Zener Diodes Working

7/29/2019 Zener Diodes Working

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SJTU J. Chen 103/06/13

Physical Operation of Diodes

Chapter 3 DiodesChapter 3 Diodes


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03/06/13 SJTU J. Chen

2

Part 1. Physical operation of diodesPart 2. Analysis of diode circuits and

applications of diodes

Content


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4

Linear and Nonlinear Devices

So far, almost all the devices we have learnt arelinear

Many signal-processing functions, however, are

implemented by nonlineardevices

Linear amplifier Nonlinear amplifier


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5

Diode and its physical structure

The diode is the simplest and most fundamentalnonlinearcircuit element

The most important region is the boundary

between n-type andp-type semiconductor, whichis calledpn junction

pn junction


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6

Symbol and characteristic for the ideal diode

(a) diode circuit symbol

+ v -

iAnode Cathode

(b) iv characteristic

---Reverse bias--- ---Forward bias---

i

0 v

(c) equivalent circuit

in the reverse direction

v < 0i=0

i

+ v -

i

(d) equivalent circuit

in the forward direction

i > 0v =0

+ v -


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How does it happen?

To answer the question, we need to know: Material, structure and the related features

(crystal and semiconductor in particular)

New particles to carry charge in addition toelectrons

New mechanism(s) of conduction in addition to

what we have known

Techniques to manufacture the devices (not

included in this course)


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Basic semiconductor concepts

Intrinsic Semiconductor ( )Doped Semiconductor ( )

Carriers ( )

Diffusion ( ), Drift ( )


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Elements and material

Periodic table


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Material and structure

Structure is another important factor to determine thephysical and chemical characteristics of the material allotrope( ), e.g.

graphite diamond


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Different features and different applications


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Material structure: crystal and noncrystal

Crystal Regular shape, fixed freezing temperature, fixed boiling

point, etc.

Why? Regular lattice structure Atoms can not tell from each other: they behave uniquely

In noncrystal, however, the atoms of the same element

usually play different roles, e.g. Polymer

aromatic hydrogen bondsaromatic hydrogen bonds

((

))


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Energy band

Energy band also reflects the material

property

Energy

level

11 =ns

s

p

2

2 } 1n

(a) Singular atom

Energy

level

s1

s

p

2

2

distance

(b) Splitting of energy levels for 8 atoms


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Transition

The movement of electrons between the energy

bands is call transition.

Transition is always accompanied with energy

change (absorption or emission of photons

and/or phonons, temperature change, etc.)


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e.g.

Ev( )

Ec( )

Eg = Ec - Ev

Photon energy h=Eg(h: Planck const. : Freq.)

Absorption of a photon


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Silicon and Germanium


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Silicon /

IV element

Each atom is bound with four neighbors via Covalent

Bond

Its atomic structure is tetrahedron( )

Monocrystalline silicon polycrystalline silicon


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Atomic structure of silicon

Tetrahedron( )


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2-D representation of the silicon crystal

+4

+4

+4

+4

+4

+4

+4

+4

ValenceValence

electronselectronsSiliconSilicon

atomsatomsCovalentCovalent

bondbond


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26

Carriers

Free electron ---produced by thermal

ionization. It can move freely in the lattice

structure so as to form current

Hole---empty position in broken covalent

bond. It can also move freely to form

current
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Carrier concentration in thermalequilibrium

where

(k: Boltzmann constant)

At room temperature (T=300K) for Si,

inpn ==

kTE

iGeBTn

=

32

10 31.5 10 ( )in cm

Carrier concentration


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strongly depends on temperature. The

high the temperature is, the dramatically

great the carrier concentration is

At room temperature only one of everybillion atoms is ionized

Silicons conductivity is between that of

conductors and insulators. Actually thecharacteristic of intrinsic silicon approaches

to insulators

in

Important notes

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31

Conductivity of the semiconductor can besignificantly changed by doping.

There are two types of doped semiconductors: n type

andp type.

They are used to formpn junction.

Doped semiconductor

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32

Doped semiconductorn type

P

Si

Si Si Si Si Si Si Si




Si Si Si Si SiSi+

Free E

DonorDonor

bound chargebound charge


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Carrier concentration forn type

In a n type Si, the following relationships hold (atroom temperature):

and

0 0

0 0

n i n

n n i

n n p

n p n

>> >>

+ >>

0

2

0 /

n D

n i D

n Np n N


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Acceptor--- trivalent impurity provides holes (usuallyentirely ionized)

Negative bound charge--- impurity atom accepting hole

give rise to negative bound charge

Majority carriers---holes (mostly generated by ionized

acceptor and a tiny small portion by thermal ionization)

Minority carriers--- free electrons (only generated by

thermal ionization.)

p type semiconductor

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Thermal equilibrium equation

Electric neutral equation

2

0 0p p ip n n =

0 0p p Ap n N= +

Carrier concentration forp type

whereNA is the acceptor concentration

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39

Carrier concentration forp type

In ap type Si, the following relationships hold (atroom temperature):

and

0 0

0 0

p i p

p p i

p n n

p n n

>> >>

+ >>

0

2

0 /

p A

p i A

p Nn n N

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Majority carrier is only determined by the

impurity. It is independent of temperature.

