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MOSFET CURRENTSOURCES FOR IC

BIASINGBYSNEHA GHANATEM-TECH(VLSI SYSTEM DESIGN)EMAIL: sneha.bandhavi@gmail.com

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FLOW-LINEIdeal Current sources

Ideal v/s Actual Current sources

Constant current sources and MOS currentsteering circuits

MOSFET characteristics

MOSFET modes of operation

MOSFET as a constant current source/sink

Design of constant current source using

current mirror

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Ideal Current source and its

Characteristics

SOU

RCE

LOA

D

A

B

Is

Transfer of energy from sourceto load

Symbol for Ideal currentsource

Z Loadimpedance

Icurrent

Is

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Ideal vs. Actual Current

Source

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A Constant current source is basic building block in IC design

Ideally the output impedance of current source should be infiniteand can generate constant current over wide range of voltages

CONSTANT CURRENTSOURCES/SINKS

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CURRENT SOURCE

USING MOSFET

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MOSFET as a Constant CurrentSource/Sink

Three regions ofoperation

Cutoff mode Linear mode Saturation mode

+-

Vgs

= 0

n+ n+

+-

Vgd

p-type body

b

g

s d

+

-

Vgs

> Vt

n+ n+

+

-

Vgd

= Vgs

+-

Vgs

> Vt

n+ n+

+-

Vgs

> Vgd

> Vt

Vds

= 0

0 < Vds

< Vgs

-Vt

p-type body

p-type body

b

g

s d

b

g

sd

Ids

+-

Vgs

> Vt

n+ n+

+-

Vgd

< Vt

Vds

> Vgs

-Vt

p-type body

b

g

s d Ids

cutoffmode

linear mode ofoperation

saturationmode

thGSGS vVV

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Basic MOSFET modes ofoperation

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< VGS-VT0

>= VGS-VT0

Basic MOSFET Equations

MOSFET IDS-VDSCharacteristics

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Design of Constant Current Sourceusing Reference Generator

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MOS CurrentMirror

The motivation behind a current mirror is toduplicate a (scaled version of the) Referencecurrent to other locations.

Current mirrorconcept

Generation ofrequired VGS

Current MirrorCircuitry( ) 2

2

1

THGS

REF

REF VVL

WKI

= ( ) 2

1

1

2

1

THGScopy VVL

WKI

=

VGS

VGS

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Current Mirror using N-

MOSFET

),( DGGSD

REFout

VVfI

II

=

M

1

M

2W1/L1

W2/L2

V

S

S

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Basic Current

Mirror

( )

( )

REF

REF

ILW

LWIou t

/

/2

=

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Current Mirror Using n-MOSFET

Basic Currentmirror USING N-channel MOSFET

M

1

M

2W1/L1 W2/L2

V

S

S

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Current Mirror Using n-MOSFET

1

2

)

)

LW

LW

I

I

REF

o

=

basic current sink using n-channel MOSFET currentmirror circuit

M

1

M

2W1/

L1

W2/

L2

V

S

S

Iout =f(VGS)= IREF Since

VGS1=VGS2Io =(W/L)2[VGS2-VTHN ]IREF (W/L)1[VGS1-VTHN ]

IfL1=L2 Io/IREF =

W2/W1

IREF= VDD-VGS-VSS RVmin across current sink isset by requirement that M2remain in saturationVmin=Vds,sat=VGS-

VthL1,L2,W1,W2 and VGS parameters are available for designers to set the currents in current

( )( ) REF

ILW

LWI

1

2

0

/

/=

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Current mirror using p-channel

MOSFET

( )

( )REF

I

LW

LWIout

1

2

/

/=

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THANK

YOU