Diodes and Diode Circuits - Notes

 Page 1


S
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Chapter 9: 
Diodes and Diode Circuits
 
 
S
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2 
2
9.1 Diode Characteristics
• A diode is simply a pn junction, but its 
applications are extensive in electronic circuits.
• Three important characteristics of a diode are:
– Forward voltage drop
• Forward Bias, about .7 volts
– Reverse voltage drop.
• Depletion layer widens, usually the applied voltage
– Reverse breakdown voltage.
• Reverse voltage drop that will force current flow and in most 
cases destroy the diode.
 
 
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3 
3
Diode Elements
• A diode has two 
leads connected to 
the external circuit.
• Since a diode 
behaves differently 
depending upon 
forward or reverse 
bias, it is critical to 
be able to distinguish 
the leads.
•The anode connects 
to the p-type 
material, the cathode
to the n-type material 
of the diode.
 
 
Page 2


S
l
i
d
e
 
1 
Chapter 9: 
Diodes and Diode Circuits
 
 
S
l
i
d
e
 
2 
2
9.1 Diode Characteristics
• A diode is simply a pn junction, but its 
applications are extensive in electronic circuits.
• Three important characteristics of a diode are:
– Forward voltage drop
• Forward Bias, about .7 volts
– Reverse voltage drop.
• Depletion layer widens, usually the applied voltage
– Reverse breakdown voltage.
• Reverse voltage drop that will force current flow and in most 
cases destroy the diode.
 
 
S
l
i
d
e
 
3 
3
Diode Elements
• A diode has two 
leads connected to 
the external circuit.
• Since a diode 
behaves differently 
depending upon 
forward or reverse 
bias, it is critical to 
be able to distinguish 
the leads.
•The anode connects 
to the p-type 
material, the cathode
to the n-type material 
of the diode.
 
 
S
l
i
d
e
 
4 
4
Ideal Diodes
• In an ideal diode, current flows freely through the 
device when forward biased, having no resistance.
• In an ideal diode, there would be no voltage drop 
across it when forward biased.  All of the source 
voltage would be dropped across circuit resistors.
• In an ideal diode, when reverse biased, it would 
have infinite resistance, causing zero current flow.
 
 
S
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e
 
5 
5
Practical Diodes
• A practical diode does offer some resistance to 
current flow when forward biased.
• Since there is some resistance, there will be some 
power dissipated when current flows through a 
forward biased diode. Therefore, there is a 
practical limit to the amount of current a diode can 
conduct without damage.
• A reverse biased diode has very high resistance.
• Excessive reverse bias can cause the diode to 
conduct.
 
 
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6 
6
Practical Diode Forward Bias
 
 
Page 3


S
l
i
d
e
 
1 
Chapter 9: 
Diodes and Diode Circuits
 
 
S
l
i
d
e
 
2 
2
9.1 Diode Characteristics
• A diode is simply a pn junction, but its 
applications are extensive in electronic circuits.
• Three important characteristics of a diode are:
– Forward voltage drop
• Forward Bias, about .7 volts
– Reverse voltage drop.
• Depletion layer widens, usually the applied voltage
– Reverse breakdown voltage.
• Reverse voltage drop that will force current flow and in most 
cases destroy the diode.
 
 
S
l
i
d
e
 
3 
3
Diode Elements
• A diode has two 
leads connected to 
the external circuit.
• Since a diode 
behaves differently 
depending upon 
forward or reverse 
bias, it is critical to 
be able to distinguish 
the leads.
•The anode connects 
to the p-type 
material, the cathode
to the n-type material 
of the diode.
 
 
S
l
i
d
e
 
4 
4
Ideal Diodes
• In an ideal diode, current flows freely through the 
device when forward biased, having no resistance.
• In an ideal diode, there would be no voltage drop 
across it when forward biased.  All of the source 
voltage would be dropped across circuit resistors.
• In an ideal diode, when reverse biased, it would 
have infinite resistance, causing zero current flow.
 
 
S
l
i
d
e
 
5 
5
Practical Diodes
• A practical diode does offer some resistance to 
current flow when forward biased.
• Since there is some resistance, there will be some 
power dissipated when current flows through a 
forward biased diode. Therefore, there is a 
practical limit to the amount of current a diode can 
conduct without damage.
• A reverse biased diode has very high resistance.
• Excessive reverse bias can cause the diode to 
conduct.
 
 
S
l
i
d
e
 
6 
6
Practical Diode Forward Bias
 
 
S
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7 
7
Practical Diode Forward Bias
 
 
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8 
8
Practical Diode Forward Bias
 
 
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9 
9
Reverse Bias
 
 
Page 4


S
l
i
d
e
 
1 
Chapter 9: 
Diodes and Diode Circuits
 
 
S
l
i
d
e
 
2 
2
9.1 Diode Characteristics
• A diode is simply a pn junction, but its 
applications are extensive in electronic circuits.
• Three important characteristics of a diode are:
– Forward voltage drop
• Forward Bias, about .7 volts
– Reverse voltage drop.
• Depletion layer widens, usually the applied voltage
– Reverse breakdown voltage.
• Reverse voltage drop that will force current flow and in most 
cases destroy the diode.
 
