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Basic FET Amplifiers

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C H A P T E R 6 Basic FET Ampli®ers 6.0 PREVIEW In the last chapter, we described the operation of the FET, in particular the MOSFET, and analyzed and designed the dc response of circuits containing these devices. In this chapter, we emphasize the use of FETs in linear ampli®er applications. Although a major use of MOSFETs is in digital applications, they are also used in linear ampli®er circuits. There are three basic con®gurations of single-stage or single-transistor FET ampli®ers. These are the common-source, source-follower, and com- mon-gate con®gurations. We investigate the characteristics of each con®gura- tion and show how these properties are used in various applications. Since MOSFET integrated circuit ampli®ers normally use MOSFETs as load devices instead of resistors because of their small size, we introduce the technique of using MOSFET enhancement or depletion devices as loads. These three con- ®gurations form the building blocks for more complex ampli®ers, so gaining a good understanding of these three ampli®er circuits is an important goal of this chapter. In integrated circuit systems, ampli®ers are usually connected in series or cascade, forming a multistage con®guration, to increase the overall voltage gain, or to provide a particular combination of voltage gain and output resis- tance. We consider a few of the many possible multistage con®gurations, to introduce the analysis methods required for such circuits, as well as their properties. JFET ampli®ers are also considered. These circuits, again, tend to be specialized, so the JFET discussion is brief. 6.1 THE MOSFET AMPLIFIER In Chapter 4, we discussed the reasons linear ampli®ers are necessary in analog electronic systems. In this chapter, we continue the analysis and design of linear ampli®ers that use ®eld-effect transistors as the amplifying device. The term small signal means that we can linearize the ac equivalent circuit. We will de®ne what is meant by small signal in the case of MOSFET circuits. The term linear ampli®ers means that we can use superposition so that the dc analysis and ac 313
Transcript
Page 1: Basic FET Amplifiers

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Page 35: Basic FET Amplifiers

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Page 37: Basic FET Amplifiers

* /

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Transition point

Cut-off point

Q-point

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vGSD (V)VGSQ = 1.18 V 1.36 V

1.4 1.60 1.0 1.2

5

4

3

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1

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Page 38: Basic FET Amplifiers

, -!

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vDS (sat) = – VTNLvGS = 0

Slope = 1rO

Transitionpoint

(a) (b)

$&! /( %& $ %'& 0

Page 39: Basic FET Amplifiers

* /

Q-point

vO

vGSD

I IIII III IV

C

B

A

VTND0

VDD – VTNL

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(c)

$&! /' %& $ 1 %'& 1 %&

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Q-pointLoad curve

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(a) (b)

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Page 40: Basic FET Amplifiers

, -!

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Page 41: Basic FET Amplifiers

2' 9 "8! ;%

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Page 42: Basic FET Amplifiers

, -!

/2 # " $ &! !

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Page 43: Basic FET Amplifiers

* /

:&- /"+:34

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Page 44: Basic FET Amplifiers

, -!

/"+5" ! $

! ! ! ! 0

! " ! !

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' "

B (

&"

5 :.

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$ " ! $

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Ri1 Ro1 Ri2 Ro2

AV1

+

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Stage 1

AV2AV1

Stage 2

$&! /5 34 $

+– R2

vi

R1

CC

CS

RD1

RS1 RS2

M1

vo

RL =4 kΩ

CC2

V + = 5 V

V – = –5 V

M2

Ri

RSiRo

$&! /6 ! $

Page 45: Basic FET Amplifiers

* /

:!$ )"! / *+!%,!- -

&"* /"+9

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Page 46: Basic FET Amplifiers

, -!

/":7 $ "

/0 /2

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0 2

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V + = 5 V

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Page 47: Basic FET Amplifiers

* /

/0 /2 ( $$ ! !" % !

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7+ 75 022

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Page 48: Basic FET Amplifiers

, -!

5( "#$

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Vo

gm1Vgs1

Vgs2

–+

+

gm2Vgs2

Vi RD1 RL

Ri

RSi

+– R1R2 RS2

$&! 1

Page 49: Basic FET Amplifiers

* /

,

02#02*0 22*

, 7/.7/.20/05+

0 7/.25+ 077

077 +

/0.

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0 ! $ "

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& "

Vgs1

Vo

gm1Vgs1

+

gm2Vgs2 = gm1Vgs1

Vi RD+– R3R2

$&! 1(

Page 50: Basic FET Amplifiers

, -!

!% 3& 4 !% $

# /"+9 $ ! ( /"0:" * $ " 3$ &! /"0:"4 3" 0.2 '4

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076 076" ? 0 2 0620 2 26 0 2 77060" 34 E * + ' 0 2 2 0 2 076 277 '" 34 *

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vi

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Page 51: Basic FET Amplifiers

* /

! & ! " " ?

