Diodes Applications
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Transcript of 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