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Electrical Engineering
Principles & Applications

Chapter 10:
Diodes
Slide 1

Outcomes
• Explain diode operation and analyze
simple diode-circuits
• Design basic rectifier circuits, choppers
and peak detectors

Slide 2

Diodes- Few Applications
• Convert AC power into DC power for radios,
televisions, computers, … and many other
electronic devices
• Convert radio frequency signals into audible
signals in radios

Slide 3

Basic Diode Concepts
• Like resistors and capacitors, diodes are two terminal
devices
– The 2 terminals are called Anode and Cathode

• It conducts current in one direction only (from anode to
cathode)
• Unlike capacitors & inductors, current in diodes is
directly related to voltage
– But this current is not linearly related to voltage

• Diodes only consume power They are said to be passive
vd
devices
+
id
anode

cathode

Diodes are nonlinear, two terminal, passive electrical devices
Slide 4

Typical Volt-Ampere Characteristic
• Forward-bias region
– Anode voltage is positive w
...
t
cathode, current flows easily
through the diode

• Reverse-bias region
– Anode voltage is negative w
...
t
cathode, no current flows through
the diode

• Reverse-breakdown region
– Anode voltage is large and
negative w
...
t
...
6 V for silicon diodes at a temperature of 300 K)
– This bias voltage is temperature dependent and reduces linearly at a
rate of 2mV/K
Diodes can be used as temperature sensors

• In reverse-bias, the current is about 1 nA
– When the breakdown voltage is reached, large current will flow

Slide 7

Shockley Equation
Used to relate the current through the diode to the voltage across it
in the forward-bias region

  vD
iD = I s exp
 nV
  T

 
 − 1

 

vD = nVT ln[ + 1]
iD
Is

id

+

vd

-

• Is is the saturation current in the order of 10-14 A
• n is a coefficient typically between 1 and 2
• VT is called the thermal voltage usually 26 mV at a temperature of
300 K

KT
VT =
q

With vD typically less than 1 volt, the current through the diode can be
approximated as:
vD

iD ≅ I s e
Slide 8

nV T

Example
At a temperature of 300 K, the diode current is iD=0
...
6 V
...
Find the saturation current Is
...
65V and at 0
...


i D = I s  exp


 vD

 nV
T




 − 1




Is =

iD
exp(vD / nVT ) − 1

10 −4
=
exp(0
...
026) − 1
= 9
...
6 5 0 V, we have:

iD = I s exp(vD / nVT ) − 1 = 9
...
650 / 0
...
6841 mA
Similarly for vD = 0
...
681

mA

Notice the difference
in current
for 0
...
93

Vss = 2 V
R=1k

Slide 12

Zener Diodes
Diodes that are intended to operate in the
breakdown region are called Zener diodes

Can be used in circuits that produce constant output
voltage while supplied by a variable supply voltage
Ex: A constant voltage device supplied by an automobile battery

Slide 13

Zener-Diode Voltage-Regulator Circuits
A voltage regulator circuit provides a nearly
constant voltage to a load from a variable source

VSS + RiD + vD = 0

A simple regulator circuit that provides a nearly
constant output voltage v0 from a variable supply
Slide 14

Example
R = 1 k , Vss = 15 V then Vss = 20 V
A 5 volts change in the input resulted in
a change of 0
...
5 –V change

VSS + RiD + vD = 0

Slide 15

Vo= -vD

10 V

Load-Line Analysis of Complex Circuits
Any circuit containing resistors, voltage and/or current
sources and a single 2-terminal nonlinear element can be
analyzed by the load-line technique

Thevenin
equivalent
for the
Linear
portion

Slide 16

Example
Find vL and Is if Vss = 24 V, R=1
...
2)=20 V
RT = (R || RL)=1 k
V T + R T iD + v D = 0
Draw the load line and
locate the operating point

v L = − v D = 10

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