.

2 0

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& 3/":84 $ ) "

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%

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Page 52: Basic FET Amplifiers

, -!

0

.

2 0

0 0

$ &! $ ) /":+ ( $ " $ &! $ ) $ " <! $$! ! $ !"

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2

0 2

Vgs gmVgs ro

+

G D

S

$&! 1/ 5*

vi

CC1

CS

vo

RL = 4 kΩ

R2 = 180 kΩ RS = 2.7 kΩ

R1 = 420 kΩRD = 2.7 kΩ

CC2

VDD = 20 V

+–

$&! 11 ! 5* '

Page 53: Basic FET Amplifiers

* /

0

.

2

* &

057

057 +27

27 0228 0

+ 2

272:2 082 2/+ 7

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gmVgs ro

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RLVi RDR1R2+–

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Page 54: Basic FET Amplifiers

, -!

:!$ )"! 6 *+!%,!- - ) $

$ ! "

$ /":8 ?

02 . +6 77060" -

$ ! , 797"

&%- $ &! /":5"

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vi

CC1

vo

RL = 10 kΩRG = 50 kΩ

+10 V

–10 V

RS

CC2

+–

$&! 12 5*

Vgs

gmVgs ro

–+

RS RLVi

Vo

RG+–

$&! 15 5*

Page 55: Basic FET Amplifiers

* /

( !

" (

( " $ ! $

! ! "

!% 3& 4 !% $

% @ ) /"::

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Page 56: Basic FET Amplifiers

, -!

$$ ! " 34 - ! $ !

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2" 34 * $ ! * "

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3 /"04

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+–

vi

CSQ1

vo

RL = 2 kΩ

R2 = 430 kΩ

R1 = 70 kΩ RS1 = 4 kΩ

RS2 = 4 kΩRD1 = 4 kΩ

CC2

CC1

VDD = 20 V

Q2

$&! 16 ( .,

Page 57: Basic FET Amplifiers

* /

$ &! !

$! "

* $ ! $

" 3 /".4

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Page 58: Basic FET Amplifiers

, -!

7

!% ! "8!

1 756 0 +7 62 7" 34

- $$ )* 7:;6 7:

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( , . 026 0 276 7" 34

- $$ )* :7 6 70 " 34 * & ! "

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1 * $ ! /"0 0;6 :7 ' 07 '"

?: /"0 ?

756 770:60 0 /7 62" E 076" 34 - $

$$ )* 7: /6" 34 * " 34 % $ ! , A

?2 $ ! & 3/"+4"1 & 3/".4 " & 2 & ! + " ( ( " J<? = $ 2 0

2 02 2"K

5 = , /"8 ' $ 0 .6 06"

!% ' .""#&! "8!

6 * $ ! $ /"0. 0;6 :7 ' 07 '"

2 ' 02 :7 '"

$ /"0. ( $ 077 ' : '" - $ ! , 07" 7"

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34 - & ! " 34 $ ! " 34 - ( ! "

( ,/"00 0:6 2 ' * : ' 7: ' 277 '" 34 0 2 . 26

262 7" 34 - $ ! "

' @ , /"00 '"

Page 59: Basic FET Amplifiers

* /

?/ $ ,/"0+ 06 7:62 77060" ? :6

:6 * 07 '" 34 - ! ( $ ! " 34 $ ! "

:1 $ ,/"0: ? 06 +62 7" 076

* 2 '" 34 - 2 /6" 34- $ ! " 34 (! !"

? $ ,/"0: $ ! , /"0:" :6

* 2 '" 34 2:6" 34 - $ ! "

?2 * , $ ,/"08 . 26 7 * 07 '" 34

- ! /6" 34 - ! $ ! " 34 * !

+– R2

vi

Rin R1

CC1

CC2RD

VDD

vo

RS

RL

$&!

VDD

vi

vo

RG = 1 MΩ+–

RL

RS

RD

CC1

CC2

$&! 1

V +

V –

vi

vo

RG =200 kΩ

+–

RL

IQ

RD

CC1

CC2

CS

$&! /

Page 60: Basic FET Amplifiers

, -!

34 ! ! & 34A

:5 $ ,/"08 , $ ? . 0:6 :62 7" * 2 '"34 - 0 :6" 34 - $ ! , " 34 % (

! A

?:6 - $ ,/"09 $ 7" & ! / 256 076" 22;6" E * 0 ' , 0 077 '" 02 "

?( $ ,/"27 ? . 0:6 262 77060" !

* 27 '" 70*" 34 - $ " 34 - $ * $ !

