Diodes
ELZ 206 - Elektronik I
Microelectronic Circuits – Fourth Edition
Adel S. Sedra, Kenneth C. Smith, 1998 Oxford University Press
Dr. Mehmet Siraç Özerdem
Department of Electrical and Electronics Engineering
Dicle University
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2004 by Oxford University Press, Inc.
The ideal diode
diode circuit symbol
i–v characteristic
equivalent circuit in the
forward direction
equivalent circuit in the
reverse direction
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Dr. Mehmet Siraç Özerdem
The ideal diode - Example
The two modes of operation of ideal diodes and the use of
an external circuit to limit the forward current and the
reverse voltage.
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Input waveform
Rectifier circuit
Equivalent circuit when vI
0
Equivalent circuit when vI ≤ 0
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Output
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Rectifier circuit
Transfer
characteristic
Output waveform
Input waveform
Input
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Rectifier circuit
vD waveform ?
Input waveform
vD waveform
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Example
vD waveform
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vD waveform
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Example
Diode logic gates
OR gate
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AND gate
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Example #
D1, D2 : ideal
V=?
I=?
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Example # #
D1, D2 : ideal
V=?
I=?
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2004 by Oxford University Press, Inc.
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Dr. Mehmet Siraç Özerdem
The i–v characteristic of a silicon junction diode.
Microelectronic Circuits - Fifth Edition
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The diode i–v relationship with some scales expanded
and others compressed in order to reveal details.
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Dr. Mehmet Siraç Özerdem
Illustrating the temperature dependence of the diode
forward characteristic. At a constant current, the
voltage drop decreases by approximately 2 mV for
every 1 C increase in temperature.
Microelectronic Circuits - Fifth Edition
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2004 by Oxford University Press, Inc.
Physical Operation of Diodes
Simplified physical structure of the junction diode.
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Two-dimensional representation of the silicon crystal. The circles represent
the inner core of silicon atoms, with +4 indicating its positive charge of +4q,
which is neutralized by the charge of the four valence electrons. Observe
how the covalent bonds are formed by sharing of the valence electrons.
At 0 K, all bonds are intact and no free electrons are available for current
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conduction.
At room temperature, some of the covalent bonds are broken by
thermal ionization. Each broken bond gives rise to a free
electron and a hole, both of which become available for current
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conduction.
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Diffusion and Drift
A bar of intrinsic silicon (a) in which the hole concentration
profile shown in (b) has been created along the x-axis by some
unspecified mechanism.
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n type
semiconductor
A silicon crystal doped by a pentavalent element. Each
dopant atom donates a free electron and is thus called a
donor. The doped semiconductor
becomes
type.
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p type
semiconductor
A silicon crystal doped with a trivalent impurity. Each
dopant atom gives rise to a hole, and the semiconductor
becomes p type.
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Analysis of Diode Circuits
A simple circuit used to
illustrate the analysis of
circuits in which the diode is
forward conducting.
For solution
Graphical analysis
Iterative analysis
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Graphical Analysis
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Example
ID, VD=?
VDD=5V
R=1k ohms
Assuming that ID=1mA at VD=0.7V and its voltage drop
changes 0.1V for every decade change in current.
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Dr. Mehmet Siraç Özerdem
Simplified diode models
Approximating the
diode forward
characteristic with
two straight lines:
the piecewise-linear
model.
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Piecewise-linear model of the diode forward
characteristic and its equivalent circuit representation.
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Example
ID, VD=?
VDD=5V
R=1k ohms
Use piecewise linear model (VD0=0.65V, rD=20 ohms)
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Example - solution
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Dr. Mehmet Siraç Özerdem
Simpler model of the diode for forward region
Development of the
constant-voltage-drop model
of the diode forward
characteristics. A vertical
straight line (B) is used to
approximate the fast-rising
exponential. Observe that
this simple model predicts
VD to within 0.1 V over the
current range of 0.1 mA to
10 mA.
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The constant-voltage-drop model of the diode forward
characteristics and its equivalent-circuit representation.
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Example
ID = ?
VDD = 5V
R = 1k ohms
Use simpler piecewise linear model (VD0=0.65V)
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Modeling the Diode Forward Characteristic
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Modeling the Diode Forward Characteristic
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Dr. Mehmet Siraç Özerdem
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The small-signal model and its application
Development of the diode small-signal model.
Note that the numerical values shown are for a
diode with n = 2.
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Application
The small-signal approximation allows one to seperate the dc
analysis from the signal analysis
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Example
V + = 10V (DC) + 1V peak amplitude sinus (AC)
ID = ?
rd = ?
vd (peak-to-peak) = ?
Assume that VD=0.7V → ID=1mA, n=2
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Dr. Mehmet Siraç Özerdem
Example - solution
Circuit for
calculating the
dc operating
point.
Circuit for
Example
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Small-signal
equivalent circuit
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Example
A string of diodes is used to provide a
constant voltage of about 2.1V.
n=2, VT = 25mV
a) Diode current (no load)
b) rd = ?
c) Input (10±1 V) → Output (2.1± ? V)
d) Calculate the change of output
voltage ∆vo = ? (with load)
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Example
A string of diodes is used to provide a
constant voltage of about 2.8V.
n=2, VT = 25mV
a) Diode current
b) rd = ?
c) Input (15±1 V) → Output (2.8± ? V)
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Operation in the reverse breakdown region – zener diode
Circuit symbol for
a zener diode.
The diode i–v characteristic
with the breakdown region
shown in some detail.
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Dr. Mehmet Siraç Özerdem
Example
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Example
6.8V zener diode is specified to have Vz=6.8V
at Iz=5mA.
rz=20 ohms and IZK=0.2mA.
a) Vo = ? (no load)
b) ∆Vo = ? for a ±1V change in V+ (no load)
c) RL=2k ohms, ∆Vo = ?
d) RL=0.5k ohms, ∆Vo = ?
e) RLmin = ? For which the diode still
operates in breakdown region.
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Example - solution
The circuit with the
zener diode replaced
with its equivalent
circuit model.
Circuit for
Example
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Design of the zener shunt regulator
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Example
It is required to design a zener shunt
regulator to provide an output
voltage of approximately 7.5V
The raw supply varies between 15
and 25V
The load current varies over the
range 0 to 15mA.
(Vz=7.5V at a current Iz=20mA
and rz=10 ohms)
a) R= ?
b) Line and load regulation = ?
c) ∆Vo = ? for full change in Vs.
d) ∆Vo = ? for full change in IL.
Dr. Mehmet Siraç Özerdem
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Example
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Example
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Rectifier Circuits
Block diagram of a dc power supply
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The Half-Wave Rectifier
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Transfer characteristic assuming
a constant-voltage-drop model
for the diodes
Full-Wave Rectifier
Full-wave rectifier
utilizing a transformer
with a center-tapped
secondary winding
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The Bridge Rectifier
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The rectifier with a filter capacitor
A simple circuit used
to illustrate the effect
of a filter capacitor.
Diodes: ideal
The circuit provides a dc
voltage equal to the peak of the
input sine wave.
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Dr. Mehmet Siraç Özerdem
The rectifier with a filter capacitor
- more practical situation -
Voltage and current
waveforms in the peak
rectifier circuit with CR
@ T. The diode is
assumed ideal.
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Limiter Circuits
General transfer characteristic
for a limiter circuit.
Applying a sine wave to a limiter can
result
clipping
offPress,
its Inc.two peaks.
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Limiter Circuits
Soft limiting.
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Dr. Mehmet Siraç Özerdem
Limiter Circuits
A variety of
basic limiting
circuits.
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2004 by Oxford University Press, Inc.
27