Lecture 10: Folded-Cascode Amplifiers Current Mirror Op …class.ece.iastate.edu/ee435/lectures/EE...

1

EE 435

Lecture 10:

Folded-Cascode Amplifiers

Current Mirror Op Amps

2

Basic Op Amp Design

• Fundamental Amplifier Design Issues

• Single-Stage Low Gain Op Amps

• Single-Stage High Gain Op Amps

• Other Basic Gain Enhancement Approaches

• Two-Stage Op Amp

Where we are at:

3

M1

M3

-A

VIN

VOUT

High output impedance quarter-circuits

Regulated Cascode Amplifier

or “Gain Boosted Cascode”

Quarter Circuit

• A is usually a simple amplifier, often the reference op amp with + terminal

connected to the desired quiescent voltage

• Assume biased with a dc current source (not shown) at drain of M3

Review from last lecture:

4

Gain-Boosted Telescopic Cascode Op Amp

VDD

V OU

T

CL

VB2

VB3

V S

S

VB5 M 1

1

A1A2

A3A4

IT

VINVIN

M1M2

M3 M4

M5

M7

M6

M8

Advantages:

Significant increase in dc gain

Limitations:

:• Signal swing (4VDSAT+VT between VDD and VSS)

• Reduction in GB power efficiency

- some current required to bias “A” amplifiers

• -additional pole in “A” amplifier -may add requirements for some compensation

• Area Overhead for 4 transistors and 4 amplifiers-actually minor concern since performance will usually

justify these resources

(with or w/o current mirror counterpart circuits)


5

Are there other useful high output impedance

circuits that can be used for the quarter circuit?

V1G M1V1 G 1

V1G M1 G 1

G2

CL

2

Vd

Vo++

1

0

1 2

1 2

1

2

2

MV

L

M

L

GA

G G

G GBW

C

GGB

C


6

Implementation of Biased Folded

Cascode Amplifier?

VDD

VIN

M1

M3 M

5

VB1 V

B3

Biased Folded CascodeImplementation of Biased

Folded Cascode

VDD

VIN

M1

M3V

B1IB1

IB2 V

OUT

VSS


7

VDD

VIN

M1

M3M5

VB1 VB3

Analysis of Biased Folded Cascode

go1gM1V1V1

go3gM3V3V3go5 gM5V5 V5

CL

VOUT

VIN

VX

0

OUT m

OUT o3 L m3 3 X o3

X o1 o3 o5 m1 1 m3 3 OUT o3

3 X

1 IN

OUT o3 L m3 o3 3

m1 IN 3 m3 o1 o3 o5 OUT o

1

o3INL o1 o

m3

3

5

V g sC g V V g

V g g g g V g V V g

V V

V V

V g sC g g V

g V V g +g g g +V

V g

gVsC g g

g

g

GM

GO

How can this be seen by inspection?

• First observe if all go’s are 0, GM=gm1

• Then observe M3 “cascodes” the impedance g01+go5

8

Biased Folded Cascode Quarter Circuit

VDD

VIN

M1

M3 M

5

VB1 V

B2

CL

VOUT

OUT m1

o3INL o1 o5

m3

V g

gVsC g g

g

m3m1

V0

o1 o5 o3

ggA

g g g

m1

L

gGB

C

9

Basic Amplifier Structure Comparisons(ideal current source biasing)

Common Source

Cascode

Regulated

Cascode

Folded Cascode

Small Signal Parameter Domain

m3m1

V0

o1 o5 o3

ggA

g g g

mV0

o

gA

g

m3m1V0

o1 o3

ggA

g g

m3m1V0

o1 o3

ggA A

g g

m

L

gGB

C

m

L

gGB

C

m

L

gGB

C

m

L

gGB

C

10

Basic Amplifier Structure Comparisons

EB3EB131

VOVV

1

λλ

4A

EB3EB1351

VOVVλλθλ

4θA

Common Source

Cascode

Regulated

Cascode

Folded Cascode

EB

VOV

1

λ

2A

EBLDD V

1

CV

2PGB

Practical Parameter Domain

EB1LDD V

1

CV

2PGB

EB3EB131

VOVV

A

λλ

4A

EB1LDD V

θ-1

CV

2PGB

Θ=pct power in A

EB1LDD V

θ

CV

2PGB

Θ=fraction of current

of M5 that is in M1

11

Biased Folded-Cascode Amplifier

VDD

VIN

M1

M3 M

5

VB1 V

B3

Quarter Circuit

M2

M4

M6

VB4

VB2

VSS

Counterpart Circuit

12

QUARTER CIRCUIT

Folded-Cascode Operational Amplifier

Op Amp

VDD

VDD

VB1

VB2

VB5

VSS

VSSVSS

VDD

VOUTVOUT

VB1

VINVIN

VB2

VB5

IT

VB3

VB4

VDD

VIN

VB1 VB3

13

Folded-Cascode Operational Amplifier (redrawn)

