MOSFETs as Current Sources - IIT...

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Analog Electronics(Course Code: EE314)

Lecture 21‐22: MOSFETs Amplifiers

Indian Institute of Technology Jodhpur, Year 2018

Course Instructor: Shree PrakashTiwari

Email: [email protected]

b h //h / /Webpage: http://home.iitj.ac.in/~sptiwari/

Course related documents will be uploaded on http://home.iitj.ac.in/~sptiwari/EE314/

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Note: The information provided in the slides are taken form text books for microelectronics (including Sedra & Smith, B. Razavi), and various other resources from internet, for teaching/academic use only

MOSFETs as Current Sources

• A MOSFET behaves as a current source when it is operating in the saturation region.

• An NMOSFET draws current from a point to ground (“sinksAn NMOSFET draws current from a point to ground ( sinks current”), whereas a PMOSFET draws current from VDD to a point (“sources current”).

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Common‐Source Stage: = 0

Amplifier circuit Small‐signal analysis circuitfor determining voltage gain, Av

Small‐signal analysis circuit fordetermining output resistance R

Dout

in

DDoxnDmv

RR

R

RIL

WCRgA

2determining output resistance, Rout

Common‐Source Stage: 0

• Channel‐length modulation results in reduced small‐signal voltage gain and amplifier output resistance.

Small signal analysis circuit Small signal analysis circuit forSmall‐signal analysis circuitfor determining voltage gain, Av

Small‐signal analysis circuit fordetermining output resistance, Rout

ODout

in

ODmv

rRR

R

rRgA

||

||

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CS Gain Variation with L• An ideal current source has infinite small‐signal resistance.

The largest Av is achieved with a current source as the load.

• Since is inversely proportional to L, Av increases with L.

D

oxn

D

Doxn

omv I

WLC

I

ILW

CrgA

2

2

CS Stage with Current‐Source Load

• Recall that a PMOSFET can be used as a current source from VDD.

Use a PMOSFET as a load of an NMOSFET CS amplifier.

21

211

||

||

OOout

OOmv

rrR

rrgA

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PMOS CS Stage with NMOS Load

• An NMOSFET can be used as the load for a PMOSFET CS amplifier.

21

212

||

)||(

OOout

OOmv

rrR

rrgA

CS Stage with Diode‐Connected Load

Amplifier circuit Small‐signal analysis circuitincluding MOSFET output resistances

:0

:0 If 121 ||||

1OOmv rr

ggA

:0

Av is lower, but it is less dependent on process parameters n and Cox and drain current (ID).

2

1

21 /

/1

LW

LW

ggA

mmv

122

2

||||1

OOm

out

m

rrg

R

g

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CS Stage with Diode‐Connected PMOS Load

:0

21

211

2

||||1

||||1

ooout

oom

mv

rrg

R

rrg

gA

:0

1mg

CS Stage with Degeneration

Amplifier circuit Small‐signal analysis circuitfor determining voltage gain, Av

Sm

Dv

Rg

RA

1 :0 If

Find Av when is not 0

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Example

• A diode‐connected device degenerates a CS stage.

21

11

mm

Dv

gg

RA

Rout of CS Stage with Degeneration

• Degeneration boosts the output impedance:


SX Riv 1

Current flowing down through ro is

SXmX

SXmXmX

Rigi

Rigivgi

1

SOmOSSmOX

X

XSXSXmXO

RrgrRRgri

v

vRiRigir

1

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Output Impedance Examples

211

11

mmOout g

grR 1211 OOOmout rrrgR

CS Stage with Gate Resistance

• For low signal frequencies, the gate conducts no current.

Gate resistance does not affect the gain or I/O impedances.