Minority carrier is strongly affected by

temperature.

If the temperature is high enough, the

characteristic of doped semiconductor will

decline to that of intrinsic semiconductor

Conclusion on the doped semiconductor

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Onp type semiconductor (substrate), n type

semiconductor can be formed by injecting

donors with into the specific area.

or reversely.

AD NN >>

Doping compensation

NA

ND+

ND

NA+

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The boundary between n andp typesemiconductor is thepn junction.

This is the basic step for VLSI fabrication

technology.

Doping compensation


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Semiconductor materials

III-V:

Gallium arsenide (GaAs)---used

for microwave circuits

InP---used for optoelectronics

II-VI: used for luminescence, IF, etc.

IV:

Silicon---todays IC technology is based entirely on silicon

Germanium---early used

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There are two mechanisms for holes and freeelectrons to move in the silicon crystal.

Drift The carrier motion is generated by the electrical field

across a piece of silicon. This motion will produce driftcurrent.

Diffusion The carrier motion is generated by the different

concentration of carrier in a piece of silicon. The diffused

motion of carriers from higher concentration to lower onewill give rise to diffusion current

Carriers movement


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50

iff i d diff i


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diffusion

A bar of intrinsic silicon (a) in which the hole concentration profile

shown in (b) has been created along thex-axis by some unspecified

mechanism.

Diffusion and diffusion current


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Einstein relationship exists between thecarrier diffusivity and mobility:

where VT is thermal voltage( ), At

room temperature

q

kTV

DDT

p

p

n

n ===

25TV mv;

Einstein relationship

53

J i


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Thepn junction under open-circuit

condition

I-V characteristic ofpn junction

Terminal characteristic of junction diode.

Physical operation of diode.

Junction capacitance

pn Junction

54

J i


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Usually thepn junction is asymmetric,p+

n orpn+

The superscript + denotes the region of

more heavily doped in comparison with theother region

pn Junction


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i d i i di i


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(a) thepn junction without

applied voltage (open-

circuited terminals)

(b) the potential distribution alongan axis perpendicular to the

junction.

pn Junction under open-circuit condition

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P d f f i j i


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The procedure of formingpn: the dynamic equilibrium of

drift and diffusion movements for carriers in the silicon:

Procedure of formingpn junction

Diffusion

Space charge

Drift

Equilibrium


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P d f f i j ti


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Space charge region Recombining of electrons and holes results in thedisappearance of carriers (depletion)

Bound charges are no longer neutralized by majority

carriers and are then uncovered.

There is a region close to the junction where majority

carriers on both side are depleted and there are

uncovered bound charges of different polarity

This region is called carrier-depletion region( ) or space charge region( ). It acts asa barrier( ) preventing the majority carriersfrom further diffusion


60

P d f f i j ti


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Drift Electric field is established across the space

charge region.

Direction of electronic field is from n side top

side. It helps minority carriers drift through the

junction. The direction of drift current is from n

side top side.


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P d f f i j ti


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Equilibrium Two opposite currents across the junction is equal

in magnitude.

No net current flows across thepn junction.

Equilibrium condition is maintained by the

barrier voltage.


62

J ti b ilt i lt


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The junction built-in voltage( )

It depends on doping concentration and

temperature

Its TC is negative.

2ln

i

DATo

n

NNVV =

Junction built-in voltage

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Width of the depletion region


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Width of the Depletion Region:

Depletion region exists almost entirely on the slightlydoped side.

Width depends on the voltage across the junction.

o

DA

depo VNNq

W )11

(2

+=

))11(2 VVNNq

W oDA

dep +=

Width of the depletion region

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I V Characteristics


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The diode iv relationship with some scales expanded

and others compressed in order to reveal details

I-V Characteristics


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Th j ti d f d bi
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Thepnjunctionexcited by a constant-

current source

supplying a currentIin

the forward direction.

The depletion layer

narrows and the barrier

voltage decreases by V

volts, which appears as

an external voltage in

the forward direction.

