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
G-NumberB. Mazhari, IITK2
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
G-NumberB. Mazhari, IITK3
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
G-NumberB. Mazhari, IITK4
VDD
m3 m4IREF
m3 m4
IBias
vVbias1
IBias
m1 m2vO
M5
vSS
G-NumberB. Mazhari, IITK5
Generation of Bias Voltages from Reference current
VDD
m3 m
VDD
IREF
m3 m4Vbias2
vVbias1
m1 m2vO
M5
vSS
G-NumberB. Mazhari, IITK6
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
G-NumberB. Mazhari, IITK7
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
G-NumberB. Mazhari, IITK8
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
G-NumberB. Mazhari, IITK9
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
G-NumberB. Mazhari, IITK11
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
G-NumberB. Mazhari, IITK12
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
G-NumberB. Mazhari, IITK13
Cascode Current Mirror
VOI
Io
VOIref
M1o
M1M4
M2M3
M2
M2M3
G-NumberB. Mazhari, IITK14
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
G-NumberB. Mazhari, IITK17
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
G-NumberB. Mazhari, IITK18
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
G-NumberB. Mazhari, IITK19
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
G-NumberB. Mazhari, IITK20
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
G-NumberB. Mazhari, IITK21
;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
G-NumberB. Mazhari, IITK22
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
G-NumberB. Mazhari, IITK23