lecture 9: high-gain single-stage op ampsclass.ece.iastate.edu/ee435/lectures/ee 435 lect 9...

1

EE 435Lecture 9:

High-Gain Single-Stage Op Amps

-

Regulated Folded Cascode Op Amp -

Current-mirror op amps

-

OTA Applications

2

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

3

What circuit is this?

B1

1

3

OUT

BB

IN

SS

Folded Cascode Amplifier

VDD

VIN M1

M3VB1IB1

IB2 VOUT

VSS

Biased Folded Cascode

Review from Last Time

4

Folded Cascode Op Amp

Needs CMFB Circuit for VB4Either single-ended or differential outputsCan connect counterpart as current mirror to eliminate CMFBFolding caused modest deterioration of AV0 and GB energy efficiencyModest improvement in output swing


5

Folded Gain-boosted Telescopic Cascode Op Amp

Needs CMFB Circuit for VB4Either single-ended or differential outputsCan connect counterpart as current mirror to eliminate CMFBFolding caused modest deterioration in GB efficiency and gainModest improvement in output swing

L

m1

2CgGB =

( )m91

o9o7

m33

o3O5O1

m1

o

gAgg

gAggg

2g

A++

−≈

V IN

V IN

VDD

V SS

IT

VB4

M1 M2

M7

M9 M 10

M5 M6

M3 M4

V OUT

CL

VIN

VB2

VB3

V SS

IT2

M8

VB1

A1 A2

A3 A4


6

Operational Amplifier Structure Comparison

Reference Op Amp

Telescopic Cascode

Regulated Cascode

Folded Cascode

Folded Regulated Cascode

31

1

21

OO

mVO gg

gA+

=L

m

CgGB2

1=

Small Signal Parameter Domain

L

T

CISR

2=

L

T

CISR

2=

m5

o5o7

m3

o3o1

m1

o

ggg

ggg

2g

A+

=L

m

CgGB2

1=

3m9

o9o7

1m3

o3o1

m1

o

Aggg

Aggg

2g

A+

≈L

m

CgGB2

1=L

T

CISR

2=

( )m9

o9o7

m3

o3o5o1

m1

o

ggg

gggg2

g

A++

=L

m

CgGB2

1=L

T

CISR

2=

( )9m9

o9o7

3m3

o3o5o1

m1

o

Aggg

Agggg

2g

A++

=L

m

CgGB2

1=L

T

CISR

2=


7

Summary of Folded Amplifier Performance

•

+

Modest improvement in output signal swing (from 5 VDS SAT

to 4VDS SAT

)

•

- Deterioration in AV0 (maybe 30% or more)

•

- Deterioration in GB power efficiency (can be significant)

•

- Minor increase in circuit size


8

Folded Gain-boosted Cascode Amplifier

with ideal current source bias

modest improvement in output swing

L

m1

2CgGB =

( )m3

o3o1

m1o

gAgg

gA −≈

9

Folded Gain-boosted Cascode Amplifier

( )Ag

gggsC

gV

V

m3

o3o5o1L

m1

IN

OUT

++

−≈

modest improvement in output swingL

m1

CgGB =

( ) o3o5o1

m3m10 ggg

AggA+

−≈

10

Basic Amplifier Structure Comparisons

L

m1

CgGB =

o3

m3

O1

m1VO g

gggA =

Common Source

Cascode

Regulated CascodeFolded Cascode


O

mVO g

gA =L

m

CgGB =


Agg

ggA

o3

m3

O1

m1VO ≈

L

m1

CgGB =

( ) o3

m3

O5O1

m1VO g

ggg

gA+

=L

m1

CgGB =

( ) Agg

gggA

o3

m3

O5O1

m1VO +

=L

m1

CgGB =

11

Basic Amplifier Structure Comparisons

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛=

EB3EB131VO VV

1λλ4A

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛+

≈EB3EB1351

VO VVλλθλ4θA

⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛=

EB1LDD Vθ

CV2PGB

Common Source

Cascode

Regulated Cascode

Folded Cascode


⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠⎞

⎜⎝⎛=

EBVO V

1λ2A ⎟⎟

⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛=

EBLDD V1

CV2PGB

Practical Parameter Domain

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛=

EB1LDD V1

CV2PGB

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛≈

EB3EB131VO VV

Aλλ4A ( )

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛=

EB1LDD Vθ-1

CV2PGB

Θ=pct power in A

Θ=fraction of current of M5

that is in M1

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛+

≈EB3EB13512

2VO VVλλλθ

A4θA( )

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛=

EB1

12

LDD Vθ-1θ

CV2PGB

Θ1

=pct of total power in A

Θ2

=fraction of current of M5

that is in M1

12

Folded Gain-boosted Telescopic Cascode Op Amp

Needs CMFB Circuit for VB4Either single-ended or differential outputsCan connect counterpart as current mirror to eliminate CMFBFolding caused modest deterioration in GB efficiency and gainModest improvement in output swing

