FET ( Field Effect Transistor)

1. Unipolar device i. e. operation depends on only one type of charge carriers (h or e)

2. Voltage controlled Device (gate voltage controls drain current)

3. Very high input impedance (109-1012 )

4. Source and drain are interchangeable in most Low-frequency applications

5. Low Voltage Low Current Operation is possible (Low-power consumption)

6. Less Noisy as Compared to BJT7. No minority carrier storage (Turn off is faster) 8. Self limiting device9. Very small in size, occupies very small space in ICs10. Low voltage low current operation is possible in MOSFETS 11. Zero temperature drift of out put is possiblek

Few important advantages of FET over conventional Transistors

Types of Field Effect Transistors (The Classification)

» JFET

MOSFET (IGFET)

n-Channel JFET

p-Channel JFET

n-Channel EMOSFET

p-Channel EMOSFET

Enhancement MOSFET

Depletion MOSFET

n-Channel DMOSFET

p-Channel DMOSFET

FET

Figure: n-Channel JFET.

The Junction Field Effect Transistor (JFET)

Gate

Drain

Source

SYMBOLS

n-channel JFET

Gate

Drain

Source

n-channel JFETOffset-gate symbol

Gate

Drain

Source

p-channel JFET

Figure: n-Channel JFET and Biasing Circuit.

Biasing the JFET

Figure: The nonconductive depletion region becomes broader with increased reverse bias. (Note: The two gate regions of each FET are connected to each other.)

Operation of JFET at Various Gate Bias Potentials

P P +

-

DC Voltage Source

+

-+

-

N

N

Operation of a JFET

Gate

Drain

Source

Figure: Circuit for drain characteristics of the n-channel JFET and its Drain characteristics.

Non-saturation (Ohmic) Region:

The drain current is given by

2

2 2

2DS

DSPGSP

DSSDS

VVVV

V

II

2

2 PGSP

DSSDS

VVV

II

2

1 and

P

GSDSSDS V

VII

Where, IDSS is the short circuit drain current, VP is the pinch off voltage

Output or Drain (VD-ID) Characteristics of n-JFET

Saturation (or Pinchoff) Region:

PGSDSVVV

PGSDSVVV

Figure: n-Channel FET for vGS = 0.

Simple Operation and Break down of n-Channel JFET

Figure: If vDG exceeds the breakdown voltage VB, drain current increases rapidly.

Break Down Region

N-Channel JFET Characteristics and Breakdown

Figure: Typical drain characteristics of an n-channel JFET.

VD-ID Characteristics of EMOS FET

Saturation or Pinch off Reg.

Locus of pts where PGSDS VVV

Figure: Transfer (or Mutual) Characteristics of n-Channel JFET

2

1

P

GSDSSDS V

VII

IDSS

VGS (off)=VP

Transfer (Mutual) Characteristics of n-Channel JFET

JFET Transfer CurveThis graph shows the value of ID for a given value

of VGS

Biasing Circuits used for JFET

• Fixed bias circuit

• Self bias circuit

• Potential Divider bias circuit

JFET (n-channel) Biasing Circuits

2

1

P

GSDSSDS V

VII

0, GGSGSGGGG IFixedVVRIV

DDSDDDS

P

GSDSSDS

RIVV

V

VII

and

12

S

GSDS

SDSGS

R

VI

RIV

0

For Self Bias Circuit

For Fixed Bias Circuit

Applying KVL to gate circuit we get

and

Where, Vp=VGS-off & IDSS is Short ckt. IDS

JFET JFET BiasingBiasing Circuits Count… Circuits Count…

or Fixed Bias Ckt.

JFET Self (or Source) Bias Circuit

2

1 and

P

GSDSSDS V

VII

S

GS

P

GSDSS R

V

V

VI

2

1

021

2

S

GS

P

GS

P

GSDSS R

V

V

V

V

VI

This quadratic equation can be solved for VGS & IDS

The Potential (Voltage) Divider Bias

01

2

S

GSG

P

GSDSS R

VV

V

VI

DSGSI V gives equation quadratic this Solving and

A Simple CS Amplifier and Variation in IDS with Vgs

FET Mid-frequency Analysis:

g

s

rd gmv vi = v

ii io

vo

d

s

+ +

_ _

mid-frequency CE amplifier circuit

RD RL RTh vs

+

_

is

' 'o o ivi m L L d D L vs vi

i s s i

ii Th Th 1 2

i

Analysis of the CS mid-frequency circuit above yields:

v v ZA = = -g R , where R = r R R A = = A

v v R + Z

vZ = = R , where R = R R

i

L

o iI vi

i L

o oo d D P vi I

o iseen by R

i Z A = = A

i R

v pZ = = r R A = = A A

i p

A common source (CS) amplifier is shown

to the right.

Rs Ci

RL

Co

CSS vi

vo

+

+

vs

+

_ _

_

io

ii

D

S

G

VDD

VDD

R1

RSS

RD

R2

The mid-frequency circuit is drawn as follows:

• the coupling capacitors (Ci and Co) and the

bypass capacitor (CSS) are short circuits

• short the DC supply voltage (superposition)• replace the FET with the hybrid- model

The resulting mid-frequency circuit is shown below.

FET Mid-frequency Analysis:

g

s

rd gmv vi = v

ii io

vo

d

s

+ +

_ _

mid-frequency CE amplifier circuit

RD RL RTh vs

+

_

is

' 'o o ivi m L L d D L vs vi

i s s i

ii Th Th 1 2

i

Analysis of the CS mid-frequency circuit above yields:

v v ZA = = -g R , where R = r R R A = = A

v v R + Z

vZ = = R , where R = R R

i

L

o iI vi

i L

o oo d D P vi I

o iseen by R

i Z A = = A

i R

v pZ = = r R A = = A A

i p

A common source (CS) amplifier is shown

to the right.

