Lab 8_Current Mirror

7/29/2019 Lab 8_Current Mirror

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Analog Integrated Circuits

Laboratory

Analog ICs;Hong-Yi Huang2-1Current Source

Hong-Yi Huang

Nano Integrated Circuits and Systems Lab.

Graduate Inst itute of Electrical Engineering


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Outline

NMOS Current Source

Basic NMOS Current Mirror

Cascode Current Mirrors

Improve Current Mirror

High-Swing Cascode Current Mirror


Wilson Current Mirror

Low-Voltage Current Mirror

Experiment Steps

Questions


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NMOS Current Source


For NMOS get into saturation region

VGS Vt, VGD Vt

VGD = VBIAS VDS Vt , we get VDS VBIAS-Vt

Define VBIAS-Vt is VMIN

So VDS VMIN is a Current Source

NL

KWhere

I

1

I

V1r

CHDD

DSds

K=dimensional factor

NCH=doping concentration


4/18

NMOS Current Source


Let VBIAS = 0.8V

VMIN=VBIAS Vt 0.4V


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Basic NMOS Current Mirror


In fact, in analog IC design, using Current Mirror (as seen in the figure above) to get

current source from a current reference is a typical method.

2

tGS1o )VV(L

WCu

2

1I

1

oxn -

2tGS2REF )VV(

L

WCu

2

1I

2

oxn -

REF

2

1o I

L/W

L/WI


6/18

If we take Short Channel Effect into analysis

Basic NMOS Current Mirror _ Second Order Effect

)V1()VV(L

WCu

2

1I

1

oxn DS12

tGS1o -

)V1()VV(L

WCu

2

1I

2

oxnREF DS22

tGS2 -


REF

2

1 I)V1(L/W

)V1(L/WI

DS2

DS1o

From the formula, if we want to reduce the Short Channel Effect, increase the

channel length will increase rO, so that the current source become more ideal.


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Basic NMOS Current Mirror _ Second Order Effect


If we increase the channel length then we get a more constant current source.


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Cascode Current Mirror

MN1MN2

MN3MN4

IOIREF

rO

+

+

+

+

+

+

-

-VR

Vd

Vb

Vc

Va

Vo

rO3

rO1

gm3va

gmb3va

io

Vo

+

-

+

-Va

+

-

Vc


Use cascode structure to enhance ro.

Vo = Va + Vc = Va + ro3[ Io + gmb3Va +gm3Va ] and Va = Io ro1

Vo = Io[ ro1 + ro3 + gmb3 ro1 ro3+ gm3 ro1 ro3 ]

We get ro = Vo / Io =ro1 + ro3 + gm3 ro1 ro3 (1+3)

Where 3 = gmb3 / gm3

- - - -


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Now we analyze the VMIN of Cascode

Current Mirror

VDS(sat) = VGS Vt

VGS = VDS(sat) + Vt

= =

Cascode Current Mirror


sa

Vo = VR - Vt

We get Vo =2VDS(sat) + Vt = VMIN

In this case, VMIN 2 0.15 + 0.4 = 0.7V


10/18


MN3Wn/Ln=4/1

IO

rO

+

Vo

+

-

+

VGS4

IREF

MN4Wn/Ln=4/4

MN1Wn/Ln

MN3Wn/Ln

4 1


In order to reduce VMINadjust the Channel Length of MN4 as four times to the

other NMOSaccording to the current formula in saturation

(VGS4 Vt) = 2(VGS2 Vt) = 2VDS(sat)so VGS4 = 2VDS(sat) + Vt

we getVo = VGS4 Vt

Vo = (2VDS(sat) + Vt ) Vt =2VDS(sat) = VMIN

--


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From the simulation, we can see that the VMIN of High-Swing Cascode Current

Mirror is smaller than Cascode Current Mirror.


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From the small signal analysis we get the output resistance to be

ro

= = Vo

/ Io

= ro3

+ ro2

+[ ]

if we increase the ro3 means we can increase output resistance obviously.

2o2m

3o1om3m13o3m3

rg1

1rrggr)1(g


13/18



We increase the width of MN3 in order to increase the output resistance, from the

simulation, the larger width produce smaller slope current curve, means we

increase the output resistance.


14/18


Now, we introduce a Low-Voltage Current Mirror.

M4

Vb

VDD

X

A

IREF

M3

M1

Iout

B

VOUT V2V

VV2VVVVVVV

minOUT

tminb

4t2GSb2t2GS4GS

1DS3GS2DS4GSb VVVVV

3GS4GS VVIf


M2 REFOUT2GS1GS IIVVAnd

From the small signal, the output resistance

ro = Vo / Io = 1 + ro1(1 + gm3ro3 + gmb3ro3) .


15/18


From above-mentioned, we can increase the output resistance ro by increasing the

width of MN3MN4.


m=1m=2m=10


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Experiment Steps

Use a single MOS as a current source as page 3, simulate output current ID. Try to

increase the channel length to see what change in ID.

Use a simple type current mirror as page 5, get the output current Io from the

reference current IREF. Try to increase the channel length to see what change in ID.

Use a cascode current mirror as page 8, get the output current Io from the reference

current IREF. Try to increase the channel length to see what change in ID.


Use high swing cascode current mirror as page 10, get the output current Io from

the reference current IREF. Try to increase the channel length to see what change in

ID.

Use wilson current mirror as page 12, get the output current Io from the reference

current IREF. Try to increase the channel width of MN3 to vary gm3 and to see what

change in ro.


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Steps

Use low voltage current mirror as page 14, get the output current Io from the

reference current IREF

. Try to increase the channel width of MN3 to vary gm3

and to

see what change in ro.



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Questions

Assuming IREF=100uA, how can we use IREF to generate 500uA300uA current

source (sink) respectively ?

An advantage of the High-Swing Cascode Current Mirror is that its VMIN is 2VDS(sat),

but what is the drawback for this structure?

Can you describe the voltage operation range of a Low-Voltage Current Mirror?


Documents

Lab 8_Current Mirror