PDF Advanced Current Mirrors and Opamps - KNTU

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Advanced Current Mirrors and Opamps

Hossein Shamsi

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Wide-Swing Current Mirrors

It is proven that if both of the following conditions are satisfied, then all transistors will be biased in the saturation region.

 



 









eff tn

eff out

nV V

V n

V 1

Typically n is chosen identical to 1. So the output swing will be:

eff

out

V

V  2

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Enhanced Output-Impedance Current Mirrors

 A 

r r

g

R

_out



_m₁ _ds₁ _ds₂

1 

(4)

Implementation of Enhanced Output-Impedance Current Mirror

1

3 eff

eff tn

out

V V V

V   

2

3 3

2 1

1

ds m

ds ds

m out

g r r

r g

R 

Advantage:

•High Output-Impedance Disadvantages:

•Low Output-Swing

•Imprecise Current Mirror

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Implementation of Enhanced Output-Impedance Current Mirror

1

3 eff

eff tn

out

V V V

V   

2

3 3

2 1 1

ds m

ds ds m out

g r r

r g R 

Advantages:

•High Output-Impedance

•Precise Current Mirror Disadvantage:

•Low Output-Swing

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Implementation of Enhanced Output-Impedance Current Mirror

2

_eff1

out

V

V  2

3 3

2 1 1

ds m

ds ds m out

g r r

r g

R 

Advantages:

•High Output-Swing Disadvantage:

•Imprecise Current Mirror

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Wide-Swing Current Mirror with Enhanced Output-Impedance

2

_eff1

out

V

V  2

3 3

2 1 1

ds m

ds ds m out

g r r

r g R 

Advantages:

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Modified Wide-Swing Current Mirror with Enhanced Output-Impedance

2

_eff1

out

V

V  2

3 3

2 1 1

ds m

ds ds m out

g r r

r g R 

Advantages:

•Precise Current Mirror

•High Output-Swing

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Current Mirror Symbol

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Folded-Cascode Opamp

•This opamp is useful when we want to drive capacitive loads.

•One of the most important parameters of this modern opamp is its transconductance value.

•Therefore, some designers refer to this modern opamp as Operational Transconductance Amplifier (OTA).

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Remember the wide-swing current mirror

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Opamp Gain

i m sc

sc

i

m

i g v

i i

g v i

2 2

2

2  



 







₃ ₁



6 6

10 8

8 ds ds m ds ds ds

m

out

g r r g r r r

r 

out m

V sc

out

o

r i A g r

v   

₁

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Competition between Two Current Sources

If Ibias1>Ibias2  Q1:Triode Q3:Active I=Ibias2 If Ibias2>Ibias1  Q1:Active Q3:Triode I=Ibias1 Assume that:

4 3

2 1

 

 



 

 

 





 

 



 

 

 





L W L

W

L W L

W

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Slew Rate

Assuming I_bias2>I_D4 and V_in+>>V_in-

L D

C SR^  I ⁴

Assuming I_bias2>I_D4 and V_in->>V_in-

L D

C SR^  I ⁴

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Lemma

9

1 g

m

R 

Wide-swing current mirror

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Frequency Response

3 8

2 9

1 5

2

1

1 0

1

1 1

1

1 1

p m

p

L m ta

out m

L out p

g C g C

g C

C g r

g A

C r







 



 









 

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Another Schematic for Folded-Cascode Opamp







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Folded-Cascode Opamp without wide-swing current source

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(pmos-input)

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(pmos-input)

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What is the DC voltage of the output node?

In a single-ended opamp, the DC voltage of the output node is determined by the feedback circuit around the opamp.

 dc  V_out

 

R

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Fully Differential Folded-Cascode Opamp

The bias voltages Vb1, Vb4, and Vb5 must be chosen so that the following relation is satisfied!!!

I_D9=0.5I_D12+I_D3

The opamp is completely symmetric. So we can use the half-circuit of the opamp for AC analysis.

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Fully Differential Folded-Cascode Opamp (half-circuit)

p m

p

L mi ta

i out m

L out p

g C

C g r

g A

C r

7 2

0 1

1 1 1





 



 









 

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What is the DC voltage of the output nodes?

In a single-ended opamp, the DC voltage of the output node is determined by the feedback circuit around the opamp.

 dc  V_out



_CM _x



_o _o _o _x _CM

o CM

x CM

V V

V RI

V V

R V I V













 

 







 





2

2 2

The DC voltage of the output nodes can not be determined!!!

CM i

i

V V

V

_



_



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Common-Mode Feedback Circuit (CMFB Circuit)

In a fully differential opamp, the CMFB circuit is employed inside the opamp to adjust the common-mode voltage of the output nodes identical to a

predetermined voltage, Vref.

CM x

o

V V

V KCL

and

KVL   2 

ref o

o

V V

Circuit V

CMFB  



^ ^

2

CM ref

x

ref o

o

V V V

V V

V

 



_



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Ideal CMFB Circuit (1)

Vref denotes the desired output common-mode voltage.

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Ideal CMFB Circuit (2)

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Single-ended Telescopic Opamp

i m

sc

g v

i 

₁

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Slew Rate

L SS

C SR I

SR^  ^ 

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Frequency Response

3 6

2 7

1 3

2 0

1

1 1

1

1 1

p m

p

L m ta

out m

L out p

g C g C

g C

C g r

g A

C

r

ⁱ

i







 



 









 

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Input common-mode range and output swing

tp B

out tn

B V V V V

V ₁    ₂ 

3 1

9

1 eff cmi B eff

eff

tn

V V V V V

V     

Output swing:

Input common-mode range:

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Telescopic Opamp without wide-swing current source

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Fully Differential Telescopic Opamp

The bias voltages Vb3 and Vb4 must be chosen so that the following relation is satisfied!!!

I_D7=0.5I_D9

The opamp is completely symmetric. So we can use the half-circuit of the opamp for AC analysis.

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Fully Differential Telescopic Opamp (half-circuit)

p m p

L m ta

out m

L out p

g C

C g r

g A

C r

3 2

1

1 0

1

1 1 1



















 

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Ideal CMFB Circuit (2)

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Fully Differential Two-Stage Opamp (nmos input)

L m p

M m ta

m m

C C

g C

g

R g R g A

 



1 5 2

1

2 5 1 1 0



C1 denotes the parasitic capacitance.

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Fully Differential Two-Stage Opamp

(pmos input)

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Fully Differential Two-Stage Opamp (First Stage: Telescopic Cascode)

C1 denotes the parasitic capacitance at node X.

Miller compensation method is utilized.

L m p

M m ta

m m

C C

g C

g

R g R g A

 



1 9 2

1

2 9 1 1 0



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Gain-Boosting Opamp

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Gain-Boosting Opamp

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Differential Pair as an Auxiliary Opamp

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Folded-Cascode Circuit as an Auxiliary Opamp

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Comparison of Performance of Various Opamp Topologies