Diode As Circuit Elements

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Diode As Circuit Elements
Section 3.1-3.3
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Key Concepts

• Diode models
• Exponential model
• Derivation of n
• Ideal model
• Constant-voltage model

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Choosing a Diode Model
Use the ideal model to develop a quick, rough
understanding of a circuit. If the ideal model
is not adequate, uses the constant voltage model,
which is sufficient for most cases.
Occasionally, we will use the exponential model
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ISReverse Saturationleakage current
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Obtain an Exponential Diode Model Empirically
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Ideal Model of a Diode
(exponential model)
(ideal model)
An ideal diode will turn on even for the
slightest forward bias voltage. (VD0) An ideal
diode will turn off even for the slightest
reverse bias voltage. (VDlt0)
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Behavior of Ideal Diode
Ideal diode VanodegtVcathode Diode is
on VanodeltVcathode Diode is off An ideal current
experieincing VanodeVcathode, carries no current
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I/V Characteristics
A short--cant get a V to develop across a diode.
An Opencant get a current to flow.
A diode
VanodegtVcathode Diode is on VanodeltVcathode
Diode is off An ideal current experieincing
VanodeVcathode, carries no current In practice,
consider a slightly positive or negative voltage
to determine the response of a diode.
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Example 1 An OR Gate Realized By Diodes
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Analysis of an OR Gate
Logic 13 V Logic 00V

• Observations
• If D1 is on, VAVOUT and VOUT1
• If D2 is on, VBVOUT and VOUT1.
• VOUT is 0 if and only if D1 and D2 are 0
• This is an OR gate.

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Example of an OR Gate
VA3 V VB0 V VOUT2.424 V3V
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In Class Exercise
VA
VB
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In Class Exercise
If VA0, node 1 is 0. If VB0, node 1 is
0. If both VA and VB are 1 or 3V, no current
can flow through R1, node 1 is 3 V. So we have
an AND gate.
Assume node 2 is 3V.
VA
VB
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Simulation Example
VA3V VB0V VOUT0.575 V
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Simulation Example
VA3V VB3V VOUT3 V
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In-Class Exercise
What is logic implemented by A, B and LED?
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Solution
0 V
0 V
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Basic Gates
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Decimal to Binary Converter
Big idea If you deprive an LED with current,it
will turn off. If you provide an LED
with current, it will turn on.
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Constant Voltage Model
If VD is less than VD, On, the diode behaves like
an open circuit. The diode will behave like an
open circuit for VDVD,on
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Analysis of an OR Gate
Logic 13 V Logic 00V

• VA3V, VB0V
• Observations
• D1 is ON.
• VD10.7V
• So VOUT2.3 V
• D2 is OFF

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Example of an OR Gate
VA3 V VB0 V VOUT2.424 V3V (ideal diode
model) 2.3 V (constant
voltage model)
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Cascade Two Stages of AND gates
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Cascade an AND with an OR
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Limitations of Diode Logic Circuit
Vout1.23 V
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Level Shift
Vout of 606 mV
Vout can depend on input voltage combination!