voltage and current references bandgap references · microsoft powerpoint - ic_slide6.ppt...

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Voltage and Current ReferencesBandgap References

The objective of reference generation is to establish a dc voltage or current with following characteristics:

•Independent of the supply and process.•Having a well-defined behavior with temperature.

•Temperature independent.

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Supply-Independent Biasing

In the design of current sources, we need to a golden current source, IREF. (Figure (a))If instead of IREF we use a resistor, the generated current sources will be supply-dependent. (Figure (b))

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A Simple Circuit to Establish Supply-Independent Current Supply-Independent Biasing

The current is independent of the supply voltage but it is still a function of process and temperature.

=0, =0

4

Start-up Problem

Iout=0 is another operating point of the above circuit because it satisfies the following equation. We call this problem as the start-up problem.

Addition of the Start-up devices

Temperature-Independent References

Negative-TC Voltage

We assume that IC is held constant.

Continued

Positive-TC Voltage

Continued

Bandgap References

2121

CCYoutXout

YX IIRVV

RVVVV

Continued

3

22321

21

2322

111

RVIIRVVV

IIIRRVV

IRVVBE

CCBEBEBE

CC

CBEout

CBEout

23

22232 1 BEBEBECout VV

RRVIRRV

Continued

23

22

3

2 ln11 BETBEBEout VnVRRVV

RRV

TVn

qk

RR

TV BEout

2

3

2 ln10

KmVn

qk

RR

5.1ln1

3

2

Continued

VmVKmVVnV

RRV BETout 2.17503005.1ln1 2

3

2

Assuming VBE2=750mV, we have:

Compatibility with CMOS Technology

Modified Bandgap Circuit

Bandgap Circuit

The drawback of the above circuit is that VDS1VDS2. Therefore ID1 is not accurately identical to ID2.

nVRRVV TEBout ln1

1

22

Bandgap Circuit (Vout=2.5V)

The drawback of the above circuit is that VDS1VDS2. Therefore ID1 is not accurately identical to ID2.

nVRRVVV TEBEBout ln12

1

243

PTAT Current Generation

The advantage of the above circuit is that VDS1=VDS2. Therefore ID1 is accurately identical to ID2.

1

lnRnVI T

Employing PTAT to Generate Temperature-Independent Voltage

The advantage of the above circuit is that VDS1=VDS2. Therefore ID1 is accurately identical to ID2.

1

23lnRnVRVV T

EBout

Generation of a PTAT Current Using a Simple Amplifier

Generation of a Temperature-Independent Voltage

Generation of a Temperature-Independent Voltage

nVRR

VRR

V TEBout ln1

41

2

4

This circuit is useful to generate low reference voltages.

Generation of a Temperature-Independent Voltage

This circuit is useful to generate low reference voltages.

NVRRV

RRV TEBREF ln

1

22

2

3

Temperature-independent Current Generation

Constant-Gm Biasing

The resistor Rs is an off-chip resistor.

Continued

This opamp utilizes the constant-Gm biasing. Rb is an off-chip resistor.

13

152

2

13

10 ,1112

LWLW

KKR

LWCμ

Ib

oxn

D