" 34 $ ! *

A

vivoRG =

100 kΩ+–

RLRD

RS

CC1

CC2

+5 V

–5 V

CS

$&! 2

+– R2

vi

Rin R1

CC1

RD

VDD = 18 V

vo

RS

RL

CC2

$&! 6

vivo

RG = 500 kΩ+–

RLRD

IQ

CC1

CC2

+9 V

–9 V

CS

$&! (

Page 61: Basic FET Amplifiers

* /

:( ,/"20 ? . 26 7:62 7" 34 - 2 /6" 34 - $ ! , "

?:(( - $ ,/"22 ! $ ! , 07 * 27 ' 277 '" $ 0 076" E

26 7"

!% / &!#;! "8!

(' $ $ +6 :7 '" - $ ! " *

$ ! 2: ' "

(/ $ ,/"2+ 262 . 26 77260" ? * + ' + ' 0 02+ 2 .9/ '" 34 * " 34

- $ , , "

vivo

RG = 200 kΩ+–

RD

RS

CC

+10 V

–10 V

CS

$&! (

+– R2

vi

Rin R1

CC1

RD

VDD = 20 V

vo

RS

RL

CC2

CS

$&! ((

+– R2

vi

R1

CC1

RS

VDD = 10 V

vo

RL

CC2

ii io

Ro

$&! (/

Page 62: Basic FET Amplifiers

, -!

(1 * $ ,/"2: 026 062 77060" 0 $ ! , "

( $ ,/"2:

? 06 0 /762 7" $ !

, 79:" 34 - & $$ )* +" 34 - & )* /7"

?:(2 $ ,/"28 1 $ ! ? 26 :62 77060" - :" $ ! , "

(5 * ,/"28" E 07 ' 7" $! * , 797" - " - ! ! * 2 ' "

:(6 $ ,/"28 ?

26 +62 7" - 277"- $ ! "

' $ ,/".7 : ? . 26 :62 7" 34

- " 34 - ! * $ ! $ $ * !"

+5 V

–5 V

vi

CC2voRG =

500 kΩRL =4 kΩIQ

+–

CC1

Ro

$&! (1

+5 V

–5 V

vi

CC2

RS

voRG =200 kΩ

RL =2 kΩ

+–

CC1

Ro

$&! (2

Page 63: Basic FET Amplifiers

* /

' * $ ,/".0" ! ! # " ! !

0

*

! !

0 *

:'( ,/".2 06

062 7" :6 .77 '"34 - 08 .6" 34 - $ ! , "

!% 1 .""#%! .8$&%

'' $ ,/".. 1 ? 06 .62 7" 34 - " 34 *

" 34 $ ! , "

'/ * , $ ,/".+" $ ? . 06 7:62 7" 34 - 78: /6" 34 -

V – = –5 V

V + = 5 V

vi

CC2

RL

vo

RG =200 kΩ

IQ

+–

CC1

Ro

$&! '

VDD

vi

CC2

RS RL

RGvo+

CC1

$&! '

+– R2

vi

R1CC1

RS

VDD

vO

Ri

Ro

$&! '(

Page 64: Basic FET Amplifiers

" 34 - ! : "

'1 /".+ ( ? 26 +62 7" ? 076 076 077 ' * 2 ' :" 34 026" 34 * " 34 - $ !

, "

' $ /".8 ( , ? . 26 262 7" ?

076 076 277 ' * 07 '" 34 -

. 076" 34 - $ ! , "

!% 2 "8! ;% :!,!

:'2 * 1 /".934" $ ? * 26 0

/762 7 )** 7:"E 076" 34 - $ ! , : $ " 34 - ! "

?'5 1 /"+.34

? 026 * 26 7:62 * 7062 * 77260" E 076" 34 - $ " 34 * ! " 34 -

$ ! "

'6 * ! " 0:6" % .6 7"5" -

$ "

/ ,/"+7 06 7:62 / * 06 * .762 /*" 7 " 34 * &

! ! " 34 - $ ! , $"

/ $ ,/"+0" 06 062 / * 06 * 7062 /*" 7 " E 96" 34 -

, -!

CC1

vi

vo

RS = 10 kΩ RD = 5 kΩ RL = 4 kΩ+–

CC2

V – = –5 V V + = +5 V

$&! ''

CC1

RL =2 kΩ

RG =50 kΩ

Ri =100 kΩ RD

vo

RS

CG

CC2

V + = +5 V V – = –5 V

io

Ri

Ro

$&! '/

VDD = 10 V

vO

vi+–

ML

MD

$&! /

Page 65: Basic FET Amplifiers

& ! * +6" 34 $ $ * ! $ ! , 00

*

"34 * $ ! * + '"

/( $ ,/"+0 ! , /"+0" 34 - $ ! * 07 '" 34 - $ "

!% 5 &%%$! "8!

