Voltage and Current References Bandgap References

1

Voltage and Current References Bandgap 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.

2

Supply-Independent Biasing

In the design of current sources, we need to a golden current source, IREF. (Figure (a))

If instead of I^REF we use a resistor, the generated current sources will be supply- dependent. (Figure (b))

3

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

I^out=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 I^C is held constant.

Continued

Positive-TC Voltage

Continued

Bandgap References

2 1

2 1

C C

Y out

X out

Y

X

I I

R V V

R V V V

V   

 





Continued

 

3 2

2 3 2

1 2

1

2 3

2 2

1 1 1

R I V

I R V

V V

I I

I R R

V V

I R V

V

C BE C

BE BE

BE C

C

C BE

out

C BE

out

       

 

 















 

₂

3 2 2

2 3

2 _{C BE}

1

_{BE BE}

out

V V

R V R

I R R

V    

 



 









Continued

 

₂

3 2 2

3

2

1 ln

1

_{BE BE T BE}

out

V n V

R V R

R V

V R  



 



 





 



 



 



  T

n V q

k R

R T

V

_{out BE}



 

 



 



 



 



₂

3

2

ln

1 0

  K

n mV q

k R R

 

 

 



 1  ln 1 . 5

3 2

Continued

  mV V

K V mV

n R V

V

_out

1 R

_T

ln

_BE2

1 . 5 300 750 1 . 2

3

2

  

 

 



 



 



Assuming V^BE2=750mV, we have:

Compatibility with CMOS Technology

Modified Bandgap Circuit

Bandgap Circuit

The drawback of the above circuit is that V^DS1VDS2. Therefore I^D1 is not accurately identical to I^D2.

  n

R V V R

V

_{out EB}

1

_T

ln

1 2

2





 



 





Bandgap Circuit (V

^out=

2.5V)

The drawback of the above circuit is that V^DS1VDS2. Therefore I^D1 is not accurately identical to I^D2.

  n

R V V R

V

V

_{out EB EB}

2 1

_T

ln

1 2 4

3





 



 







PTAT Current Generation

The advantage of the above circuit is that V^DS1=V^DS2. Therefore I^D1 is accurately identical to I^D2.

 

1

ln R

n

I  V

^T

Employing PTAT to Generate Temperature-Independent Voltage

The advantage of the above circuit is that V^DS1=V^DS2. Therefore I^D1 is accurately identical to I^D2.

 

1 2

3

ln R

n R V

V

V

_out



_EB



^T

Generation of a PTAT Current Using a Simple Amplifier

Generation of a Temperature-Independent Voltage

Generation of a Temperature-Independent Voltage

  n

R V V R

R

V

_out

R

_{EB T}

ln

1 4 1

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.

 

_



 



 

 V N

R V R

R

V_REF R _{EB T} ln

1 2 2

2 3

Temperature-independent Current Generation

Constant-G

^m

Biasing

The resistor Rs is an off-chip resistor.

Continued

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

13 15 2

2

13

10 1 ,

1 1 2



 







 





 



 



 



 



 

L W L W K K

R L

C W μ I

ox b n D