Frequency Response

of CG Stage and Source Follower

Hanwool Jeong

hwjeong@kw.ac.kr

1

CG Stage & Source Follower

2

VDD

RL

Vin

Vout

Frequency Response of CG Stage

3

Revisit Effect of Coupling Capacitorin CS Stage

1 2

1 2

||( )

1||

X

inPREV

i

V R Rs

VR R R

sC

4

+-

RP

R1

R2

RD

RS

VoutX

VinCi

1 2

1 2

( || )

1 ( || )

i

PREV i

s R R C

s R R R C

CG Stage

1/ ( ( 1/ ) 1)

m D i

m S PREV mS S i

sg R C

g R s R g R C

5

+-

RPREV

Vin

Ci

RD

RS

Vout

Vb

VDD

( ) ( ) ( )out outX

in in X

V VVs s s

V V V

1||

1 1||

S

mm D

PREV S

i m

Rg

g R

R RsC g

VX

(When RS is large)(1 )

m D i

m m PREV i

sg R C

g s g R C

Low Frequency Response of CG Amp

6

(1 )

m D i

m m PREV i

sg R C

g s g R C

1/

D

m PREV

R

g R

1(1/ )

(1 )

m PREV

m

m PREV

g R C

gg R C

With Higher Frequency

• Transistor cap should be considered.

7

CGS

CGD

Vout

RD

CDB

RPREV CSBVin

RD

Vout

RPREV

RS

RD

RPREV

RS

Ci

Poles At High Frequency

• Vb is ac ground (Why?)

• No Miller effect

• Input pole rarely creates a speed bottleneck

8

CGS

CGD

Vout

RD

CDB

RPREV CSBVin

Vb

X

,

1

( )p out

D DB GDR C C

,

1

1|| ( )

p X

PREV SB GS

m

R C Cg

9

10

Assume CDB and CSB are equal.

CG shown below,

RL

RS

11

Frequency Response of Source Follower

12

Low Frequency Response of SF

• We already did!

13

Capacitor Components for High Frequency Response of SF

14

Vin

RS Vout

VDD

High Frequency Response of SF (cont’d)

• KCL can be used at the output node:

• Ohm’s law across CGS can eliminate vgs:

15

Vin

RS

Vout

CGD CGS

CSB+CL

gmvgs

+

vgs

-

( )( ) ( )

in out gs

GD out gs m gs out SB L

S

V V vsC V v g v sV C C

R

1 1inm GD gs out GD SB L

S S S

Vg sC v V sC sC sC

R R R

( )1( )

in out gs

gs GD out gs

GS S

V V vv sC V v

sC R

1 ( )S GS GD gs in out S GD outsR C C v V V sR C V

1 ( )

in out S GD outgs

S GS GD

V V sR C Vv

sR C C

High Frequency Response of SF (cont’d)

2

1

1 ( )( )

GS

out m

in GD SB L SS GD GS GD SB L GD GS

m m

Cs

V g

V C C C Rs R C s C C C C C C

g g

16

1 1

1 ( )

in in out S GD outm GD out GD SB L

S S S GS GD S

V V V sR C Vg sC V sC sC sC

R R sR C C R

• This leads to

17

Input Impedance

• SF input resistance approaches infinity at low frequency.

• Total input capacitance would be

18

1

1in GS GD

m L

C C Cg R

19

Output Impedance

• For simplicity, other R,C are neglected.

20

+-

VX

IX

gmvgsCGSCGD

RS

Output Impedance vs. Frequency

• That is ωz = 1/RSCGS and ωp = gm/CGS.

21

1 1

1 ( / )

S GSm

GS m

sR Cg

s C g

ω ω

|Zout| |Zout|

1/RSCGS gm/CGS gm/CGS 1/RSCGS

1/gm

1/gm

22