diodes applications

7/30/2019 Diodes Applications

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Prof. Mohammed Gulam Ahamad


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Zener Diodes Zener characteristics


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Diode types Rectifier

Zener

Lower Vz due to

more heavily doped

Zener diode operates in the breakdown region


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Made of silicon pn junction Heavily doped to reduce the breakdown voltage Used for operation in the reverse breakdown

region Called reverse break down (1.2


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Vz=Nominal Zener voltage; specific for IZT;typically has some tolerance

IZT=Zener test current; related to Vz Vz=Change in Zener voltage IZ=Current going through the diode for

different values of Vz ZZ=Zener impedance (Vz/z)

z= IZ - IZT IZK=Minimum dc current

IZM=Maximum dc current PD(max)=The max. power dissipated by thediode (IZM x Vz); IZM = PDmax / Vz

Power Derating (DF is derating factor (mW /oC) PD(derated) = PD(max)@T1-DF(T2-T1)

TC=Temperature coefficient (% /oC) or (mW/oC) Vz= xTC x Vz; is the change of

temperature

Iz

IZT

ZzT=Linear

In ideal model we assume Z is zero

Minimum currentrequired to

operate in

breakdownregion

Maximum current

the diode can

handle

Range at which voltage is

maintained while current

is changing


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Varactor diodes Light-emitting diodes Schottky diodes

PIN diodes


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Operate in reversebias

Dielectric is actingas capacitor

C capacitance,A=plate area,

d=thickness

dAC =

Depletion

Region PlateSeparationCapacitanceVR


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Tuning Ratio (TR) Nominal Capacitance is typically given

(47 pF at VR=4 V)

Example: 1N5139 Applications

Tuning circuits in TV to set theresonant frequency

Bandpass filter (R+(L||C))LC

fr2

1=

(max)

(min)

R

R

V

V

C

CTR =


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Accume Vcont(min) = 2.9

Vcont(max) = 29 Hence

C2.9 = 55 pF

C29 = 17 pF

LCfr

2

1=

fmin = 679 MKhz

fmax = 1.22 MHz


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Forward-biased Recombining free electrons

with holes releases energy heat and light

The process in whichphotons emit visible light is

called electroluminescent The color of the LED

depends on the addedimpurity (differentwavelengths)

If the color is Invisible

Infrared


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Radiation intensity, Ie (power/steradian =

W/Sr) Sr is the unit of solid angular measurement

Irradiance = H = Ie / d2. (mW/cm2)

General radiation pattern

of a typical LED.Max Ie

can be obtaine

d from IF vs Ie graph


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If H = 5, what is I?


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Photodiode Reverse biased

Reverse current increases withlight

Note that in rectifier diode,reverse current increases withtemperature

Can also be used as variableresistor

Dark current is referred to thereverse current when there is nolight

Example: MDD821

Note how the diode isPlaces in the circuit


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Used for fast switching andhigh frequency

applications Uses metal-to-

semiconductor junction V forward = 0.3 No reverse leakage Rapid response to changes

(no hole current)


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Has an intrinsic region It has a constant capacitance when reverse

biased When forward-biased current can be

controlled through changing the resistance


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The circuit above Vout should not change while Vinchanges

1. What kind of diode do we need? Show how it is placed inthe circuit.

2. Is the diode reversed or forward biased?

3. Model the practical diode of the diode assuming it is a1N4733. Show the equivalent circuit after modeling.

4. Determine the range of the input voltage in which the

output voltage stays the same. What is the minimum andmax values of the output voltage?

5. Draw Vin Vout voltage plot for the above points youcalculated.

6. Use Multisim to verify your plot above.


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What color of light is being emitted? Assume IF for the light-emitting diode is

13 mA. What is the radiance of light-emitting diode? (Find H)

Find the sensitivity of the photodiode.What is its unit?

Find the reverse current through thephotodiode

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