Special Purpose Diodes

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Chapter 3

• Special Purpose Diodes

Fahmi SamsuriAnalog Electronics
Faculty of Electrical Electronic Engineering
KUKTEM
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FIGURE 3-1 Zener diode symbol.
1. ZENER DIODE
Zener Diode- is a silicon pn junction device
that differ from rectifier diodes
because it is designed for operation
in the reverse- breakdown region.
- if zener doide is forward-biased it
operates the same as a rectifier
diode. Function - to provide a stable
reference voltage for use in power
supplies voltmeter other
instruments voltage regulators.
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FIGURE 3-2 General diode V-I characteristic.
Zener breakdown- occurs in a zener diode at low
reverse voltages. - zener diode is heavily
doped to reduce the breakdown voltage. This
causes a very thin depletion region.
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FIGURE 3-3 Reverse characteristic of a zener
diode. VZ is usually specified at the zener test
current IZT and is designated VZT.

• When the reverse voltage (VR) is
  increased the reverse current (IR) remains
  extremely small up to the knee of the curve.
• From the bottom of the knee the zener breakdown
  voltage (Vz) remains essentially constant
  although it increases slightly as the zener
  currentIz increases.

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FIGURE 3-4 Zener diode equivalent circuit
models and the characteristic curve illustrating
ZZ.

• Ideal model of a zener diode in reverse
  breakdown.
• Practical model of a zener diode in reverse
  breakdown.

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Example 1 A zener diode exibits a certain
change in Vz on a portion of the linear
characteristic curve between Izk and Izm as
ilustrated below. What is the zener impedence
SOLUTION
Zz Vz 50mV 10 Iz 5mA
Answer 10 ohm
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Example 2 From the datasheet a 1N4736A zener
diode has a Zz of 3.5. The datasheet gives Vz
6.8V at a test current Iz of 37mA. What is the
voltage across the zener terminals when the
current is 50mA When the current is 25mA Figure
below represent the zener diode.
Answer When Iz50mA Vz 6.85V When Iz
25mA Vz 6.76V
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Temperature Coefficient

• The temperature coefficient specifies the
  percentage change in zener voltage for each C
  change in temperature.
• Formula for calculating the change in zener
  voltage for a given junction temperature change
  for a specified temperature coefficient is
• Vz Vz X TC X T
• Vz nominal zener voltage at 25C
• TC temperature coefficient
• T the change in temperature
• Positive TC zener voltage increases with an
  increase in temperature or
• decreases with a decrease in
  temperature.
• Negative TC zener voltage decreases with an
  increase in temperature or
• increases with a
  decrease in temperature.

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• In some cases the temperature coefficient is
  expressed in mV/ C rather than as /C.
• For this cases Vz is calculated as
• Vz TC X T
• Example 3
• An 8.2V zener diode (8.2V at 25 C) has a
  positive temperature coefficient of 0.05 / C.
  What is the zener voltage at 60 C

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FIGURE 3-7 Partial data sheet for the
1N4728-1N4764 series 1 W zener diodes.
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FIGURE 3-8 Zener regulation of a varying input
voltage.
2. Zener Diode Applications

• Zener Diode are widely used for voltage
  regulation.
• From figure when the input voltage varies
  (within limits) the zener diode maintains a
  nearly constant output voltage across its
  terminals.
• However as Vin changes Iz will change
  proportionally so that the limitations on the
  input voltage variation are set by the minimum
  and maximum current values (Izk Izm) with which
  the zener can operate.
• Resistor R is the series current-limitting
  resistor.

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Example 4 Determine minimum and maximum input
voltage VIN Suppose that 10V zener diode in
Figure below can be maintain regulation over a
range of zener current values from Izk0.25mA to
Izm100mA.(IzmPD(max)/ Vz 1W / 10V 100mA).
Answer Vin(min) 10.055V Vin(max) 32V
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Example 5 Determine the minimum and maximum
input voltage that can be regulated by the zener
diode in figure below. From the datasheet the
following information for the 1N4733 is obtained
Vz 5.1V at IZT 49mA and Zz 7 at IZT.
Power dissipation is 1W.
Continue
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FIGURE 3-11 Equivalent of circuit in Figure
3-10.
- Assume this value of Zz over the range of
current values. The equivalent circuit is shown
in figure below
Answer Vin(min)4.86V Vin(max)25.7V
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Zener regulation with a variable load.

• The zener diode maintains a nearly constant
  voltage across RL as long as the zener current is
  greater than IZK and less than IZM.
• From No Load To Full Load
• When the output terminals of the zener regulator
  are open (RL 8) the load current is zero and
  all of the current is through the zener.
• When a loaad resistor (RL) is connected part of
  the total current is through the zener and part
  through RL.
• As RL the load current IL and IZ .
• - The zener diode continues to regulate the
  voltage until IZ reaches the minimum value IZK
  and the load current is maximum. The total
  current through R remains essentially constant.