Thepn junction under forward-bias
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Total current under forward bias


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0 0 2

( ) (( ) ( )

( )( 1) ( )( 1)

( 1)

n p

T T

T

pD nD pD nD

x x x x

V Vp n n p pV Vn

i

p n p D n A

VV

s

dp x dn xI I I A J J A q qdx dx

D p D n D DqA e qAn e

L L L n L n

I e

= =

= + = + = +

= + = +

=

Total current under forward-bias

where

Is---saturation current

A---junction cross-sectional area

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I V characteristic equation


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Nonlinear (exponential relationship)

Is (saturation current) strongly depends on temperature

n=1 or 2, in general n=1

1)Tv nVs

i I e

I-V characteristic equation

e.g. assuming V1atI1 and V2 atI2, then:

For a decade changes in current, the diode voltage drop changes by

60mv (for n=1) or 120mv (for n=2)

2 2

2 1 1 1

ln 2.3 lgT T

I IV V nV nV

I I= =

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Turn on voltage


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A conduction diode has approximately a constant voltage

drop across it. Its called turn-on voltage.

Diodes with different current rating will exhibit the turn-onvoltage at different currents.

Negative TC,

VV

VV

onD

onD

25.0

7.0

)(

)(

=

= For silicon

For germanium

CmvTC /2=

Turn-on voltage

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The pn junction under reverse bias
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Thepn junction excited by aconstant-current sourceI in the

reverse direction.

To avoid breakdown,I is kept

smaller thanIS.

Note that the depletion layer

widens and the barrier voltage

increases by VRvolts, which

appearsbetween the terminals

as a reverse voltage.

Thepn junction under reverse-bias

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Carrier distribution under reverse bias
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Carrier distribution under reverse-bias

UR

pn0

nn0pp0

np0 x

p-typearea

n-type

area

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whereIsis the saturation current. It is proportional to ni2,

which is a strong function of temperature.

sIi=

)(

)(

2

00

An

n

Dp

p

i

n

pn

p

np

s

nL

D

nL

D

qAn

L

nD

L

pDqAI

+=

+=

Independent of voltage


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The pn junction in the breakdown region


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Thepnjunction excited by a reverse-current sourceI,whereI> IS

Thepn junction in the breakdown region

The junction breaks down, and a voltage VZ, with the

polarity indicated, develops across the junction.


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Breakdown mechanisms


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Remember:pn junction breakdown is not a destructive

process, provided that the maximum specified

power dissipation is not exceeded.

Breakdown mechanisms

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Zener Diode


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Circuit symbol

The diode ivcharacteristic

with the breakdown region

shown in some detail.

Zener Diode

79

Junction Capacitance


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Diffusion Capacitance Charge stored in bulk region changes with the change of

voltage acrosspn junction gives rise to capacitive effect

Small-signal diffusion capacitance


CCdd,,

80



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Depletion capacitance Charge stored in depletion layer

changes with the change of voltage

acrosspn junction, which gives rise

to capacitive effect. Small-signal depletion capacitance


UURR

UURR++UU

CCbb

PNPN

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Diffusion Capacitance


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According to the definition:

The charge stored in bulk region is obtained from the following

equations:

Qd dV

dQC =

pp

pnonn

xnonp

I

LpxpAq

dxpxpAqQn

=

=

=

])([

])([

nnn IQ =

82



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The expression for diffusion capacitance:

Forward-bias, linear relationship

Reverse-bias, almost inexistence

=

=

0

)(

)(

][

Q

T

T

Q

T

T

VV

sTd

IV

IV

eIdV

dC T


83

Depletion Capacitance


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According to the definition:

Actually this capacitance is similar to parallel plate

capacitance.

QR VVR

j dVdQC

=

=

)1(

))(11

(2

[

0

0

o

R

j

R

BA

dep

j

VV

C

vVNNq

A

W

AC

+

=

++

=


84



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A more general formula for depletion capacitanceis :

where m is called grading coefficient.

If the concentration changes sharply,

mR

j

j V

CC

)V

1(0

0

+

=

2

1~3

1=m

2

1=m


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Remember:

a) Diffusion and depletion capacitances are

incremental capacitances, only are applied under the

small-signal circuit condition.

b) They are not constants, they have relationship withthe voltage across the pn junction.


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Summary


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Summary

Si and Ge are IV elements with tetrahedronatomic structure

They can be used to manufacture various

devicesSi is dominant because

better thermal stability due to large bandgap

abundant (27 % in the Earth) and cheap

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Summary


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When you find yourself competingwith silicon, dont.

Arno Penzias, joined Bell Lab in 1961 best known for his work in radio astronomy,

winning a Nobel Prize in 1978 for research thatenabled a better understanding of the origins of theuniverse.

Tingye Li, joined Bell Lab in 1957 a world-renowned scientist in the fields of microwaves,

lasers and optical communications. His innovational work

on lightwave communications has had a far-reaching

impact on information technology for decades

Summary

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Homework


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Homework

February 21 1.3 1.10 1.14 1.27

February 28

3.33 3.35 3.36 3.39 3.42


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SJTU J. Chen 8903/06/13

Part 2. Analysis of diode circuitsand applications of diodes

90

Analysis of Diode Circuit


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Models Mathematic model

Circuit model

Methods of analysis Graphical analysis

Iterative analysis

Modeling analysis

y


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92

The Diode Models


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Mathematic Model

The circuit models are derived by approximating

the curve into piecewise-line.

