lecture 9: folded-cascode amplifiers current mirror op ampsclass.ece.iastate.edu/ee435/lectures/ee...

1

EE 435

Lecture 9:

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)

Review from last lecture:

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

What circuit is this?

VSS

CL

M3

M1

IDB

VDD

VBB

VOUT

VIN

Cascode Amplifier

IB1

M1

M3

VOUT

VBB

VIN

VSS

CL

7

What circuit is this?

VSS

CL

M3

M1

IDB

VDD

VBB

VOUT

VIN

Cascode Amplifier• Cascode Amplifier– Often termed a “Folded Cascode Amplifier”

– Same small-signal performance as other

– VOUT swing VDSAt1-VDSAT2 could be small or negative

– But a biasing problem !!

IB1

M1

M3

VOUT

VBB

VIN

VSS

CL

M3

M1

VOUT

M3

M1

VOUT

VIN VIN

CLC

L

Small-signal circuit structures are identical !

8

Biased Folded Cascode Amplifier

IB1

M1

M3

VOUT

VBB

VIN

VSS

Folded Cascode Amplifier Biased Folded Cascode

Must have ID1= IB1-IB2 > 0

VDD

VIN

M1

M3VB1IB1

IB2

VOUT

ID1

VSS

9

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

10

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

11

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

12

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

13

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

14

Biased Folded-Cascode Amplifier

VDD

VIN

M1

M3 M

5

VB1 V

B3

Quarter Circuit

M2

M4

M6

VB4

VB2

VSS

Counterpart Circuit

15

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

16

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

17

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

18


(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

19

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

20


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

21


(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

22

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

23

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

24

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

25


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

26


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

27

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

28


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

29

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

30

End of Lecture 9

lecture 9: folded-cascode amplifiers current mirror op ampsclass.ece.iastate.edu/ee435/lectures/ee...

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