ANALOGUE ELECTRONICS

CIRCUITS 1

EKT 204

Introduction to FET Amplifier:

FET (Review)

1

FIELD EFFECT TRASISTOR (FET)

ADVANTAGES OF FET TYPES OF FET & ITS

OPERATION

2

Voltage-controlled amplifier: input impedance very high

Low noise output: useful as preamplifiers when noise must be very low because of high gain in following stages

Better linearity: distortion minimized Low inter-electrode capacitance: at high

frequency, inter-electrode capacitance can make amplifier work poorly. FET desirable in RF stages (high frequency)

FET Advantages

3

Types of FETFET

JFET MOSFET MESFET

n channel Enhancement mode

p channel n channel

p channel

Depletion mode

n channel

p channel 4

Junction FET (JFET)

n-channel p-channel

G np p

S

D

G pn n

S

D

D

S

G

D

S

G

Structure

Symbol

ohmic contact

5

Metal-Oxide-Semiconductor MOS (MOSFET)

DE

PL

ET

ION

EN

HA

NC

EM

EN

T

n-channel p-channel

p

p

p

p

n

n

dielectric

metal

6

JFET Operation

n

n

p p

n

n

p p

VGG

VDDVDD

depletion region

• Gate-source is reversed-biased

zero current at gate

• IDS flow through the channel and the value is determined by the width of depletion region and the width of the channel 7

MOSFET Operation

p-type

n+n+

p-type

n+n+ - - - - - - -

G GD DS S

SSSS

• No voltage applied to gate

• Current is zero

• +ve voltage applied to gate

• Electron inversion layer is created

• Current is generated between source and drain

electron inversion layer

8

FET BIASING

JFET BIAS CIRCUITS Self-bias Voltage-divider bias

MOSFET BIAS CIRCUITS Voltage-divider bias Drain-feedback bias

9

Equivalence biasing of JFET & BJT

AI

II

V

VII

G

SD

P

GSDSSD

0

12

VV

II

II

BE

EC

BC

7.0

<==>

<==>

<==>

JFET BJT

10

JFET Bias Circuits - Self-Bias

+VDD

RD

RSRG

SDSGGS RIVVV

SDDDDDS RRIVV

IG = 0

11

JFET Bias Circuits - Voltage-divider Bias

+VDD

RD

RS

R1

R2

DDG VRR

RV

21

2

S

GSGD R

VVI

VG

ID

12

MOSFET Bias Circuits - Voltage-divider Bias

+VDD

R1 RD

R2

2TNGSD

DDDDDS

VVKIwhere

RIVV

DDGS VRR

RV

21

2

13

MOSFET Bias Circuits - Drain-Feedback Bias

+VDD

RDRG

DDDDDSGS RIVVV

IG = 0

14

LOAD LINE

SELF-BIASED JFET VOLTAGE-DIVIDER

BIAS JFET

15

LOAD LINE- SELF-BIASED JFET

+VDD 9V

RD

2.2K

RS

680

RG

10M

Example

Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure.

16

ID (mA)

-VGS (V)

4 IDSS

-6

VGS(off)

2.25

-1.5-2.72

Q

For ID=0,

VGS=-IDRS=(0)(680)=0V

From the curve,

IDSS=4mA; so ID=IDSS=4mA

VGS=-IDRS=-(4m)(680)=-2.72V

ID=2.25mAVGS=-1.5V

Q point is the intersection between

the transfer characteristiccurve and the

load line

17

LOAD LINE - VOLTAGE-DIVIDER BIAS JFET

+VDD

8V

RD

680

R1

2.2M

R2

2.2MRS

3.3K

Example

Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure.

18

ID (mA)

-VGS (V)

12 IDSS

-3

VGS(off)

1.8

1.2-1.8

Q

4VGS (V)

For ID=0,

VVRR

RVV DDGGS 48

4.4

2.2

21

2

mAKR

V

R

VVI

S

G

S

GSGD 2.1

3.3

4

For VGS=0,

ID=1.8mAVGS=-1.8V

Q point is the intersection between

the transfer characteristiccurve and the

load line

19

EXERCISES (Load Line JFET)

+VDD 6V

RD

820

RS

330

RG

10M

1. Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure.

ID (mA)

-VGS (V)

IDSS = 5mA

VGS(off)=-3.5

20

2. Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure.

+VDD

12V

RD

1.8K

R1

3.3M

R2

2.2MRS

3.3K

ID (mA)

-VGS (V)

IDSS = 5mA

VGS(off)=-4V

EXERCISES (Cont.)

21

FET CHARACTERISTI

CS

JFET MOSFE

T

22

JFET CHARACTERISTICS

DRAIN CHARACTERISTIC

ohmic region

Saturation region

breakdown region

VDS

ID

VP

IDSS

VGS

VGS=0

VP=|VGS (off)|23

TRANSFER CHARACTERISTIC

N-CHANNEL

ID

VP

IDSS

-VGS

21P

GS

VV

DSSD II

JFET CHARACTERISTICS

24

JFET DATA SHEET

For MMBF5459 VGS (off) = -8.0V (max)

IDSS = 9.0 mA (typ.) 25

MOSFET CHARACTERISTICS TRANSFER CHARACTERISTIC

(Depletion MOSFET)ID

VGS(off)=VP

IDSS

-VGS

N-CHANNEL 21P

GS

VV

DSSD II

26

TRANSFER CHARACTERISTIC (Enhancement MOSFET)

ID

VTN +VGS

N-CHANNEL 2TNGSD VVKI

K in formula can be calculated by substituting data sheet values ID(on) for ID and VGS at which ID(on) is specified for VGS

MOSFET CHARACTERISTICS

27

E-MOSFET DATA SHEET

2

22

)( /48.58.05.4

75VmA

mA

VV

IK

TNGS

onD

ID(on) = 75 mA (minimum) at

VTN = 0.8 V and VGS = 4.5V

28