diodes elz 206 - elektronik i - dicle ü · pdf fileadel s. sedra, kenneth c. smith ......

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Copyright 2004 by Oxford University Press, Inc.

Diodes

ELZ 206 - Elektronik I

Dr. Mehmet Siraç Özerdem

Department of Electrical and Electronics Engineering

Dicle University

Microelectronic Circuits – Fourth Edition

Adel S. Sedra, Kenneth C. Smith, 1998 Oxford University Press


The ideal diode

diode circuit symbol

i–v characteristic

equivalent circuit in the

reverse direction

equivalent circuit in the

forward direction

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Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith

The ideal diode - Example

The two modes of operation of ideal diodes and the use of

an external circuit to limit the forward current and the

reverse voltage.



Rectifier circuit Input waveform

Equivalent circuit when vI 0 Equivalent circuit when vI ≤ 0

Output waveform

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Rectifier circuit

Input waveform

Output waveform

Input

Transfer

characteristic



Rectifier circuit

Input waveformvD waveform ?

vD waveform

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Example

vD waveform vD waveform



Example

Diode logic gates

OR gate AND gate

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Example #

D1, D2 : ideal

V = ?

I = ?



Example # #

D1, D2 : ideal

V = ?

I = ?

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The i–v characteristic of a silicon junction diode.



The diode i–v relationship with some scales expanded

and others compressed in order to reveal details.

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Illustrating the temperature dependence of the diode

forward characteristic. At a constant current, the

voltage drop decreases by approximately 2 mV for

every 1 C increase in temperature.



Simplified physical structure of the junction diode.

Physical Operation of Diodes

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Two-dimensional representation of the silicon crystal. The circles represent

the inner core of silicon atoms, with +4 indicating its positive charge of +4q,

which is neutralized by the charge of the four valence electrons. Observe

how the covalent bonds are formed by sharing of the valence electrons.

At 0 K, all bonds are intact and no free electrons are available for current

conduction.



At room temperature, some of the covalent bonds are broken by

thermal ionization. Each broken bond gives rise to a free

electron and a hole, both of which become available for current

conduction.

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

profile shown in (b) has been created along the x-axis by some

unspecified mechanism.

Diffusion and Drift



A silicon crystal doped by a pentavalent element. Each

dopant atom donates a free electron and is thus called a

donor. The doped semiconductor becomes n type.

n type

semiconductor

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A silicon crystal doped with a trivalent impurity. Each

dopant atom gives rise to a hole, and the semiconductor

becomes p type.

p type

semiconductor


A simple circuit used to

illustrate the analysis of

circuits in which the diode is

forward conducting.

Analysis of Diode Circuits

For solution

Graphical analysis

Iterative analysis

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Graphical Analysis



Example

ID, VD=?

VDD=5V

R=1k ohms

Assuming that ID=1mA at VD=0.7V and its voltage drop

changes 0.1V for every decade change in current.

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Approximating the

diode forward

characteristic with

two straight lines:

the piecewise-linear

model.

Simplified diode models



Piecewise-linear model of the diode forward

characteristic and its equivalent circuit representation.

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Example

ID, VD=?

VDD=5V

R=1k ohms

Use piecewise linear model (VD0=0.65V, rD=20 ohms)



Example - solution

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Development of the

constant-voltage-drop model

of the diode forward

characteristics. A vertical

straight line (B) is used to

approximate the fast-rising

exponential. Observe that

this simple model predicts

VD to within 0.1 V over the

current range of 0.1 mA to

10 mA.

Simpler model of the diode for forward region



The constant-voltage-drop model of the diode forward

characteristics and its equivalent-circuit representation.

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Example

ID = ?

VDD = 5V

R = 1k ohms

Use simpler piecewise linear model (VD0=0.65V)



Modeling the Diode Forward Characteristic

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Modeling the Diode Forward CharacteristicDr. Mehmet Siraç Özerdem


The small-signal model and its application

Development of the diode small-signal model.

Note that the numerical values shown are for a

diode with n = 2.

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Application

The small-signal approximation allows one to seperate the dc

analysis from the signal analysis



Example

V + = 10V (DC) + 1V peak amplitude sinus (AC)

ID = ?

rd = ?

vd (peak-to-peak) = ?

Assume that VD=0.7V → ID=1mA, n=2

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Example - solution

Circuit for

Example

Circuit for

calculating the

dc operating

point.

Small-signal

equivalent circuit



Example

A string of diodes is used to provide a

constant voltage of about 2.1V.

n=2, VT = 25mV

a) Diode current (no load)

b) rd = ?

c) Input (10±1 V) → Output (2.1± ? V)

d) Calculate the change of output

voltage ∆vo = ? (with load)

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Example

A string of diodes is used to provide a

constant voltage of about 2.8V.

n=2, VT = 25mV

a) Diode current

b) rd = ?

c) Input (15±1 V) → Output (2.8± ? V)



Circuit symbol for

a zener diode.

Operation in the reverse breakdown region – zener diode

The diode i–v characteristic

with the breakdown region

shown in some detail.

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Example



Example

6.8V zener diode is specified to have Vz=6.8V

at Iz=5mA.

rz=20 ohms and IZK=0.2mA.

a) Vo = ? (no load)

b) ∆Vo = ? for a ±1V change in V+ (no load)

c) RL=2k ohms, ∆Vo = ?

d) RL=0.5k ohms, ∆Vo = ?

e) RLmin = ? For which the diode still

operates in breakdown region.

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Example - solution

The circuit with the

zener diode replaced

with its equivalent

circuit model.Circuit for

Example



Design of the zener shunt regulator

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Example It is required to design a zener shunt

regulator to provide an output

voltage of approximately 7.5V

The raw supply varies between 15

and 25V

The load current varies over the

range 0 to 15mA.

(Vz=7.5V at a current Iz=20mA

and rz=10 ohms)

a) R= ?

b) Line and load regulation = ?

c) ∆Vo = ? for full change in Vs.

d) ∆Vo = ? for full change in IL.



Example

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Example



Rectifier Circuits

Block diagram of a dc power supply

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The Half-Wave RectifierDr. Mehmet Siraç Özerdem


Full-Wave RectifierTransfer characteristic assuming

a constant-voltage-drop model

for the diodes

Full-wave rectifier

utilizing a transformer

with a center-tapped

secondary winding

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The Bridge RectifierDr. Mehmet Siraç Özerdem


The rectifier with a filter capacitor

The circuit provides a dc

voltage equal to the peak of the

input sine wave.

A simple circuit used

to illustrate the effect

of a filter capacitor.

Diodes: ideal

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The rectifier with a filter capacitor - more practical situation -

Voltage and current

waveforms in the peak

rectifier circuit with CR

@ T. The diode is

assumed ideal.



Limiter Circuits

General transfer characteristic

for a limiter circuit.

Applying a sine wave to a limiter can

result in clipping off its two peaks.

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Limiter Circuits

Soft limiting.



A variety of

basic limiting

circuits.

Limiter CircuitsDr. Mehmet Siraç Özerdem

diodes elz 206 - elektronik i - dicle ü · pdf fileadel s. sedra, kenneth c. smith ......

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