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CS Core with Biasing

DRRRA

|| 21 RR 21 ||

Sm

D

Gv

Rg

R

RRR

RRA

1||

||

21

21Dm

Gv Rg

RRR

RRA

21

21

||

||

Diode‐Connected MOSFETs

1R

1R

Diode‐connected NMOSFET Diode‐connected PMOSFET

11

om

X rg

R 22

om

Y rg

R

Small‐signal analysis circuit Small‐signal analysis circuit

• Note that the small‐signal model of a PMOSFET is identical to that of an NMOSFET

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Summary of MOSFET Impedances

• Looking into the drain, the impedance is if th t

• Looking into the gate, the impedance is i fi it ( )

• Looking into the source, the impedance is 1/gmi ll l ith ifro if the gate

and source are (ac) grounded.

infinite (∞). in parallel with ro if the gate and drain are (ac) grounded.

Common‐Gate Amplifier Stage

• An increase in Vin decreases VGS and hence decreases ID.

The voltage drop across RD decreases Vout increases

The small signal voltage gain (A ) is positiveThe small‐signal voltage gain (Av) is positive.

Dmv RgA

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Operation in Saturation Region

• For M1 to operate in saturation, Vout cannot fall below Vb‐VTH.

Trade‐off between headroom and voltage gain.

I/O Impedances of CG Stage ( = 0)Small‐signal analysis circuit for

determining output resistance, Rout

Small‐signal analysis circuit fordetermining input resistance, Rin

Dout RR m

in gR

1

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CG Stage with Source Resistance

Small‐signal equivalent

in

mS

mX v

gR

gv

1

1

Small signal equivalent circuit seen at input

For = 0:

S

m

Dv

Rg

RA

11

1

SmDm

in

X

X

out

in

out

RgRg

v

v

v

v

v

v

For 0:

• The output impedance of a CG stage with source resistance is identical to that of CS stage with degeneration.

S ll i l l i i i f

CG Stage with Source Resistance


OSOmSSmOout rRrgRRgrR 11

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CG Stage with Biasing

• R1 and R2 establish the gate bias voltage.

• R3 provides a path for the bias current of M1 to flow.

Dm

mout RgRgR

gR

v

v

/1||

/1||3

Gmin RgRv /1||3

CG Stage with Gate Resistance

• For low signal frequencies, the gate conducts no current.

Gate resistance does not affect the gain or I/O impedances.

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CG Stage Example

Small‐signal equivalent circuit seen at input

Small‐signal equivalent circuit seen at output

11 1RR

inSmm

in

Smm

mmX v

Rggv

Rgg

ggv

21

21

21

1

1

11

11

12

111 Om

SOmout rg

RrgR

DOS

m

Omout RrRg

rgR ||||1

1

2

11

Smm

Dm

in

X

X

outv Rgg

Rg

v

v

v

vA

21

1

1

Source Follower Stage

11

|| LOout

v

Rr

v

vA

Small‐signal analysis circuit fordetermining voltage gain, Av

||1 LO

m

in Rrg

v

Equivalent circuit

Looutinm

Lomout

Rrvvg

Rrvgv

1outin vvv 1

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Source Follower Example

• In this example, M2 acts as a current source.

21

1

21

||1

||

OO

m

OOv

rrg

rrA

Rout of Source Follower

• The output impedance of a source follower is relatively low, whereas the input impedance is infinite (at low frequencies); thus, it is useful as a voltage buffer., g

LLOout Rg

Rrg

R ||1

||||1


mm gg

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Source Follower with Biasing

• RG sets the gate voltage to VDD; RS sets the drain current.(Solve the quadratic equation to obtain the value of ID.)

221

THSDDDoxnD VRIVLW

CI

Assuming = 0:

Supply‐Independent Biasing

• If Rs is replaced by a current source, the drain current IDbecomes independent of the supply voltage VDD.

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Review: MOSFET Amplifier Design• A MOSFET amplifier circuit should be designed to

1. ensure that the MOSFET operates in the saturation region,

2 allow the desired level of DC current to flow and2. allow the desired level of DC current to flow, and

3. couple to a small‐signal input source and to an output “load”.

