chapter2 diode see2063

7/31/2019 Chapter2 DIode SEE2063

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Cha ter 2

DIODE

Part 1

Resistance Levels

Chapter 2 DIODE, Part 1 Resistance LevelSEE 2063 1


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I-V Characteristics : Silicon vs. Germanium

Peak Inverse Voltage (PIV) isthe maximum reverse-biasPotential that can be a liedbefore entering the Zenerregion.

Si: PIV ~ 1000VGe: PIV ~ 400V

Si: temperature up to 200 C.Ge: temperature below 100 C.

: T~ .Ge: VT~ 0.3V



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Resistance Levels

i) DC or Static Resistance The application of DC voltage to acircuit containing a semiconductordiode will result in an o eratin ointon the characteristic curve that will notchange with time.

The dc resistance levels at the kneeand below will be greater than theresistance levels obtained for the

vertical rise section of thecharacteristics.

Resistance, RDat the operating pointThe lower the current through adiode the higher the dc resistance

level.



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Example : DC or Static Resistance

: e erm ne e c res s ance eve s orthediode at

D=(b) ID= 20mA

(c) VD= -10V

Solution



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ii) AC or Dynamic Resistance

If a sinusoidal voltage(signal) is applied,the varyinginput will move the instantaneous operating point upand down a region of the characteristics and

us e nes a spec c c ange n currenand voltage.

With no applied varying signal, the point of operation-

applied dc levels.Quiescent: still or unvarying

AC Resistance, rd

e ower t e -po nt o operat on(smaller current or lower voltage) thehigher the AC resistance.



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Example : AC or Dynamic Resistance

Solution



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iii) Average AC Resistance

produce a broad swing, the resistanceassociated with the device for this regionis called the average ac resistance.

The average ac resistance is the resistance

determined by a straight line drawn between

maximum and minimum values of inputvoltage.

minimum

The lower the level of currents used todetermine the average resistance the higherthe resistance level.



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Summary of Resistance Levels



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Cha ter 2

DIODE

Part 2

Diode Equivalent Circuits

Chapter 2 DIODE, Part 2 Diode Equivalent Circuits 1


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An equivalent circuit is a combination of elements properly chosen to best represent theActual terminal characteristics of a device, system, or such in a particular operating region.

i) Piecewise-Linear Equivalent CircuitOne technique for obtaining an equivalent circuit for a diode is to approximate the characteristicsof the device by the straight-line segments. For a sloping section,the average ac resistance isthe resistance level appearing in the equivalent circuit of the actual device.

only one direction of conduction through a device, anda reverse-bias condition will result in the open-circuit

state for the device.Since a diode does not reach the conduction stateuntil VDreaches 0.7V with the forward bias, a batteryVTopposing the conduction direction must appear in

.



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ii Sim lified E uivalent Circuit

For most applications, the resistance rav is sufficiently small to be ignored incomparison to the other elements of the network.

It states that a forward-bias silicon diode in an electronic system under dc

conditions has a drop of 0.7V across it in the conduction state at any level ofo e.



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iii Ideal E uivalent Circuit

Let us take it a step further and establish that a 0.7V level can often be ignored incomparison to the applied voltage level.



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Summary of Diode Equivalent Circuits



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Cha ter 2

DIODE

Part 3

Diode Notation and Testing

Chapter 2 DIODE, Part 3 Diode Notation and Testing 1


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The notation frequently used for semiconductor diodes.

Various type ofjunction diodes.



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ON state :

The meter has ansource (~2mA) that willdefine the voltage level

~ . .

Digital TesterOL indication:O en



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Cha ter 2

DIODE

Part 4

Load Line Analysis

Chapter 2 DIODE, Part 4 Load Line Analysis 1


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Load Line AnalysisThe a lied load will have an im act on the oint or re ion of a device. A line can

be drawn on the I-V characteristics of the device that represents the applied load.