L

m1

2CgGB =

( )m91

o9o7

m33

o3O5O1

m1

o

gAgg

gAggg

2g

A++

−≈

V IN

V IN

VDD

V SS

IT

VB4

M1 M2

M7

M9 M 10

M5 M6

M3 M4

V OUT

CL

VIN

VB2

VB3

V SS

IT2

M8

VB1

A1 A2

A3 A4

13

Operational Amplifier Structure Comparison

Reference Op Amp

Telescopic Cascode

Regulated Cascode

Folded Cascode


31

1

21

OO

mVO gg

gA+

=L

m

CgGB2

1=


L

T

CISR

2=

L

T

CISR

2=

m5

o5o7

m3

o3o1

m1

o

ggg

ggg

2g

A+

=L

m

CgGB2

1=

3m9

o9o7

1m3

o3o1

m1

o

Aggg

Aggg

2g

A+

≈L

m

CgGB2

1=L

T

CISR

2=

( )m9

o9o7

m3

o3o5o1

m1

o

ggg

gggg2

g

A++

=L

m

CgGB2

1=L

T

CISR

2=

( )9m9

o9o7

3m3

o3o5o1

m1

o

Aggg

Agggg

2g

A++

=L

m

CgGB2

1=L

T

CISR

2=

14

Summary of Folded Amplifier Performance

•

+

Modest improvement in output signal swing (from 5 VDS SAT

to 4VDS SAT

)

•

- Deterioration in AV0 (maybe 30% or more)

•

- Deterioration in GB power efficiency (can be significant)

•

- Minor increase in circuit size

15

Other Methods of Gain Enhancement

OCCOQC

QCM

V ggg

A+

−=0

L

mQC

CgGB =

Two Strategies:

1.

Decrease denominator of AV0

2.

Increase numerator of AV0

Previous approaches focused on decreasing denominator

Consider now increasing numerator

Recall:

16

gmEQ

Gain Enhancement Strategy

Use this as a quarter circuit

use the quarter circuit itself to form the op amp

gm

is increased by the mirror gain !

Mgg m1MQC =

17

gmEQ

Gain Enhancement Strategy

18

Current Mirror Op Amps

OEQ

mEQV g

gA −=0

21m

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

CgGB =

Very Simple Structure!

Still need to generate the bias current IB

19

Current Mirror Op Amps

Need CMFB t0 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

20

Is this a real clever solution?

21

Basic Current Mirror Op Amp

86 OOOEQ ggg +=2

1mmEQ

gMg =

86

1

2OO

m

VO gg

gMA

+

•−=

L

m

CgMGB2

1=

CMFB not shown

L

T

CIMSR

2•

=

22

•

Current-Mirror Op Amp offers strategy forgm

enhancement•

Very Simple Structure

•

Has applications as an OTA•

But –

how good are the properties of the

CMOA? Is this a real clever solution?

23

Seminal Work on the OTA

N.E.C. PROCEEDINGSFrom:

24

Seminal Work on the OTA

1969

N.E.C. PROCEEDINGSDecember 1969

From:

25

( )12AB IIMII −=−

34

4OUTB

3A

IIIII

II

=

+=

=

Original OTA

Current Mirror

( )ABOUT IIMI −=

26

Original OTA( )12AB IIMII −=−

2B MII =1A MII =

( )ABOUT IIMI −=

27

Original OTA

( )ABOUT IIMI −=

2B MII =1A MII =

28

Original OTA

( )ABOUT IIMI −=

2B MII =1A MII =

3-mirror OTA

29

Current Mirror Op Amp W/O CMFB

1mmEQ Mgg =

Often termed an OTA

INmOUT VgI =Introduced by Wheatley and Whitlinger

in 1969

30

OTA Circuits

•

OTA often used open loop•

Excellent High Frequency Performance

•

Gain can be made programmable with dc current•

Large or very large adjustment ranges possible

⎩⎨⎧ •

=circuitsMOSforIK

circuitsBJTforIKg

ABC

ABC

m

2 to 3 decades of adjustment for MOS

5 to 6 decades of adjustment for BJT

31

OTA Applications

Voltage Controlled Amplifier

Note: Technically current-controlled, control variable not shown here and on following slides

INmOUT VRgV •=

gm

is controllable with IABC

32

OTA Applications

INmOUT VRgV •−=

Voltage Controlled Inverting Amplifier

33

OTA Applications

m

IN gR 1

=

Voltage Controlled Resistances

m

IN gR 1

−=

34

OTA Applications

in

m

mOUT V

ggV

2

1=

Voltage Controlled Resistorless

Amplifiers

in

m

mOUT V

ggV

2

1−=

Noninverting Voltage Controlled Amplifier Inverting Voltage Controlled Amplifier

Extremely large gain adjustment is possible

35

OTA Applications

inm

OUT VsCgV = in

mOUT V

sCgV −=

Voltage Controlled Integrators

Noninverting Voltage Controlled Integrator Inverting Voltage Controlled Integrator

36

Comparison with Op Amp Based Integrators

INOUT VsRC

V ⎥⎦⎤

⎢⎣⎡−=

1

INOUT VsRC

V ⎥⎦⎤

⎢⎣⎡=

1

OTA-based integrators require less components and significantly less for realizing the noninverting integration function !

