ELE 112
Electronic Circuits
Dr. Ahmed Nader
Professor
Faculty of Engineering, Galala University
Fall 2023
1
Because learning changes everything. ®
Chapter 5
Special-Purpose Diodes
Electronic Principles
Ninth Edition
Albert Malvino, David J. Bates, Patrick E. Hoppe
© 2021 McGraw-Hill. All rights reserved. Authorized only for instructor use in the classroom.
No reproduction or further distribution permitted without the prior written consent of McGraw-Hill.
Topics Covered in Chapter 5
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Zener Diodes.
Light-Emitting Diodes.
7 Segment Displays.
Photodiodes & Optocouplers.
Schottky Diodes.
Varactor.
Special Purpose Diodes.
© McGraw-Hill
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Zener Diode
1
• Zener diodes are designed and optimized to operate
in their “breakdown” region (very sharp knee).
Zener diodes are “reverse biased.”
• A common use for zener diodes is voltage
regulation (it maintains a constant
output voltage even though the
Forward
Reverse
current through it changes).
Breakdown
© McGraw-Hill
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Power Supply
• Power Supplies combine multiple stages.
• Step-Up.
• Step-Down.
• Isolation.
© McGraw-Hill
• Half-Wave.
• Full-Wave.
• Bridge.
• Choke-Input.
• CapacitorInput.
• Integrated
Circuit.
• Discrete
Parts.
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Zener
Effect
• When a diode is heavily doped, the depletion layer
becomes very narrow. Because of this, the electric
field (= voltage/distance) is very intense. When the
field strength is intense enough, it will pull electrons
out of their valence orbits. The creation of free
electrons in this way is called Zener effect.
• Different than avalanche effect where the minority
carriers are accelerated to high enough speeds to
dislodge other minority carriers, producing a chain
effect that results in a large reverse current.
© McGraw-Hill
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Zener Diode
2
• To get breakdown operation, the source voltage VS
must be greater than the Zener breakdown voltage VZ.
• A series resistor R1 is always used to limit the Zener
current to less than its maximum current rating.
© McGraw-Hill
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Zener Diode
3
VS  VR1  VZ  0
VR1  VS  VZ
VR1  7 V
VR1
I R1 
R1
Find IR1??
I R1  7 mA
Zener diodes are typically labeled D1, D2, etc. The label of VZ
was used to emphasize the fixed voltage drop across the zener
diode.
© McGraw-Hill
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Zener Diode
4
Ammeter
Voltmeter
What happens if Vs changes from 20V to 30V?
© McGraw-Hill
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Load Lines
1
• The Q point is where the circuit is operating,
based on the voltage source.
• The Q point is the intersection of the load line
and the Zener diode V-I curve.
• When the source voltage changes, a different
load line appears with a different Q point.
© McGraw-Hill
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Load Lines
2
VS  VZ
IZ 
RS
20V
1kΩ
Calculating the ends of the load line.
© McGraw-Hill
VS
20 V
30 V
IZ (diode shorted)
20 mA 30 mA
VZ (diode open)
20 V
30 V
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Zener Diode With RL
𝑽𝑳 𝑽𝒁
𝑰𝑳 =
=
𝑹𝑳 𝑹𝑳
𝑉𝑇𝐻
1𝑘
= 𝑉𝑆
1𝑘 + 1𝑘
𝑉𝑇𝐻 = 15𝑉
VTH  VZ
This circuit will function properly (Zener diode regulates)
if the zener diode “sees” a VTH greater than the Zener
voltage rating.
VTH
© McGraw-Hill
RL

 VS
R1  RL
𝑽𝑺 − 𝑽𝒁
𝑰𝑺 =
𝑹𝟏
Should be > 0 for
proper regulation
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Example
© McGraw-Hill
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Temperature Coefficient
• When the ambient temperature changes, VZ will change
slightly. The temperature coefficient is defined as the
change in breakdown voltage per degree of increase.
Example: A Zener diode with a breakdown voltage of 4 V
may have a temperature coefficient of -2 mV/°C.
If temperature increases by 1°, the breakdown voltage
decreases by 2 mV.
• In applications requiring a highly stable reference voltage,
a Zener diode is connected in series with one or more
semiconductor diodes whose voltage drops change with
temperature in the opposite direction that VZ changes.