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Example 6 Determine the minimum and maximum
load currents for which the zener diode in figure
maintain regulation. What is the minimum value
of RL that can be used Vz12V Izk1mA and
Izm50mA. Assume an ideal zener diode where Zz0
and Vz remains a constant 12V over the range of
current values for simplicity.
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• Example 7
• For the circuit below
• Determine Vout at Izk and Izm.
• Calculate the value of R that should be used
• Determine the minimum value of RL that can be
  used.
• -From datasheet 1N4744 Vz15V_at_ IZT IZK
  0.25mA IZT 17mA ZZT 14

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FIGURE 3-15
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FIGURE 3-16 Basic zener limiting action with a
sinusoidal input voltage.
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FIGURE 3-17
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FIGURE 3-18
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FIGURE 3-19 The reverse-biased varactor diode
acts as a variable capacitor.
2. VARACTOR DIODE
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FIGURE 3-20 Varactor diode capacitance varies
with reverse voltage.
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FIGURE 3-21 Varactor diode symbol.
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FIGURE 3-22 Partial data sheet for the
1N5139-1N5148 varactor diodes.
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FIGURE 3-23 A resonant band-pass filter using
a varactor diode for adjusting the resonant
frequency over a specified range.
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FIGURE 3-25 Symbol for an LED. When
forward-biased it emits light.
3.LED
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FIGURE 3-26 Electroluminescence in a
forward-biased LED.
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FIGURE 3-27 Basic operation of an LED.
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FIGURE 3-28 Examples of typical spectral
output curves for LEDs.
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FIGURE 3-29 General radiation pattern of a
typical LED.
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FIGURE 3-30 Typical LEDs.
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FIGURE 3-31 Partial data sheet for an MLED81
1R light-emitting diode.
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FIGURE 3-32 The 7-segment LED display.
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FIGURE 3-33 Photodiode.
4. PHOTODIODE
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FIGURE 3-34 Typical photodiode characteristics.
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FIGURE 3-35 Operation of a photodiode.
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FIGURE 3-36 Partial data sheet for the MRD821
photodiode.
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FIGURE 3-37 Symbol for a current regulator
diode.
5.CURRENT REGULATOR DIODE
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FIGURE 3-38 Typical characteristic curve for a
current regulator diode.
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FIGURE 3-39 Schottky diode symbol.
6. SCHOTTKY DIODE
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FIGURE 3-40 Basic internal construction of a
Schottky diode.
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FIGURE 3-41 PIN diode.
7. PIN DIODE
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FIGURE 3-42 PIN diode characteristics.
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FIGURE 3-43 Tunnel diode symbols.
8. TUNNEL DIODE
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FIGURE 3-44 Tunnel diode characteristic curve.
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FIGURE 3-45 Parallel resonant circuit.
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FIGURE 3-46 Basic tunnel diode oscillator.
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FIGURE 3-47 Basic laser diode construction and
operation.
9. LASER DIODE
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FIGURE 3-51 Basic system concept and block
diagram of the counting and control system.
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FIGURE 3-52 Zener regulated power supply
preliminary schematic.
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FIGURE 3-53 Basic schematics of the IR emitter
and IR detector circuits.
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FIGURE 3-57 Diode symbols.
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FIGURE 3-58
Question 1 From the characteristic curve in
figure what is the approximate minimum zener
current (Izk) and approximate zener voltage at
Izk
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FIGURE 3-59
Question 2Determine the minimum input required
for regulation to be established. Assume an ideal
zener diode wih Izk 1.5mA and Vz14V.
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FIGURE 3-60
Question 3 To what value must R be adjust to
make Iz40mA Assume Vz12V at 30mA and Zz30.
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FIGURE 3-61
Question 4 A loaded zener regulator is shown ib
figure below. Vz5.1V at Iz49mA Izk 1mA Zz7
and Izm70mA. Determine the minimum and maximum
permissible load currents.
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FIGURE 3-62
Question 5 Figure below is a curve of reverse
voltage versus capacitance for a certain
varactor. Determine the change in capacitance if
Vr varies from 5V to 20V.
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FIGURE 3-63
Question 6 What capacitance value is required
for each of the varactors in figure below to
produces a resonant frequency of 1 MHz
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FIGURE 3-64
Question 7 The LED in figure (a) below has a
light producing characteristic as shown in part
(b). Neglecting the forward voltage drop of the
LED determine the amount of radiant (light)
power produced in mW.
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FIGURE 3-66
Question 8 What is the resistance of each
photodiode in figure below