diode as circuit elements section 3.1-3.3. key concepts diode models –exponential model derivation...
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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 issufficient for most cases.
Occasionally, we will use the exponential model
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IS=Reverse Saturation=leakage 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.(VD≥0)An ideal diode will turn off even for the slightest reverse bias voltage.(VD<0)
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Behavior of Ideal Diode
Ideal diode:Vanode>Vcathode: Diode is onVanode<Vcathode: Diode is offAn ideal current experieincing Vanode=Vcathode, carries no current
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I/V Characteristics
An Open—can’t get a current to flow.
A short--can’t get aV to develop across a diode. A diode
Vanode>Vcathode: Diode is onVanode<Vcathode: Diode is offAn ideal current experieincing Vanode=Vcathode, carries no currentIn 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
Observations:1.If D1 is on, VA=VOUT and VOUT=“1”2.If D2 is on, VB=VOUT and VOUT=“1”.3.VOUT is 0 if and only if D1 and D2 are “0”
This is an OR gate.
Logic 1=3 VLogic 0=0V
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Example of an OR Gate
VA=3 VVB=0 VVOUT=2.424 V≈3V
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In Class Exercise
VA
VB
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In Class Exercise
VA
VB
If VA=“0”, node 1 is “0”.If VB=“0”, 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.
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Simulation Example
VA=3VVB=0VVOUT=0.575 V
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Simulation Example
VA=3VVB=3VVOUT=3 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 withcurrent, it will turn on.
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If VD is less than VD, On, the diode behaves like an open circuit.The diode will behave like an open circuit for VD=VD,on
Constant Voltage Model
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Analysis of an OR Gate
VA=3V, VB=0VObservations:D1 is ON. VD1=0.7VSo VOUT=2.3 VD2 is OFF
Logic 1=3 VLogic 0=0V
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Example of an OR Gate
VA=3 VVB=0 VVOUT=2.424 V≈3V (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
Vout=1.23 V
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Level Shift
Vout of 606 mV
Vout can depend on input voltage combination!