EE 435 Lecture 11class.ece.iastate.edu/ee435/lectures/EE 435 Lect 11...EE 435 Lecture 11 Current Mirror Op Amps--Alternative perspective-- Loop phase-shift concerns OTA circuits 2

EE 435

Lecture 11

Current Mirror Op Amps

-- Alternative perspective

-- Loop phase-shift concerns

OTA circuits

2

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

Review from last lecture:

3

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

gm

IOUT

IABC

circuitsMOSforIK

circuitsBJTforIKg

ABC

ABC

m

2 to 3 decades of adjustment for MOS

5 to 6 decades of adjustment for BJT

4

OTA Applications

gm

VIN

VOUT

R

Voltage Controlled Amplifier

Note: Technically current-controlled, control variable

not shown here and on following slides

INmOUTVRgV

gm is controllable with IABC

5

OTA Applications

INmOUTVRgV

Voltage Controlled Inverting Amplifier

gm

VIN

VOUT

RL

6

OTA Applications

RIN

gm

RIN

gm

m

IN

gR

1

Voltage Controlled Resistances

m

IN

gR

1

7

OTA Applications

gm1

VIN

VOUT

gm2

gm1

VIN

VOUT

gm2

in

m

m

OUTV

g

gV

2

1

Voltage Controlled Resistorless Amplifiers

in

m

m

OUTV

g

gV

2

1

Noninverting Voltage Controlled Amplifier Inverting Voltage Controlled Amplifier

Extremely large gain adjustment is possible

8

OTA Applications

gm

VIN

VOUT

C

in

m

OUTV

sC

gV

in

m

OUTV

sC

gV

Voltage Controlled Integrators

Noninverting Voltage Controlled Integrator Inverting Voltage Controlled Integrator

gm

VIN

VOUT

C

9

Comparison with Op Amp Based Integrators

VIN

VOUT

CR

INOUTV

sRCV

1

VIN

VOUT

CR

R1

R1

INOUTV

sRCV

1

OTA-based integrators require less components and significantly

less for realizing the noninverting integration function !

10

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

11

• Current-Mirror Op Amp offers strategy for

gm enhancement

• Very Simple Structure

• Has applications as an OTA

• But – how good are the properties of the

CMOA?Is this a real clever solution?

12

Current Mirror Op Amp W/O CMFB

VDD

VSS

VIN

VIN

M : 1 1 : M

IT

VSS

1 : 1

VOUT

1mmEQMgg

86 OOOEQggg

86

1

OO

m

VO

gg

gMA

And can use higher output impedance

current mirrors to decrease gOEQ

VDD

VSS

VINVIN

IT

VB1

VSSVSS

M1 M2

M3 M4M5 M6

M7M8M9

VOUT

CL

L

T

C

MISR

13

SR of Current Mirror Op Amp VDD

VSS

VINVIN

IT

VB1

VSSVSS

M1 M2

M3 M4M5 M6

M7M8M9

VOUT

CL

L

T

C

MISR

VDD

VSS

VINVIN

VOUT VOUT

IT

VB1

VSSVSS

VB2

M1 M2

M3 M4M5 M6

M7M8M9

CLCL

T

L

MISR

2C

14

Fully Differential Current Mirror Op

Amp with Improved Slew Rate

1 : M MM M : 1

VSS

VIN VIN

IT

1 : 1 1 : 1

VOUT VOUT

VDD

VSS

CLCL

Need CMFB circuit and requires modest circuit

modification to provide CMFB insertion point

15

VDD

VSS

VINVIN

IT

VB1

VSSVSS

M1 M2

M3 M4

M5AM6A

M7M8AM9A

M6B

VOUT

M5B

M9B

M8B

VSSVSS

VOUT

Fully Differential Current Mirror Op Amp with

Improved Slew Rate



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

16

VDD

VSS

VINVIN

IT

VB1

VSSVSS

M1 M2

M3 M4

M5AM6A

M7M8AM9A

M6B

VOUT

M5B

M9B

M8B

VSSVSS

VOUT L

T

C

MISR


Improved Slew Rate



L

T

AmpOpCM

C

IMSR

2

Improved a factor of 2 !

but …

17

VDD

VSS

VINVIN

IT

VB1

VSSVSS

M1 M2

M3 M4

M5AM6A

M7M8AM9A

M6B

VOUT

M5B

M9B

M8B

VSSVSS

VOUT

L

T

C

MISR

L

T

AmpOpCM

C

IMSR

2


Improved Slew Rate

Improved a factor of 2 !

but …

M1IVPTDD AmpOp CM

M1IVPTDD

2

M

M

CV

PSR

LDD

AmpOpCM

12

M

M

CV

PSR

LDD21

SR actually about the same for “improved SR circuit”

and basic OTA

18

Comparison of Current-Mirror Op Amps

with Previous Structures

86

1

2

OO

m

VO

gg

gM

A

64

46

LW

LWM

Does the simple mirror gain

really provide an “almost free”

gain enhancement ?

VDD

VSS

VINVIN

VOUTVOUT

IT

VB1

VSSVSS

VB2

M1 M2

M3 M4M5 M6

M7M8M9

Ask the apple comparison question !

19



86

1

2

OO

m

VO

gg

gM

A

64

46

LW

LWM

Does the simple mirror gain really provide an “almost

free” really large gain enhancement ?VDD

VSS

VINVIN

VOUTVOUT

IT

VB1

VSSVSS

VB2

M1 M2

M3 M4M5 M6

M7M8M9

Are we comparing Apples with Apples?• In the small-signal parameter domain?

