L19: Current Mirror (21.10.2013) - IITKhome.iitk.ac.in/~baquer/L19_current_mirror.pdf · Simple...

CMOS Analog Circuits

L19: Current Mirror (21.10.2013)

B Mazhari

G-NumberB. Mazhari, IITK1

B. MazhariDept. of EE, IIT Kanpur

Current Source to Current Sink

VDD Suppose we have a good reference current, how do webias the differential pair shown below?

IREF VDD

RDRD

M

D

V M VM1Vi1 M2 Vi2

I

VSS

ISS

Need a current sink


Need a current sink

Current source to current SinkV

VDD

I

VDD

VIREF

I

VDD

IREF

ISS

m1 m2

SS

VDD

m1 m2

RDRD

Si l C t Mi

M1Vi1 M2 Vi2


Simple Current Mirror

VSS

ISS

Current Source to current Source

VDD Suppose we have a good reference current source,how do we bias the amplifier shown below?

IREFVDDDD

IBias

vO

vS Vbias1

M1

O

vS Vbias1


VDD

m3 m4IREF

m3 m4

IBias

vVbias1

IBias

m1 m2vO

M5

vSS


Generation of Bias Voltages from Reference current

VDD

m3 m

VDD

IREF

m3 m4Vbias2

vVbias1

m1 m2vO

M5

vSS


Generation of Bias Voltages from Reference current

V

V

VDD

M3

VDD

DD TP dsatV V V

V

Vbias3M3

Vbias2

M2

2 2DD TP dsatV V V DD TP dsatV V V

vO

Ibias

2

IBias

M1vS Vbias1bias1


Although biasing is fine, voltage swing of the amplifier is compromised

VDDVDD

20/1 20/1DD TP dsatV V V

5/1 20/12DD TP dV V V DD dsatV V

VI I

vO

Ibias

2DD TP dsatV V V

Vbias2Ibias Ibias

Vbias1vS How do we

generate this?

2I

g


2; SDSG TP dsat dsat

IV V V V

Important characteristics of current mirrors

Current matchingVDD

Output ResistanceIREF

21

o oR r Output ResistanceISS

22

o on DS

R rI

Voltage Swing mino dsatV Vm1 m2

Frequency/Transient Response


Current Mirrors with larger Output Resistance

Basic idea: use negative feedbackBasic idea: use negative feedback

For a given change in V

VOI

For a given change in Vo,decrease change in Io

Io

M1M2

Iref

M1M2

R

VO IO( ) VS1( )

I ( )

VGS1( )

IO( )

VGS1( )

(1 ( ) )R RG-NumberB. Mazhari, IITK

10

1 (1 ( ) )o o m mbR r g g R

HX X+ H

Xi XO

XF-

1i

OXX

H

VOIref

+vO / rO1 iOIo

M1M2

+

vO / rO1 iO

iFv

R -Rvgsgm1

(1 )R r g R

11

o oo

m

v rig R

1 (1 )o

o mo

v r g Ri


1 (1 )o o mR r g R

Different current mirror topologies use different techniques forrealizing resistor R

Io

VO

I

VO

I

VOIref Iref Iref

IoM1

IoM1

IoM1M4

VbiasM2M3 M3

M2M2M3

Wil C d R l t d C dWilson Cascode Regulated Cascode


Current Mirror Expression for R Output resistance

Simple 0 01r

CascodeCascode 03r ~ 01031 rrg m

WilsonWilsonin

m

m rrgg

032

3 ~ 103

2

31 oin

m

mm rrr

gg

g

Regulated Cascode inmo rrgr 0443 ~ 1044031 oinmm rrrgrg


Cascode Current Mirror

VOI

Io

VOIref

M1o

M1M4

M2M3

M2

M2M3


M1M1M1

M2 ro2

G-NumberB. Mazhari, IITK151 1 1 2( )o o m mb oR r g g r

Cascode Current Mirror

Voltage swing is compromisedVoltage swing is compromised

VOIref

IoM1M4 2V V V

2 2T DSATV V

M1M4 min 2o T DSATV V V T DSATV V

T DSATV V

M2M3

G-NumberB. Mazhari, IITK16GS T DSATV V V

Wilson Current Mirror

VO IO( ) VGS2( )

I

VOIref IO( )

V ( )IoM1

VGS1( )

+vO / rO1 iO

M2M3

+

vO / rO1 iO

iFRg

vgs1

-Rgm1

1 1(1 )oo m

v r g R 1 1(1 )o mo

r g Ri

3 031 ( )fR g r r


1 1 03( )o o m refR r g r r 3 03

2( )m ref

mR g r r

g

VVOIref

M1

M2M3T DSATV V

min 2o T DSATV V V


Voltage Swing is compromised again

Regulated Cascode Mirror

VOIref

+vO / rO1 iO

Io

Iref

M1

+

vO / rO1 iO

iF-Rgm1

vgs1

M3M1

(1 )ov R

M2M41 1(1 )o

o mo

r g Ri

2 3 03( )o m refR r g r r


1 1 02 3 03{ ( )}O o m m refR r g r g r r

Wideswing Cascode current mirror

V VOI

Io

VOIref

2 2T DSATV V Io

VOIref

VIoM1M4

o

M4 M1VT

M2M3

T DSATV VT DSATV V

M2M3

Vdsat

M2M3 M2M3


VOIref

2 3T DSATV V VOIref2V 3V

Io

ref

VT+Vdsat

Io(W/L)1(W/L)

(W/L)

2VT+3Vdsat

m2m4m5

M4 M1

VdsatV V

(W/L)

Vdsat

1(W/L)4

VT+2Vdsat

VT+Vdsat

M2M3T DSATV V

(W/L)

(W/L)

(W/L)m3 m1

m6

4 2GS T DSATV V V 2; DS

GS TN dsat dsatIV V V V

Problem: because VD3 is different fromV th ill b i t h i t


;GS TN dsat dsatV V V V

4 3 2 14

VD1, there will be a mismatch in current

Low voltage Cascode Current Mirror: Alternative design

VO

VT+2Vdsat

IrefIref

Io(W/L)1(W/L)

m2

T dsat

(W/L)

(W/L)1(W/L)4

m4m5VdsatVdsat

(W/L)

m1

(W/L)

dsat

m3


Summary

Current sources are useful for biasing and as load Current sources are useful for biasing and as loadelement

Simple current mirror offers limited output resistancebut has good output swing

Output resistance can be increased through use ofnegative feedbacknegative feedback

Cascode current mirror offers high output resistancegbut suffers from reduced swing


L19: Current Mirror (21.10.2013) - IITKhome.iitk.ac.in/~baquer/L19_current_mirror.pdf · Simple...

Documents

Transcript of L19: Current Mirror (21.10.2013) - IITKhome.iitk.ac.in/~baquer/L19_current_mirror.pdf · Simple...