=

s

nVv

s

nVv

s

I

eI

eIi

T

T )1(

Forward biased

Reverse biased

93

The Diode Models


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Circuit Modela) Simplified diode model

b) The constant-voltage-drop model

c) Small-signal model

d) High-frequency model

e) Zener Diode Model

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Ideal Diode Model


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Forward bias

short circuitReverse biasopen circuit


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Simplified Diode Model


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Piecewise-linear model of the diode forward characteristic and its

equivalent circuit representation.

p

97

Constant-Voltage-Drop Model


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The constant-voltage-drop model of the diode forward characteristics

and its equivalent-circuit representation.

g p

98

Small-Signal Model


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Development of the diode small-signal

model. Note that the numerical values shown

are for a diode with n = 2.

g

rd

99

Small-Signal Model


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Incremental resistance:

*The signal amplitude sufficiently small such

that the excursion at Q along the i-v curve is

limited to a short, almost linear segment.

)2,1(, == nI

nVr

DQ

Td

g

100

High-Frequency Model


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High frequency model

Related to the bias

Unable to be simply substitute by ideal ones.

g q y

101

Zener Diode Model


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ZZZZrIVV +=

0

102

Method of Analysis


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Graphical Analysis Load line

Diode characteristic

Q operating pointVisualization

103

Load line


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RL

i

v

i

v

VDD/R

VDD

Slope= -1/RL

104

Method of Analysis


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Iterative analysis

Refer to example 3.4 (p.158)

Model Analysis

Refer to example 3.6 (p.167) and 3.7

(p.169)

105

The Application of Diode Circuits


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Rectifier circuits Half-wave rectifier

Full-wave rectifier

The peak rectifier

Voltage regulatorLimiter

106

Half-Wave Rectifier


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(a) Half-wave rectifier.

(b) Equivalent circuit of the half-wave rectifier with the diode replaced byits battery-plus-resistance model.

107

Half-Wave Rectifier


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(c)Transfer characteristic of the rectifier circuit.

(d) Input and output waveforms, assuming that RrD


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In selecting diodes for rectifier design, two

important parameters must be specified:

The current-handling capability

The peak inverse voltage (PIV)

It is usually prudent, however, to select a

diode that has a reverse breakdown voltage at

least 50% greater than the expected PIV.

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Full-Wave Rectifier


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Transformer with a center-tapped secondary

winding

Bridge rectifier

110

Full-Wave Rectifier


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(a) circuit

(b) transfer characteristic assuming a constant-voltage-drop model for

the diodes

111

Full-Wave Rectifier


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(c) input and output waveforms.

112

The Bridge Rectifier


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(a) circuit

113

The Bridge Rectifier


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(b) input and output waveforms

114

Peak Rectifier


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The pulsating nature of the output voltage

produced by the rectifier circuits discussed

above makes it unsuitable as a dc supply for

electronic circuits.

A simple way to reduce the variation of the

output voltage is to place a capacitor across

the load resistor.

115

Peak Rectifier


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TCR >>

Thefilter capacitorserves to reduce substantially the

variations in the rectifier output

116

Peak Rectifier


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Voltage and current

waveforms in the peak

rectifier circuit with .

The diode is assumed ideal.

TCR >>

117

Peak Rectifier


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Waveforms in the full-wave peak rectifier.

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Self-study


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3.8 Limiting and clamping circuits

3.9 Special diode types

3.10 The SPICE diode model and simulation

examples

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SUMMARY


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The silicon junction diode is basically apn junction. Such a

junction is formed in a single silicon crystal.

The unidirectional-current-flow property makes the diode

useful

A silicon diode conducts a negligible current until the

forward voltage is at least 0.5 V. Then the current increasesrapidly, with the voltage drop increasing by 60 mV to 120 mV

(depending on the value ofn) for every decade of current

change.

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SUMMARY


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The forward conduction of practical silicon diodes

is accurately characterized by the relationship:

T

vnV

si I e=

121

SUMMARY


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In the reverse direction, a silicon diode

conducts a current on the order of 10-9A.

This current is much greater thanIs and

increases with the magnitude of reverse

voltage.

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SUMMARY


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Beyond a certain value of reverse voltage

(that depends on the diode) breakdown

occurs, and current increases rapidly with a

small corresponding increase in voltage.

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SUMMARY


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A hierarchy of diode models exists, with the

selection of an appropriate model dictated by

the application.

In many applications, a conducting diode is

modeled as having a constant voltage drop,

usually about 0.7V.

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Homework


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March 7, 2011

3.4 3.9 3 .22 3.74 3.84

Zener Diodes Working

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