© McGraw-Hill
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2nd Approx. of a Zener Diode
The zener resistance ( RZ ) is typically very small and has
a minor effect on the load voltage.
© McGraw-Hill
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Zener Diode
6
Once the Zener diode is
conducting, continue
increasing the voltage source.
Measure the diode current and
voltage at two distinct voltage
source values.
V2  V1
RZ 
I 2  I1
© McGraw-Hill
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Zener Diode
7
V1  10 V
I D1  20.24 mA
VD1  6.25 V
V1  12 V
I D1  30.47 mA
VD1  6.27 V
6.27 V  6.25 V
RZ 
30.47 mA  20.24 mA
© McGraw-Hill
RZ  1.96Ω
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2nd Approx. of a Zener Diode
2
Given:
I Z  10 mA
RZ  10Ω
VL  VZ  I Z RZ
Where VL  I Z RZ
VL  (10mA)(10Ω)
VL  100 mV
© McGraw-Hill
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Output Ripples
© McGraw-Hill
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Examples
Solution:
© McGraw-Hill
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Zener Diode Ratings
• Maximum power  PZ  max   VZ I Z ( max ) . (safety factor of 2)
• Available tolerances (VZ): 1, 2, 5 and 20 percent.
• Zener resistance, RZT , increases at the knee
of the characteristic curve.
• A derating factor: how much you have to reduce
the power rating of a device. For example,
6.67 mW per degree for temperatures above 50 °C
is typical.
© McGraw-Hill
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Other Applications
Pre-regulator (1st Zener)
• Explain the operation of the above circuits.
© McGraw-Hill
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Optoelectronics
• Optoelectronics is the technology that
combines optics and electronics. This field
includes many devices based on the action of a
pn junction.
• Examples of optoelectronic devices are:
1) LED (Current  Light)
2) Photodiodes (Light  Current)
3) Optocouplers
4) Laser diodes
© McGraw-Hill
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Light-Emitting Diodes
1
• LEDs have replaced incandescent lamps in
many applications because of: lower energy
consumption (efficiency) and longer lifetime.
• The material used for the semiconductor die
will determine the LED’s characteristics.
© McGraw-Hill
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Energy Levels
1
• Each radius equals an energy level.
 The lowest energy level is in the smallest orbit.
 The highest energy level is in the largest orbit.
• When an electron is moved from a lower
to a higher orbit, it gains potential energy
with respect to the nucleus. Outside
energy is required to lift an electron to an
outer orbit. Sources of outside energy
are: heat, light, and voltage.
© McGraw-Hill
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Energy Levels
2
• When an electron falls back to a smaller orbit,
it gives up the extra energy in the form of heat,
light, or other radiation.
Free electron  Recombination  Valence electron  Exit
© McGraw-Hill
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Light-Emitting Diodes
2
• In a forward-biased diode, free electrons
cross the pn junction and fall into holes.
Falling from a higher to a lower energy level,
they radiate energy in the form of light.
• LEDs have very low reverse bias voltage limit
Typical peak inverse voltage (PIV) is  5V.
© McGraw-Hill
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Light-Emitting Diodes
3
• The shorter lead is
the Cathode.
• The flat side is the
Cathode.
• Visible (red, etc.) or
Infrared Light
• The color of the light, which corresponds to the
wavelength energy of the photons, is primarily
determined by the energy band gap of the
semiconductor materials that are used.
© McGraw-Hill
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Light-Emitting Diodes
4
• The typical forward voltage drop for LEDs is
between 1.5 V and 2.5 V.
• The brightness of a LED depends on the
current. The amount of light emitted is often
specified as its luminous intensity and is rated
in candelas (cd) for example 70mcd at 20mA.
VS  VD
IS  ID 
RS
PD  VD  I D 
© McGraw-Hill
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Light-Emitting Diodes
Solve for VRS , I D1 , and PD1
5
VS  VRS  VD1  0
VRS  VS  VD1
VRS  12 V  2 V
VRS  10 V
10V
VRS
I RS 
I RS 
1kΩ
RS
Note: Since the forward voltage range for VD1 is 1.5 V to
2.5 V, we use 2 V.
© McGraw-Hill
PD1  20 mW
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Applications
© McGraw-Hill
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Applications
•
2
What is the purpose of the capacitor?