37

Properties of OTA-Based Circuits•

Can realize arbitrarily complex functions

•

Circuits are often simpler than what can be obtained with Op Amp counterparts

•

Inherently offer excellent high frequency performance•

Can be controlled with a dc voltage or current

•

Often used open-loop rather than in a feedback configuration (circuit properties depend directly on gm

)•

Other high output impedance op amps can also serve as OTA

•

Linearity is limited•

Signal swing may be limited but can be good too

•

Circuit properties process and temperature dependent

38

•

Current-Mirror Op Amp offers strategy forgm

enhancement•

Very Simple Structure

•

Has applications as an OTA•

But –

how good are the properties of the

CMOA? Is this a real clever solution?

39

Current Mirror Op Amp W/O CMFB

1mmEQ Mgg =86 OOOEQ ggg +=

86

1

OO

mVO gg

gMA+•

−=

Can use higher output impedance current mirrors to decrease gOEQ L

T

CMISR =

40

SR of Current Mirror Op Amp

L

T

CMISR =T

L

MISR2C

=

41

Fully Differential Current Mirror Op Amp with Improved Slew Rate

Need CMFB circuit and requires modest circuit modification to provide CMFB insertion point

42



This circuit was published because of the claim for improved SR (Fig 6.15 MJ)

43

L

T

CMISR =



L

TAmpOpCM C

IMSR2•

=

Improved a factor of 2 !

but …

44

L

T

CMISR =

L

TAmpOpCM C

IMSR2•

=


Improved a factor of 2 !but …

( )M1IVP TDD AmpOp CM +=

( )M1IVP TDD 2+=

[ ]⎥⎦⎤

⎢⎣

⎡+⎟⎟

⎠

⎞⎜⎜⎝

⎛=

MM

CVPSR

LDD

AmpOpCM 12

⎥⎦⎤

⎢⎣⎡+⎟⎟

⎠

⎞⎜⎜⎝

⎛=

MM

CVPSR

LDD 21

SR actually about the same for “improved SR circuit”and basic OTA

45

Comparison of Current-Mirror Op Amps with Previous Structures

86

1

2OO

m

VO gg

gMA

+

•−=

64

46

LWLWM =

Does the simple mirror gainreally provide an “almost free”gain enhancement ?

46

Reference Op Amp

L

T

CISR

2=

31

1

2OOL

m

ggsC

g

)s(A++

=

31

1

21

OO

mVO gg

gA+

=

L

m

CgGB2

1=

⎟⎟⎠

⎞⎜⎜⎝

⎛⎥⎦

⎤⎢⎣

⎡+

=EB131

V0 V1

λλ1A

⎥⎦

⎤⎢⎣

⎡•⎟⎟⎠

⎞⎜⎜⎝

⎛=

EB1LDD V1

C2VPGB

LDDCVPSR

2=

Consider single-ended output performance :

47


86

1

2OO

m

VO gg

gMA

+

•−=

4

6

m

m

ggM =

Does the simple mirror gain really provide an “almost free”

gain enhancement ?

86

1

4

6

2OO

m

m

m

VO gg

ggg

A+

•−=

64

46

LWLWM =

Gain Enhancement Potential Less Apparent but stillImproved by gm6

/gm4

ratio

48


86

1

2OO

m

VO gg

gMA

+

•−=

( ) ( ) ( ) EB1MMEB1TM8M6EB1

T

QDM8M6EB

T

V V2λ1

λλV1

2IMλλV

M2I

IλλV

MI

A ≅+

=+

=+

⎟⎠⎞

⎜⎝⎛

=8681

0

22

21

Does the simple mirror gain really provide an “almost free”

gain enhancement ?

Consider how the gain appears in the practical parameter domain

This is exactly the same as was obtained for the simple differential amplifier!For a given VEB1

, there is NO gain enhancement !

49


LEB

T

L

m

L

mEQ

CVMI

C

gM

CgGB

1

1

22 ===

( )M1IVP TDD +=

How does the GB power efficiency compare with previous amplifiers ?

GB efficiency decreased for small M !!

⎥⎦⎤

⎢⎣⎡+⎟⎟

⎠

⎞⎜⎜⎝

⎛==

M1M

CV2VP

C2VMIGB

LDDEB1LEB1

T

GB for Telescopic Cascode and Ref Op Amp !

50


( )M1IVP TDD +=

⎥⎦⎤

⎢⎣⎡+

=M1

MC2V

PSRLDD

How does the SR compare with previous amplifiers ?

L

T

CIMSR

2•

=

L

TAmpOpfRe C

ISR2

=

SR Improved by factor of M !but …

LDD

OpAmpfRe CVPSR

2=

SR Really Less than for Ref Op Amp !!

51


How does the Current Mirror Op Amp really compare with previous amplifiers or with reference amplifier?

Perceived improvements may appear to be very significant

Actual performance is not as good inalmost every respect !

52

End of Lecture 9

lecture 9: high-gain single-stage op ampsclass.ece.iastate.edu/ee435/lectures/ee 435 lect 9...

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