lecture 19marked

8/12/2019 Lecture 19marked

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EE105 Fall 2007 Lecture 19, Slide 1 Prof. Liu, UC Berkeley

Lecture 19

OUTLINE

Common-gate stage

Source follower

Reading: Chapter 7.3-7.4

ANNOUNCEMENTS For Problem 4 of HW10, use VDD= 1.8V and VTH= 0.4V

Note: Midterm #2 will be held on Thursday 11/15


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Diode-Connected MOSFETs

Note that the small-signal model of a PMOSFET is identical to

that of an NMOSFET

1

1

1

o

m

X rg

R 2

2

1

o

m

Y rg

R

Diode-connected NMOSFET

Small-signal analysis circuit Small-signal analysis circuit

Diode-connected PMOSFET


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Summary of MOSFET Impedances

Looking intothe drain, theimpedance isroif the gateand sourceare (ac)

grounded.

Looking intothe gate, theimpedance isinfinite ().

Looking into thesource, theimpedance is 1/gmin parallel with roifthe gate and drainare (ac) grounded.


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Common-Gate Amplifier Stage

An increase in Vin

decreases VGS

and hence decreases ID.

The voltage drop across RDdecreasesVoutincreases

The small-signal voltage gain (Av) is positive.

Dmv RgA


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Operation in Saturation Region

For M1to operate in saturation, V

outcannot fall below V

b-V

TH.

Trade-off between headroom and voltage gain.


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I/O Impedances of CG Stage ( = 0)

Dout RR

m

ing

R 1

Small-signal analysis circuit for

determining output resistance, Rout


determining input resistance, Rin


7/16EE105 Fall 2007 Lecture 19, Slide 7 Prof. Liu, UC Berkeley

CG Stage with Source Resistance

S

m

D

v

Rg

RA

1

in

m

S

mX v

gR

gv

1

1

1

1

Sm

Dm

in

X

X

out

in

out

RgRg

v

v

v

v

v

v

Small-signal equivalent

circuit seen at input

For = 0:



OSOmSSmOout rRrgRRgrR 11

The output impedance of a CG stage with source resistance isidentical to that of CS stage with degeneration.





CG Stage with Biasing

R1

and R2establish the gate bias voltage.

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

Dm

Sm

m

in

out RgRgR

gR

v

v

/1||

/1||

3

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.



CG Stage Example

DOS

m

Omout RrR

grgR ||||

11

2

11

SmmDm

in

X

X

outv

Rgg

Rg

v

v

v

vA

21

1

1


circuit seen at input


circuit seen at output

inSmmin

S

mm

mm

X vRgg

v

Rgg

ggv

21

21

21

1

1

11

11

1

2

111

1O

m

SOmout rg

RrgR


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Source Follower Stage


determining voltage gain, Av

1

||1

||

LO

m

LO

in

out

v

Rrg

Rr

v

vA

Equivalent circuit

Looutinm

Lomout

Rrvvg

Rrvgv

1

outin vvv 1


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Source Follower Example

In this example, M2acts as a current source.

21

1

21

||1

||

OO

m

OOv

rrg

rrA


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Routof 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.

L

m

LO

m

out Rg

Rrg

R ||1

||||1




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Source Follower with Biasing

RGsets the gate voltage to VDD; RSsets the drain current.(Solve the quadratic equation to obtain the value of ID.)

221

THSDDDoxnD VRIV

LWCI

Assuming l= 0:


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Supply-Independent Biasing

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

lecture 19marked

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