Kirchoffs voltage law

Load



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Example 1: Determine (a) VDQ and IDQ (b) VR



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Example 2: Change R to 2kOhm. Determine (a) VDQ and IDQ (b) VR



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Example 3: Repeat example 1. Determine (a) VDQ and IDQ using approximate



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Example 4: Repeat example 2. Determine (a) VDQ and IDQ using approximate



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Example 5: Repeat example 1. Determine (a) VDQ and IDQ using ideal diode model



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Approximate and Ideal Diode Models

rav was not employed because rav is typically muchLess than the other series elements of the network.

rav was not employed because rav is typically muchLess than the other series elements of the network.



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Cha ter 2

DIODE

Part 4

Series Diodes Configurations

with DC Inputs

Chapter 2 DIODE, Series Diode Configurations 1


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Series Diode Configuration with DC Inputs

Series diode configuration

(1) Determining the state of the diode (2) Substituting the equivalent

model for the on diode



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(1) Determining the state of the diode

(2) Substituting the equivalent

model for the off diode



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Example 6: Determine VD, VRand ID.

Solution

e app e vo tage esta s es a current nthe clockwise direction to match the arrow

of the symbol and the diode is in the ONs a e .




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Example 7: Repeat example 6 with the diode reversed.Determine V V and I .

Solution

in the diode symbol. Diode is in OFF stateresulting to open circuit.


1. An open circuit can have any voltage across its terminals, but the current is always 0 A.

2. A short circuit has a 0V drop across its terminals, but the current is limited only by theSurrounding network.

ource notat on



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Example 8: Determine VD, VRand ID.

The level of the applied voltage is

OFF Operating pointSeries diode configuration

insufficient to turn the silicon diodeON. The point of operation on thecharacteristics shown in the figureestablishing the open circuit equivalent.



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Example 9: Determine VD, and ID.

o u on:

The resulting current has the same directionas e arrow ea s o e sym o s o o o es.

Here, E=12V > (0.7V + 0.3V)= 1V.

Determining the state of the diode and



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Example 10: Determine VD, Voand ID.The combination of aShort circuit in seriesWith an open circuit alwaysResults in an open circuit

= .

e erm n ng e s a e o e o e ansubstituting the equivalent model for the diodeSeries diode configuration



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Example 11: Determine I, V1 V2and Vo .

Series diode configuration Determining the state of the diode andsubstituting the equivalent model for the diode



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Cha ter 2

DIODE

Configurations

w t nputs

Chapter 2 DIODE, Parallel and Series Diodes Conf. 1


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Example 12: Determine Vo , I1 , ID1 and ID2 .

Parallel diode configuration

Determining the state of the diode and

substitutin the e uivalent model for the diode

Assuming diodes of similar characteristics,



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Example 13: Determine I.

Parallel diode configurationDetermining the state of the diode and

substituting the equivalent model for the diode



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Example 14: Determine Vo . The applied voltage will turn both diodes on. , , .

the silicon diode would not match 0.3V across the Gediode as required by the fact that the voltage acrossparallel elements must be the same.

Parallel diode configuration

Determining the state of the diode andsubstituting the equivalent model for the diode


E l 15 D i I I d I


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Example 15: Determine I1,I2and ID2 .

Series and Parallel diode configuration

eterm n ng t e state o t e o e an

substituting the equivalent model for the diode



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Cha ter 2

DIODE

Chapter 2 DIODE, AND / OR Gates 1

E ample 16 Determine V


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Example 16: Determine V0 .

Redrawn OR Gates

Positive logic OR GatesFirst, assume that D1 is on and D2 is off.

Vo = E - VD = 10V 0.7V = 9.3V (level 1)With 9.3V at the cathode (-) side of D2 and0V at the anode (+) side, D2 is in the off state.

ur assump on s correc .


Example 17: Determine V


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Example 17: Determine V0 .

Positive logic AND Gates

Redrawn OR Gates

With 10V at the cathode side of D1, it is assumed that D1 is in the off state even though

1 .D2 is assumed to be in the on state due to the low voltage at the cathode side and theAvailability of the 10V source through the 1kOhm resistor.

The voltage at Vo is 0.7V due to the forward-biased diode D2. With 0.7V at the anode ofD1 and 10V at the cathode, D1 is definitely in the off state.


Ideal Diode


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Ideal Diode

Chapter 2 Ideal Diode 1

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