Proper “DC biasing” is required!(DC analysis using large‐signal MOSFET model)

• Key amplifier parameters: (AC analysis using small‐signal MOSFET model)(AC analysis using small‐signal MOSFET model)– Voltage gain Av vout/vin– Input resistance Rin resistance seen between the input node

and ground (with output terminal floating)– Output resistance Rout resistance seen between the output

node and ground (with input terminal grounded)

MOSFET Models

• The large‐signal model is used to determine the DC operating point (VGS, VDS, ID) of the MOSFET.

• The small‐signal model is used to determine how the output responds to an input signaloutput responds to an input signal.

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Comparison of Amplifier Topologies

Common Source

• Large Av < 0

Common Gate

• Large Av > 0

Source Follower

• 0 < Av ≤ 1‐ degraded by RS

• Large Rin

– determined by biasing circuitry

• Rout RD

d &

‐degraded by RS

• Small Rin‐ decreased by RS

• Rout RD

• ro decreases Av & Routbut impedance seen

• Large Rin– determined by

biasing circuitry

• Small Rout ‐ decreased by RS

• r decreases A &• ro decreases Av & Routbut impedance seenlooking into the draincan be “boosted” by source degeneration

but impedance seenlooking into the drain can be “boosted” by source degeneration

ro decreases Av & Rout

Common Source Stage

D

Gv

R

R

RRR

RRA

21

21

1||

||

0

Dout

in

Sm

G

RR

RRR

Rg

21

21

||

||0

SOmODout RrgrRR

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Common Gate Stage

Dm

GmS

mSv Rg

RgR

gRA

/1||

/1||

0

Sm

in Rg

R1

Dout RR SOmODout RrgrRR

0

Source Follower

Sm

Sv

Rg

RA

1SO

SOv

Rrg

RrA

||1

||

0 0

mg

Gin RR

Som

out

Gin

m

Rrg

R

RR

g

||||1

Sm

out Rg

R ||1

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CS Stage Example 1

• M1 is the amplifying device; M2 and M3 serve as the load.

Equivalent circuit for small‐signal analysis, showing resistances connected to the drain

1233

1233

1

||||||1

||||||1

OOOm

out

OOOm

mv

rrrg

R

rrrg

gA

CS Stage Example 2

• M1 is the amplifying device; M3 serves as a source (degeneration) resistance; M2 serves as the load.

E i l t i it f ll i l l iEquivalent circuit for small‐signal analysis

01

3

31

2

||11

O

mm

Ov

rgg

rA

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CS Stage vs. CG Stage• With the input signal applied at different locations, these circuits

behave differently, although they are identical in other aspects.

Common source amplifier Common gate amplifierCommon source amplifier Common gate amplifier

02

01

Sm

Ov

Rg

rA

2

1

1 12221 ||)1( OOSOmmv rrRrggA

Composite Stage Example 1

• By replacing M1 and the current source with a Theveninequivalent circuit, and recognizing the right side as a CG stage, the voltage gain can be easily obtained. g g y

12

11

mm

Dv

gg

RA

01 02

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Composite Stage Example 2• This example shows that by probing different nodes in a circuit,

different output signals can be obtained.

• Vout1 is a result of M1 acting as a source follower, whereas Vout2out1 1 g , out2

is a result of M1 acting as a CS stage with degeneration.

221

221

||11

||1

Omm

Om

in

out

rgg

rg

v

v

221

4332

||11

||||1

Omm

OOm

in

out

rgg

rrg

v

v

01

Short‐Circuit Transconductance

• The short‐circuit transconductance is a measure of the strength of a circuit in converting an input voltage signal into an output current signal:p g

0

outvin

outm v

iG

• The voltage gain of a linear circuit is(Rout is the output resistance of the circuit)

outmv RGA

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What Next• Cascode stage

• Current Mirror

• Frequency ResponseFrequency Response

MOSFETs as Current Sources - IIT...

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