• In the practical parameter domain?

• Does it matter if we are making a comparison?

20

Reference Op Amp

VDD

VB1

M1M2

VB2

M3 M4

VINVIN

CL

M9

CL

VOUTVOUT

IT

L

T

C

ISR

2

31

1

2

OOL

m

ggsC

g

)s(A

31

1

2

1

OO

m

VO

gg

gA

L

m

C

gGB

2

1

EB131

V0

V

1

λλ

1A

EB1LDDV

1

C2V

PGB

LDDCV

PSR

2

Consider single-ended output performance :

21



86

1

2

OO

m

VO

gg

gM

A

4

6

m

m

g

gM

Does the simple mirror gain really provide an

“almost free” gain enhancement ?

86

1

4

6

2

OO

m

m

m

VO

gg

g

g

g

A

64

46

LW

LWMI

OUT

IIN

M6

M4

Gain Enhancement Potential Less Apparent but still

Improved by gm6/gm4 ratio

22



86

1

2

OO

m

VO

gg

gM

A

EB1MMEB1T

M8M6EB1

T

QDM8M6EB

T

V

V2λ

1

λλV

1

2

IMλλV

M2

I

IλλV

MI

A

8681

0

22

2

1

Does the simple mirror gain really provide an

“almost free” gain enhancement ?

VDD

VSS

VINVIN

VOUTVOUT

IT

VB1

VSSVSS

VB2

M1 M2

M3 M4M5 M6

M7M8M9

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 !

23



LEB

T

L

m

L

mEQ

CV

MI

C

gM

C

gGB

1

1

22

M1IVPTDD

VDD

VSS

VINVIN

VOUT VOUT

M : 1 1 : M

IT

VSS VSS

M1M2

How does the GB power efficiency compare with

previous amplifiers ?

GB efficiency decreased for small M !!

M1

M

CV2V

P

C2V

MIGB

LDDEB1LEB1

T

GB for Telescopic Cascode and Ref Op Amp !

24



M1IVPTDD

VDD

VSS

VINVIN

VOUTVOUT

M : 1 1 : M

IT

VSS VSS

M1M2

M1

M

C2V

PSR

LDD

How does the SR compare with previous

amplifiers ?

L

T

C

IMSR

2

L

T

AmpOpfRe

C

ISR

2

SR Improved by factor of M !but …

LDD

OpAmpfRe

CV

PSR

2

SR Really Less than for Ref Op Amp !!

25



VDD

VSS

VINVIN

VOUT VOUT

M : 1 1 : M

IT

VSS VSS

M1M2

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 in

almost every respect !

But performance is comparable to

other circuits and the circuit structure

is really simple

Widely used architecture as well but

maybe more for OTA applications

26

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:

27

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:

28

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

So what happened with the Current Mirror

approach to increasing the numerator?

Current-Mirror Op Amps – Another

Perspective !

VDD

VSS

VINVIN

VOUT VOUT

IT

VB1

VSSVSS

VB2

M1 M2

M3 M4M5 M6

M7M8M9

Differential Half-Circuit


Perspective !


M2

M4

M6

M8

VOUT

VIN

Cascade of n-channel common source amplifier

with p-channel common-source amplifier !


Perspective !

M2

M4

M6

M8

VOUT

VIN

86

6

4

2

2

1

OO

m

m

m

V

gg

g

g

gA

86

1

4

6

86

1

22

OO

m

m

m

OO

m

VO

gg

g

g

g

gg

gM

A




From Current Mirror Analysis :

32

Comparison of Different Circuit Designs• An objective comparison of different design approaches is often a

critical part of the design process

• Different objective functions or different comparison approaches often

lead to different conclusions

• Textbooks and the technical literature do not always identify the most

appropriate objective functions

• Critical to identify metrics that capture the important characteristics of

a design when making comparisons but this is often a challenging task

?


Perspective !

VDD

VSS

VINVIN

VOUT VOUT

IT

VB1

VSSVSS

VB2

M1 M2

M3 M4M5 M6

M7M8M9



Perspective !


M2

M4

M6

M8

VOUT

VIN




Perspective !

M2

M4

M6

M8

VOUT

VIN

86

6

4

2

2

1

OO

m

m

m

V

gg

g

g

gA

86

1

4

6

86

1

22

OO

m

m

m

OO

m

VO

gg

g

g

g

gg

gM

A


Cascade of n-channel common source amplifier with p-channel common-source amplifier !

From Current Mirror Analysis :

• Current mirror op amp often used open-loop as an OTA

• If feedback is applied, there may be concerns about becoming

underdamped or oscillatory

• Current-mirror op amp offers no improvement in performance over

the reference op amp

• Current-mirror op amp can be viewed as a cascade of two common-

source amplifiers, one with a low gain and the other with a larger

gain

• Current-mirror op amp is useful as an open-loop programmable

transconductance amplifier (OTA)

• Current-mirror op amp will work in feedback applications as well but

performance would often be better with alternative Op Amp

architectures

• If used in feedback applications, excessive phase shift may cause

feedback circuit to become under-damped or oscillatory

Current Mirror Op Amp Summary

End of Lecture 11

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EE 435 Lecture 11class.ece.iastate.edu/ee435/lectures/EE 435 Lect 11...EE 435 Lecture 11 Current Mirror Op Amps--Alternative perspective-- Loop phase-shift concerns OTA circuits 2