These transistors pair-wise form a current source

and one in each pair can be removed

VB5

VIN VIN

VOUT VOUT

VDD

VSSVSS

IT

VB4

VB3

VB2

VB1

M1 M2

M7

M9

M8

M10

M5 M6

M3 M4

M11M12

VDD

VB5

IT2

14

Folded Cascode Op Amp

Needs CMFB Circuit for VB4

Either single-ended or differential outputs

Can connect counterpart as current mirror to eliminate CMFB

Folding caused modest deterioration of AV0 and GB energy efficiency

Modest improvement in output swing

VIN

VIN

VOUT V

OUT

VDD

VSS

VSS

IT

VB4

VB3

VB2

VB1

M1

M2

M7

M9

M8

M10

M5 M

6

M3 M

4

IT2

15


(Single-ended Output)

VIN

VIN

VDD

VSS

VSS

IT

VB3

VB2

M1

M2

M7

M9

M8

M10

M5 M

6

M3 M

4

VOUT

CL

IT2

VB1

1mmEQgg

OEQL

mEQ

V

gsC

gsA

m9

O9

O7

m3

O3

O5O1OEQ

g

gg

g

gggg

L

m

C

gGB 1

L

mEQ

C

gGB

OEQ

mEQ

V

g

gA

0

m9

O9

O7

m3

O3

O5O1

m

V0

g

gg

g

ggg

gA

1

16

Operational Amplifier Structure Comparison

Reference

Op Amp

Telescopic

Cascode

Regulated

Cascode

Folded

Cascode

31

1

2

1

OO

m

VO

gg

gA

L

m

C

gGB

2

1


L

T

C

ISR

2

L

T

C

ISR

2

m5

o5o7

m3

o3o1

m1

o

g

gg

g

gg

2

g

A

L

m

C

gGB

2

1

3m9

o9o7

1m3

o3o1

m1

o

Ag

gg

Ag

gg

2

g

A

L

m

C

gGB

2

1L

T

C

ISR

2

m9

o9o7

m3

o3o5o1

m1

o

g

gg

g

ggg

2

g

A

L

m

C

gGB

2

1L

T

C

ISR

2

17


3

EB775

1

EB331EB1

V0

A

Vλλ

A

VλλV

2A

EB1LDD V

1

C2V

θ-1PGB

LDDC2V

θ-1PSR

Reference

Op Amp

Telescopic

Cascode

Regulated

Cascode

Folded

Cascode


EB131

V0

V

1

λλ

1A

EB1LDDV

1

C2V

PGB

LDDC2V

PSR

LDDC2V

PSR

EB575EB331EB1

V0VλλVλλV

2A

EB1LDDV

1

C2V

PGB

Θ=pct power in A

EB979EB3351EB1

V0Vλλθ1VλλθλV

2θA

EB1LDD V

θ

C2V

PGB

LDDC2V

PθSR

Θ=fraction of

current of M5

that is in M1

18


(Single-ended Output)

L

m

C

gGB 1

m9

O9

O7

m3

O3

O5O1

m

V0

g

gg

g

ggg

gA

1

What is a practical design parameter set?

How many degrees of freedom are there?

DOF ?

{IT,W1/L1,W5/L5,W3/L3,W9/L9,W7/L7,VB1,VB2,VB3}

9 DOF

Practical Design Parameters

{P,θ,VEB1,VEB3,VEB5,VEB7,VEB9,VB2,VB3}

where θ=IT/(IT+IT2)

VIN

VIN

VDD

VSS

VSS

IT

VB3

VB2

M1

M2

M7

M9

M8

M10

M5 M

6

M3 M

4

VOUT

CL

IT2

VB1

19

Textbook reference:

Some of the material we have been discussing appears in

Chapter 3, some in Chapter 5, and some in Chapter 6 of the

Martin and Johns text

In particular, the telescopic and folded cascode structures are

referred to as advanced op amps and appear in later chapters of

the text

20

Folded Gain-boosted

Cascode Amplifier

with ideal current source bias

modest improvement in output swing

L

m1

2C

gGB

m3

o3o1

m1o

gA

gg

gA

IB1

M1

M3

VOUT

IB2

VIN

-A

21

Folded Gain-boosted

Cascode Amplifier

-A

VDD

VIN

M1

M3 M

5

VB2

CL

VOUT

IBIAS

Ag

gggsC

g

V

V

m3

o3o5o1L

m1

IN

OUT

modest improvement in output swingL

m1

C

gGB

o3o5o1

m3m10

ggg

AggA

22


L

m1

C

gGB

o3

m3

O1

m1VO

g

g

g

gA

Common Source

Cascode

Regulated

Cascode

Folded Cascode

Folded Regulated

Cascode

O

mVO

g

gA

L

m

C

gGB


Ag

g

g

gA

o3

m3

O1

m1VO

L

m1

C

gGB

o3

m3

O5O1

m1VO

g

g

gg

gA

L

m1

C

gGB

A

g

g

gg

gA

o3

m3

O5O1

m1VO

L

m1

C

gGB

23


EB3EB131

VOVV

1

λλ

4A

EB3EB1351

VOVVλλθλ

4θA

EB1LDD V

θ

CV

2PGB

Common Source

Cascode

Regulated

Cascode

Folded Cascode

Folded Regulated

Cascode

EB

VOV

1

λ

2A

EBLDD V

1

CV

2PGB


EB1LDD V

1

CV

2PGB

EB3EB131

VOVV

A

λλ

4A

EB1LDD V

θ-1

CV

2PGB

Θ=pct power in A

Θ=fraction of current

of M5 that is in M1

EB3EB13512

2VO

VVλλλθ

A4θA

EB1

12

LDD V

θ-1θ

CV

2PGB

Θ1=pct of total power in A

Θ2=fraction of current of

M5 that is in M1

24

Folded Gain-boosted Telescopic

Cascode Op Amp

Needs CMFB Circuit for VB4

Either single-ended or differential outputs

Can connect counterpart as current mirror to eliminate CMFB

Folding caused modest deterioration in GB efficiency and gain

Modest improvement in output swing

L

m1

2C

gGB

m91

o9o7

m33

o3O5O1

m1

o

gA

gg

gA

ggg

2

g

A

V I

NV I

N

VDD

V S

S

IT

VB4

M1

M2

M7

M9 M 1

0

M5 M

6

M3 M

4

V OU

T

CL

VIN

VB2

VB3

V S

S

IT2

M8

VB1

A1A2

A3A4

25


Reference

Op Amp

Telescopic

Cascode

Regulated

Cascode

Folded

Cascode

Folded

Regulated

Cascode

31

1

2

1

OO

m

VO

gg

gA

L

m

C

gGB

2

1


L

T

C

ISR

2

L

T

C

ISR

2

m5

o5o7

m3

o3o1

m1

o

g

gg

g

gg

2

g

A

L

m

C

gGB

2

1

3m9

o9o7

1m3

o3o1

m1

o

Ag

gg

Ag

gg

2

g

A

L

m

C

gGB

2

1L

T

C

ISR

2

m9

o9o7

m3

o3o5o1

m1

o

g

gg

g

ggg

2

g

A

L

m

C

gGB

2

1L

T

C

ISR

2

9m9

o9o7

3m3

o3o5o1

m1

o

Ag

gg

Ag

ggg

2

g

A

L

m

C

gGB

2

1L

T

C

ISR

2

26

Summary of Folded Amplifier Performance

• + Modest improvement in output signal

swing (from 5 VDS SAT to 4VDS SAT)

• + Can directly feed output back to input

to create buffer

• - Deterioration in AV0 (maybe 30% or

more)

• - Deterioration in GB power efficiency

(can be significant)

• - Minor increase in circuit size

27

Other Methods of Gain Enhancement

OCCOQC

QCM

V

gg

gA

0

L

mQC

C

gGB

Two Strategies:

1. Decrease denominator of AV0

2. Increase numerator of AV0

Previous approaches focused on decreasing denominator

Consider now increasing numerator

Recall:

VIN

VDD

VSS

VOUT

Quarter

Circuit

Counterpart

Circuit

CL

VBB

28

Determination of op amp characteristics

from quarter circuit characteristics

M1

OV

d L 1 2

GV 2AV sC G G

FVIN

IXX

VOUT

VSS

CL

Small signal differential amplifier

GsC

GsA

L

MVQC

)(

Small signal Quarter Circuit

F

P

VDD

VBB

OUTV

OUTV

d

2

V d

2

V

IBIAS

CLCL

or VSS

• Note that the counterpart circuit is simply serving as the biasing current source

• Could use counterpart circuits (or other circuits) from other quarter circuits for “P”

• Counterpart circuits connected as one-port

• Can think of making differential op amp directly from quarter circuit

29

Differential input op amp directly from

quarter circuit

M1

OV

d L 1 2

GV 2AV sC G G

F

VIN

IXX

VOUT

VSS

CL GsC

GsA

L

MVQC

)(

F

P

VDD

VBB

OUTV

OUTV

d

2

V d

2

V

IBIAS

CLCL

or VSS

BB

M1

OV

d L 1 I

GV 2AV sC G G

F

VDD

IBB

OUTV

OUTV

d

2

V d

2

V CLCL

VSS

IBB

F

VDD

IBB

OUTV

OUTV

d

2

V d

2

V CLCL

IBB

IBIAS

30

gmEQ Gain Enhancement Strategy

Consider using the quarter circuit itself to

form the op amp

gm is increased by the mirror gain !