?:/' ,/"+. ? 0 70622 0762 0 26 2 26 0 2 7"

? 076 0 + ' 277 '" 34 - 0 7+ 2 2 0 +6 2 :6" 34 * $ ! , " 34 - (

! "

:// ,/"+. ! , /"+." ? 076 0 0 ' 277 '

2 2 ' 2 7: '" 34 - $ /2 0 7+" 34 - ! 22 0" 34 - $ ! "

* /

RL

viCC

vo

VDD

+–

ML

+

VGG+

–vDSL

MDRo

$&! /

+–vi

CC vO

R2

Rin

R1

RD1

RD2RS1 CS1

CS2

VDD

M1 M2

RS2

$&! /'

Page 66: Basic FET Amplifiers

:/1 * ,/"+: 0 2 27762 0 2 756 0 2 7" 34 - 2 86 +77 '" 34 - ! 0 2

0" 34 * $ ! , "

/ ,/"+/ ? 0 2 +62 0 2 26 0 2 7" 34 - 0 20 2" 34 - 0 2" 34 - ! $ ! , "

:/2 /":7 ( ?

0 2 06 0 2 262 0 2 7" 34 E 02 ' 0 2 . :77 '" - . 0 2 2:6" 34 - $ ! , "

:/5 ! /":7 (

076 076" ? 0 2 +62 0 2 0:6 0 2 7" 34 E 2 ' 7"0" -

: 0 2 .:6" 34 - $ !$ "

!% 6 7 < "8!

/6 * ) /":. / . .6 77060" E 076" 34 - + /6" 34 - $"34 - $ ! , $!

! +"

1 ) ,/":7 ? 2 . 26 7" - , , "

:1 ) $

,/":0 ? 5 . +26 7" E 276 * 0/ '" - 26 0 2 077 ' $ 2 2"

, -!

+–

vi

vO

CC

R2

RinR1

M1 M2

RS1 =20 kΩ

RS2 =6 kΩ

VDD = 10 V

Ro

$&! /1

CC2 CC3vo

RS2 =10 kΩ

RS1 =10 kΩ

RD =5 kΩ

RL =2 kΩ

–10 V +10 V

+10 V

–10 V

viRG =400 kΩ

+–

CC1

M2

M1

$&! /

Page 67: Basic FET Amplifiers

?:1( * $ ) ,/":2 07 . :6 77060" E 026 * 7: '" 34 - 077 ' $ 2 2 34 - $ ! , "

1' $ ) $ ,/":. ? 2 . 08:6 7" 34 - "

34 - $ , , " 34 - ($ ! "

:1/ $ ) $ ,/":+ 5 . +6 7" - +

8:6 , . 0 2 +77 '"

* /

vi

CC

RD = 8 kΩ

VDD

vo

RS2 =250 Ω

RG =50 kΩ

RS1 =100 Ω RL =

4 kΩ

CC2

CS

iiio

+–

$&! 1

vi

CC1

Rin vo

RL

R2 RS

R1

RD

CC2

VDD

+–

iiio

$&! 1

vi

CC1

Rin

vo

RLR2 RS

R1

CC2

VDD

+–

ii

io

Ro

$&! 1(

+–vi

vo

R2 = 110 kΩ

R1 = 90 kΩ

RL =10 kΩ

RS = 5 kΩ

CC2

CC1

VDD = 10 V

iiio

$&! 1'

Page 68: Basic FET Amplifiers

. 4 49 7

11 * /"22 ! (

/"/" = ! $ $ $ ! "

1 = ! $ ! $

/"25" ! ( /"8"

12 $ /".+ ! ( /"07" = $

! 3' 4" ! "

15 , ( /"22 "

- $ ! " ! ( 2":6 & ! "

16 @ , /":5 ( /"2+"

:9 7

J1? "K

?: $ /"0/ ! ! 27 ! "

! :6 0 ' ? 756 0

+7 62 77060", )* ! & " - )* 70

?: $ /".8 ! 076 277 277" ? 0

.7 62 . 26

7" * : '" - ! '$$' :6"

, -!

vi

CC1

vOR2

RD

R1 RS

VDD = 20 V

+–

$&! 1/

Page 69: Basic FET Amplifiers

?:( $ /".2 " ! 026 ? 0:6 0

+7 62 7"

* 077" - 277% ! " $ "

?:' 1

/"+.34 ! :6 ? / 06 /* 26 0

+7 62 77060 7.:602"- $ ! , 277 " = $ & $

/"

?:/ /":7 ? 06 0

+762 7" - $

! 07" - ( ! "

* /

Page 70: Basic FET Amplifiers

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