Rs Ci

RL

Co

CSS vi

vo

+

+

vs

+

_ _

_

io

ii

D

S

G

VDD

VDD

R1

RSS

RD

R2

The mid-frequency circuit is drawn as follows:

• the coupling capacitors (Ci and Co) and the

bypass capacitor (CSS) are short circuits

• short the DC supply voltage (superposition)• replace the FET with the hybrid- model

The resulting mid-frequency circuit is shown below.

Procedure: Analysis of an FET amplifier at mid-frequency:

1) Find the DC Q-point. This will insure that the FET is operating in the saturation

region and these values are needed for the next step.

2) Find gm. If gm is not specified, calculate it using the DC values of VGS as follows:

3) Calculate the required values (typically Avi, Avs, AI, AP, Zi, and Zo. Use the formulas for

the appropriate amplifier configuration (CS, CG, CD, etc).

DSSDm GS P2

GS P

Dm GS T

GS

GS

2IIg = = V - V (for JFET's and DM MOSFET's)

V V

Ig = = V - V (for EM MOSFET's)

V

(Note: Uses DC value of V )

K

PE-Electrical Review Course - Class 4 (Transistors)

Example 7:

Find the mid-frequency values for Avi, Avs, AI, AP, Zi,

and Zo for the amplifier shown below. Assume that

Ci, Co, and CSS are large.

Note that this is the same biasing circuit used in Ex. 2, so VGS = -0.178 V.The JFET has the following specifications:

DSS = 4 mA, VP = -1.46 V, rd = 50 k

10 k Ci 8 k

Co

CSS vi

vo

+

+

vs

+

_ _

_

io

ii

D

S

G

18 V 18 V

800 k

2 k

500

400 k

FET Amplifier Configurations and

Relationships:

'' ' m L

vi m L m L 'm L

'L d D L d D L SS L

i Th SS Thm

o d D d D SSm

i i ivs vi vi vi

s i s i s i

i i iI vi vi vi

L L L

P vi I vi I

CS CG CD

g RA -g R g R

1 g R

R r R R r R R R R

1Z R R R

g

1Z r R r R R

g

Z Z ZA A A A

R + Z R + Z R + Z

Z Z ZA A A A

R R R

A A A A A

vi I

Th 1 2

A A

where R = R R

VCC

RD

S

R2

RSS

Rs Ci

RL

Co

C2

vi vo

+

+

vs

+

_

_ _

io ii

Common Gate (CG) Amplifier

R1

D

G

Note: The biasing circuit is the same for each amp.

Rs Ci

RL

Co

CSS vi

vo

+

+

vs

+

_ _

_

io

ii

D

S

G

VDD

VDD

R1

RSS

RD

R2

Common Source (CS) Amplifier

Rs C i

vi

+

vs

+

_

_

ii G

VDD

VDD

R1

RSS

R2

Common Drain (CD) Amplifier (also called “source follower”)

RL

C o

vo

+

_

io

D

S

Figure: Circuit symbol for an enhancement-mode n-channel MOSFET.

Figure: n-Channel Enhancement MOSFET showing channel length L and channel width W.

Figure: For vGS < Vto the pn junction between drain and body is reverse biased and iD=0.

Figure: For vGS >Vto a channel of n-type material is induced in the region under the gate. As vGS increases, the channel becomes thicker. For small values of vDS ,iD is proportional to vDS.

The device behaves as a resistor whose value depends on vGS.

Figure: As vDS increases, the channel pinches down at the drain end and iD increases more slowly. Finally for vDS> vGS -Vto, iD becomes constant.

Current-Voltage Relationship of n-EMOSFET

Locus of points where

Figure: Drain characteristics

Figure: This circuit can be used to plot drain characteristics.

Figure: Diodes protect the oxide layer from destruction by static electric charge.

Figure: Simple NMOS amplifier circuit and Characteristics with load line.

Figure: Drain characteristics and load line

Figure vDS versus time for the circuit of Figure 5.13.

Figure Fixed- plus self-bias circuit.

Figure Graphical solution of Equations (5.17) and (5.18).

Figure Fixed- plus self-biased circuit of Example 5.3.

Figure The more nearly horizontal bias line results in less change in the Q-point.

Figure Small-signal equivalent circuit for FETs.

Figure FET small-signal equivalent circuit that accounts for the dependence of iD on vDS.

Figure Determination of gm and rd. See Example 5.5.

Figure Common-source amplifier.

For drawing an a c equivalent circuit of Amp.

•Assume all Capacitors C1, C2, Cs as short circuit elements for ac signal

•Short circuit the d c supply

•Replace the FET by its small signal model

Analysis of CS Amplifier

LgsmLoo

gs

ov

RvgRiv

v

vA

gain, Voltage

dDLLmgs

ov

rRRRgv

vA ,

Dd

DdDdo Rr

RrRrZ

imp., put Out

21 imp., Input RRRZ

Gin

A C Equivalent Circuit

Simplified A C Equivalent Circuit

Analysis of CS Amplifier with Potential Divider Bias

)R||(rgAv Ddm

DR10r D,m

dRgAv

)R||(rgAv Ddm

This is a CS amplifier configuration therefore the input is on the gate and the output is on the drain. 21 R||RZi

Dd R||rZo

DdD 10RrRZo