Transcript Practice with Circuits

ECE 4991 Electrical and Electronic
Circuits
Chapter 9
Where We Are
• Chapter 2 - The basic concepts and practice at
analyzing simple electric circuits with sources
and resistors
• Chapter 3 – More harder networks to analyze and
the notion of equivalent circuits
• Chapter 4 – Capacitors and inductors added to
the mix
• Chapter 5 – Analyzing transient situations in
complex passive networks
• Chapter 8 – New subject – the wonders of
operational amplifiers as system elements
• Chapter 9 – Introduction to semiconductors – the
basics and diodes – more network analysis
• Chapter 10 – Bipolar junction transistors and how
they work – now you can build your own op amp
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What’s Important in
Chapter 9
1.
2.
3.
4.
5.
6.
Definitions
Semiconductor basics
Diode behavior
Ideal diode model
Offset diode model
Diodes in circuits
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1. Definitions
•
•
•
•
Semiconductor
Diode
Majority Carrier
Minority Carrier
•
•
•
•
Forward bias
Reverse bias
Ideal model
Offset model
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2. Semiconductor Basics
• Metals are a “sea of electrons” –
conduct electricity extremely well
• Insulators have no available charge
carriers to make an electric current
• Semiconductors are in between
• Silicon
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Semiconductor Basics
• Silicon can be “doped” to provide
charge carriers – either negative or
positive
– N-type or P-type
• One-part-per-million doping is normal
– ~ 1 x 1016 /cm3
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What’s a diode?
• A volume of n-type silicon touching a
volume of p-type silicon forms a
“diode”
• Electric current flows easily in one
direction but not in the reverse
direction
• This is the electronic symbol for a diode
(P N)
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Why do they work the way
they do?
-
P side
Dn ni2 
 qVa
Jn  x  q
exp 

Ln N a 
 kT
+
+
+
+
+
+
+
+
+
+
N side
 x 
 
  1 exp   
 
 Ln 
x0
Which leads to the diode equation

 qVa
I  I 0 exp 
 kT

 
  1
 
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How to make a diode
•
•
•
•
Start with p-type silicon
Grow silicon dioxide (glass)
Pattern and etch a hole in the oxide
Dope the exposed silicon n-type and
diffuse it into the substrate
• Apply metal contacts to top and bottom
SiO
SiO
2
2
n-type Si
p-type Si
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3. Diode behavior
• Plot diode current
versus diode bias
x 10
-15

 qVa
I  I 0 exp 
 kT

 
  1
 
Ideal Diode I-V Characteristic Near the Origin
12
10
Current (A)
8
6
4
2
-3kT/q
+3kT/q
0
-1
-0.8
-0.6
-0.4
Applied Voltage (V)
-0.2
0
0.2
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There are two types of
diode reverse breakdown
• Avalanching
– Caused by strong electric fields in the diode
– Impact ionization – chain reaction
• Zener Breakdown
– Quantum mechanical effect
– Can be set and controlled very tightly
– Used in circuits to set voltages – usually in
the 5 to 25 volt range.
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Diode Breakdown
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4. Ideal Diode Model
• Forward bias
– Diode turns on hard at
zero volts
• Reverse bias
x 10
-15
Ideal Diode I-V Characteristic Near the Origin
12
– Zero current
independent of bias
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Current (A)
8
6
Ideal Diode Model
4
2
-3kT/q
+3kT/q
0
-1
-0.8
-0.6
-0.4
-0.2
Applied Voltage (V)
0
0.2
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5. Offset Diode Model
Offset Diode Model
• Forward bias
– Diode turns on hard at
0.6 volts
• Reverse bias
– Zero current
independent of bias
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For Either Model…
Reverse breakdown is
abrupt, at either the
Zener or the avalanche
voltage
Or
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6. Diodes in Circuits
• Diodes are often found in LCR circuits
• Use a simple model for the diode in the
circuit
– Ideal
– Offset
• Circuit analysis by hand involves some
initial guesswork
– Is it conducting or not?
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An ideal diode in a circuit
+
+
V
R
R
R
D
V
VD < 0
VD  0
D
+
D
V
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An offset model diode behaves like an
ideal diode in series with a battery
Offset model diode
=
+
Ideal diode
0.6 volt battery
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An offset model diode in a
circuit
R
+
+
V
R
R
VD  0.6 V
D
V
VD < 0.6 V
+ 0.6 V
D
+
D
V
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A Zener diode in a circuit
R
0 < VD < VZ
R
+
R
VD  VZ
+
V
D
+
+VZ V
V
D
V
R
D
+
D
V
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Procedure for Circuit
Analysis
1. Think about the circuit, then assume a
conduction state (on, off, or zenering)
2. Substitute in the assumed diode model
3. Solve the circuit
4. Figure out the resultant diode current
and voltage
5. If consistent with assumption, congrats!
If not, try again with another assumption
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Practice with Circuits
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Practice with Circuits
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Practice with Circuits
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Practice with Circuits
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