Mgg m1MQC

M1

1 : M

IB

VIN

VOUT

CL

Consider this quarter circuit

Folding is required to establish the correct

bias current direction

Could have done this for other quarter

circuits as well but there is a particularly

important reason we are following this

approach with this quarter circuit – What

is it ?

OQCgOutput conductance of QC:

31


1 : M

VIN

IB

VOUT

M1

Redraw to absorb IB in the quarter circuit

BBOEQ OQC OIg g g

MQC M1g = g M

32


M : 1

IB

M1

1 : M

IB

M2V

IN

VOUT

VIN

VOUT

IT

1 : M

VIN

IB

VOUT

M1

33


OEQ

mEQV0

g

gA

2

1m

mEQ

gMg Premise: Transconductance gain increased by mirror gain M

Premise: If output conductance is small, gain can be very high

Premise: GB very good as well

L

mEQ

C

gGB

Very Simple Structure!

M : 1

IB

M1

1 : M

IB

M2VIN

VOUT

VIN

VOUT

IT

CLCL

Still need to generate the bias current IB

BBOEQ OQC OIg g g

-

OUTV0 + +

IN IN

VA =

V - V

(for VIN+=Vd/2)

34


VDD

VSS

VINVIN

VOUT VOUT

M : 1 1 : M

IT

VSS VSS

M1M2

VDD

VSS

VINVIN

VOUT VOUT

IT

VB1

VSSVSS

VB2

M1 M2

M3 M4M5 M6

M7M8M9

Need CMFB to establish VB2

Can use higher output impedance current mirrors

Can use current mirror bias to eliminate CMFB but loose one output

Basic Current Mirror Op Amp

35

Is this a real clever solution?

36

Basic Current Mirror Op Amp

86 OOOEQggg

VDD

VSS

VINVIN

VOUT VOUT

IT

VB1

VSSVSS

VB2

M1 M2

M3 M4M5 M6

M7M8M9

CL CL

2

1m

mEQ

gMg

O8O6

m1

VOgg2

gM

A

L

m

C

gMGB

2

1

CMFB not shown

L

T

C

IMSR

2

37

• Current-Mirror Op Amp offers strategy for

gm enhancement

• Very Simple Structure

• Has applications as an OTA

• Based upon small signal analysis,

performance appears to be very good !

• But – how good are the properties of the

CMOA?Is this a real clever solution?

38

Seminal Work on the OTA

N.E.C. PROCEEDINGS

From:

39

Seminal Work on the OTA

1969 N.E.C. PROCEEDINGS

December 1969

From:

40

VIN V

INQ1

Q2

I1

I2

IABC

Q3 Q

4

I3

I4

IOUT

12AB IIMII

IA I

B

34

4OUTB

3A

II

III

II

Original OTA

Current

Mirror

ABOUT IIMI

41

Original OTA

VIN V

INQ1

Q2

I1

I2

IABC

Q3 Q

4

I3

I4

IOUT

12AB IIMII

IA I

B

2B MII 1A MII

ABOUT IIMI

42

Original OTA

ABOUT IIMI

VIN V

INQ1

Q2

I1

I2

IABC

Q3 Q

4

I3

I4

IOUT

IA I

B

2B MII 1A MII

43

Original OTA

ABOUT IIMI

VIN V

INQ1

Q2

I1

I2

IABC

Q3 Q

4

I3

I4

IOUT

IA I

B

2B MII 1A MII

3-mirror OTA

44

Current Mirror Op Amp W/O CMFBV

DD

VSS

VIN

VIN

M : 1 1 : M

IT

VSS

1 : 1

VOUT

IOUT

VIN

gm

1mmEQMgg

Often termed an OTA

INmOUTVgI

Introduced by Wheatley and Whitlinger in 1969

45

End of Lecture 10

Lecture 10: Folded-Cascode Amplifiers Current Mirror Op …class.ece.iastate.edu/ee435/lectures/EE...

Documents

Transcript of Lecture 10: Folded-Cascode Amplifiers Current Mirror Op …class.ece.iastate.edu/ee435/lectures/EE...