 Same as resistor but with average power dissipation = ??
•
What is the purpose of the diode?
 Provide a path during the negative half cycle
© McGraw-Hill
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7 Segment Display
1
• Seven-segment displays are LEDs arranged in a
pattern to display decimal numbers.
• The LEDs are labeled A through G.
© McGraw-Hill
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7 Segment Display
2
• The LEDs share either a Common Anode (CA) or a
Common Cathode (CC).
• Common Anode Design: Single (common) power
connection and multiple ground connections.
© McGraw-Hill
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7 Segment Display
3
• Common Cathode Design: Single (common) ground
connection and multiple “high” connections.
© McGraw-Hill
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Light-Emitting Diodes
6
 High-power LEDs are now available with
continuous power ratings of 10 W and above.
These power LEDs can operate in the hundreds
of mAs to over 1 A of current.
 An increasing array of applications are being
developed including automotive interior and
exterior lighting, architectural indoor
and outdoor area lighting.
luminous intensity
𝐄𝐟𝐟𝐢𝐜𝐢𝐞𝐧𝐜𝐲 =
𝐏𝐨𝐰𝐞𝐫
© McGraw-Hill
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Photodiodes
When light energy bombards a pn junction, it can
dislodge valence electrons (like thermal energy).
The incoming light produces free electrons and holes.
The stronger the light, the greater number of minority
carriers. The more light striking the junction, the
larger the reverse current in a diode.
A photodiode has been
optimized for its sensitivity to
light (the reverse current is in
range of 10μA).
Used in digital cameras.
© McGraw-Hill
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Optocouplers
• An optocoupler is formed when a LED and a
Photodiode are used together.
• When input voltage is varying, amount of light is
fluctuating. Thus, output voltage is varying.
• Provides electrical isolation between the input and
output circuits.
© McGraw-Hill
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Remote Control
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Schottky Diode
1
• The Schottky diode is used for high frequency
rectification (>300 MHz), where fast switching times
are required. Common diodes used in HF rectification
produce reverse conduction called “tails.”
• The Schottky diode has very small reverse recovery
time tr: The time it takes to turn off a forward-biased
diode (recombination of stored charges).
© McGraw-Hill
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Schottky Diode
2
• Uses a metal such as gold, silver, or platinum on
one side of the junction (no holes) and doped
silicon (typically n-type) on the other side.
• It has no depletion layer  no stored charges at
junction and no reverse recovery time.
• The Schottky diode has a barrier potential of only
0.25 V. It has low PIV, typically 50 V or less
• A majority carrier device
• No recombination occurs
© McGraw-Hill
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Various Special Purpose Diodes
1
Varactor (Variable Capacitor) Diode
•
•
•
•
Used in reverse bias
Exhibits variable capacitance.
Used to tune resonant circuits.
Applications include radio and television tuners. A
parallel LC tank circuit has only one frequency at
which maximum impedance occurs (resonance).
𝐶𝑇 =
𝐶𝑗0
1+
© McGraw-Hill
𝑉𝑅
2∅𝐹
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Bandpass Filter
• The RLC series resonant
circuit provides a bandpass
filter when the output is taken
off the resistor.
• The center frequency is:
0 
1
LC
• The filter will pass frequencies
from 𝜔1 𝑡𝑜 𝜔2
• It can also be made by feeding
the output from a lowpass to a
highpass filter.
© McGraw Hill
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Various Special Purpose Diodes
Laser: Emits coherent light.
2
intrinsic
• Used for burning data in DVD players.
PIN: Operates as a variable resistor at RF and
microwave frequencies.
Step-recovery: Snaps off when reverse biased.
• Frequency multipliers.
Back: Conducts better when reverse biased.
• Small-signal rectifiers.
Tunnel: Has a negative resistance region.
• Used in high-frequency oscillators.
© McGraw-Hill
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Summary
© McGraw-Hill
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Important Formulas
VZ  VL
1st Approx.
1
VS  VZ
IS 
RS
VL
IL 
RL
IZ  IS  IL
2nd Approx.
© McGraw-Hill
VL  VZ  VL
VL  I Z RZ
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