 
S
l
i
d
e
 
3 
3
Diode Elements
• A diode has two 
leads connected to 
the external circuit.
• Since a diode 
behaves differently 
depending upon 
forward or reverse 
bias, it is critical to 
be able to distinguish 
the leads.
•The anode connects 
to the p-type 
material, the cathode
to the n-type material 
of the diode.
 
 
S
l
i
d
e
 
4 
4
Ideal Diodes
• In an ideal diode, current flows freely through the 
device when forward biased, having no resistance.
• In an ideal diode, there would be no voltage drop 
across it when forward biased.  All of the source 
voltage would be dropped across circuit resistors.
• In an ideal diode, when reverse biased, it would 
have infinite resistance, causing zero current flow.
 
 
S
l
i
d
e
 
5 
5
Practical Diodes
• A practical diode does offer some resistance to 
current flow when forward biased.
• Since there is some resistance, there will be some 
power dissipated when current flows through a 
forward biased diode. Therefore, there is a 
practical limit to the amount of current a diode can 
conduct without damage.
• A reverse biased diode has very high resistance.
• Excessive reverse bias can cause the diode to 
conduct.
 
 
S
l
i
d
e
 
6 
6
Practical Diode Forward Bias
 
 
S
l
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d
e
 
7 
7
Practical Diode Forward Bias
 
 
S
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8 
8
Practical Diode Forward Bias
 
 
S
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9 
9
Reverse Bias
 
 
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1
0 
10
Reverse Bias
 
 
S
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1
1 
11
Exceed Breakdown Voltage
 
 
S
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1
2 
12
Current versus Voltage
• In a practical diode, there 
is very little forward 
current until the barrier 
voltage is reached.
• When reverse biased, only 
a small amount of current 
flows as long as the 
reverse voltage is less than 
the breakdown voltage of 
the device.
Current Vs Voltage 
Curve for a practical 
diode
 
 
Page 5


S
l
i
d
e
 
1 
Chapter 9: 
Diodes and Diode Circuits
 
 
S
l
i
d
e
 
2 
2
9.1 Diode Characteristics
• A diode is simply a pn junction, but its 
applications are extensive in electronic circuits.
• Three important characteristics of a diode are:
– Forward voltage drop
• Forward Bias, about .7 volts
– Reverse voltage drop.
• Depletion layer widens, usually the applied voltage
– Reverse breakdown voltage.
• Reverse voltage drop that will force current flow and in most 
cases destroy the diode.
 
 
S
l
i
d
e
 
3 
3
Diode Elements
• A diode has two 
leads connected to 
the external circuit.
• Since a diode 
behaves differently 
depending upon 
forward or reverse 
bias, it is critical to 
be able to distinguish 
the leads.
•The anode connects 
to the p-type 
material, the cathode
to the n-type material 
of the diode.
 
 
S
l
i
d
e
 
4 
4
Ideal Diodes
• In an ideal diode, current flows freely through the 
device when forward biased, having no resistance.
• In an ideal diode, there would be no voltage drop 
across it when forward biased.  All of the source 
voltage would be dropped across circuit resistors.
• In an ideal diode, when reverse biased, it would 
have infinite resistance, causing zero current flow.
 
 
S
l
i
d
e
 
5 
5
Practical Diodes
• A practical diode does offer some resistance to 
current flow when forward biased.
• Since there is some resistance, there will be some 
power dissipated when current flows through a 
forward biased diode. Therefore, there is a 
practical limit to the amount of current a diode can 
conduct without damage.
• A reverse biased diode has very high resistance.
• Excessive reverse bias can cause the diode to 
conduct.
 
 
S
l
i
d
e
 
6 
6
Practical Diode Forward Bias
 
 
S
l
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d
e
 
7 
7
Practical Diode Forward Bias
 
 
S
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d
e
 
8 
8
Practical Diode Forward Bias
 
 
S
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e
 
9 
9
Reverse Bias
 
 
S
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d
e
 
1
0 
10
Reverse Bias
 
 
S
l
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d
e
 
1
1 
11
Exceed Breakdown Voltage
 
 
S
l
i
d
e
 
1
2 
12
Current versus Voltage
• In a practical diode, there 
is very little forward 
current until the barrier 
voltage is reached.
• When reverse biased, only 
a small amount of current 
flows as long as the 
reverse voltage is less than 
the breakdown voltage of 
the device.
Current Vs Voltage 
Curve for a practical 
diode
 
 
S
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d
e
 
1
3 
13
9.2 Power Supply Applications
• Nearly all computers have some sort of power 
supply.
• Power supply circuits must:
– Convert the ac line voltage into a dc voltage
required by the circuit.
– Reduce the ac voltage to a lower value.
– Continuously adjust the dc output voltage to 
keep it constant under varying load conditions.
 
 
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1
4 
14
Half-wave Rectifier
•The term rectify is used to 
describe the conversion of 
ac into dc.
• In the circuit shown, only 
one-half of the input 
waveform is allowed to 
pass through to the 
output.
•This is called half-wave 
rectification.
 
 
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1
5 
15
Circuit Operation
• During the positive alternation, the diode is forward biased 
and the full applied voltage is dropped across the load 
resistor.
• During the negative alternation, the diode is reverse biased 
and acts like an open circuit.  No voltage is present across 
the load resistor.
• The output voltage is actually pulsating dc.
• An application for a half-wave rectifier is shown on the 
following slide.