vertical bipolar

transistors, that use wells as their bases and the substrates as their

collectors.

In an n-well process, the vertical bipolar transistors are PNP type with

their collectors connected to ground.

In a p-well process, the vertical bipolar transistors are NPN transistors,

with their collectors connected to the positive power supply

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Other Possible Implementation

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TC of Bandgap Reference Voltage


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TC of Bandgap Reference Voltage

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unit 1 - voltage and current references1.pdf

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