Transcript Lecture4

LECTURE 4
DIODE
LED
ZENER DIODE
DIODE LOGIC
Diode Conduction & Current Flow
• Diodes allow current to flow in one direction only
• The diode conducts when the anode is biased positively with
respect to the cathode
• Semiconductor diodes always include special marks on the
packages
• The circuit schematic symbol forms an arrow that points in
the direction of current flow when the anode is biased
positively with respect to the cathode, the “forward
conduction region”.
Current-Voltage Characteristics
• The current voltage characteristic of a typical semiconductor
diode
• Note that the reverse saturation current is exaggerated for
clarity, and the reverse breakdown voltage Vbr is usually much
large than the forward turn-on Vt.
Current-Voltage Characteristics
• Above the forward threshold voltage Vt the
current increases exponentially. Under reverse
bias only a tiny reverse saturation current
flows until breakdown is encountered at which
point the current increases rapidly
• There are basically three parameters that
describe the operation: Vt, Vbr, and the
maximum current that the device can sustain
during operation
Current-Voltage Characteristics
• The impact of these model parameters varies
with application
– In the AM “crystal” radio, the signal levels are very
small  a diode with a small turn-on voltage is
desirable
– In a power-supply application, devices that can
sustain large forward currents and reverse
voltages are often preferred
Current-Voltage Characteristics
• The turn-on voltage is largely governed by the choice
of semiconductor material and the device design
– Silicon-based PN junction diodes have threshold voltages
(Vt)of 0.6-0.8V, typically 0.7V
– Germanium PN junction diode have threshold voltage of
(Vt) 0.3V.
• The current-handling capacity is often limited by
thermal considerations (the device can be destroyed
with too much current and/or poor heat sinking and)
but this can be manipulated by geometrical factors
such as the device cross section, so devices of various
sizes with a wide range of current-handling capacities
are available
Diode Packages
Example part number of a diode: 1N4148, 1N400x
Any part number beginning with “1N” is always
a diode. However, not all diode part numbers
begin with “1N”!
Diode Packages
• The 1N4148 is usually listed under the
heading of “small-signal diodes” or
“switching” diodes.
• The 1N400x is usually listed under the heading
of “rectifier” diodes
• What is the difference between the two?
– voltage and current handling capability
– the 1N4148 can handle currents of up to 300 mA
and peak reverse voltages of 75V
– the1N400x can handle up to 1A and voltages
ranging from 60V (1N4001) to 1200V (1N4007).
Diode Packages
• So the 4148 tends to be used in applications
where the currents are small, for example in an
audio amplifier circuit
• The 400x is designed for abuse and is a good
choice in power-supply applications
• There are lots of other diodes available on the
market; this discussion is just intended to make
you aware that diode selection involves a number
of considerations beyond threshold voltage
Light-Emitting Diodes and Displays
• Light emitting diodes (LEDs) are diodes made with direct
bandgap semiconductors (GaAs, GaN, etc.)
• LEDs are designed to generate light when enough current
passes through the device
• LEDs are functional similar to any diode, but they typically
have a much larger threshold voltage
– ~2V for red and green LEDs
– ~4V for blue and white LEDs
• The packaging is also necessarily different, with a variety of
sizes and lens configurations for various display
requirements
Light-Emitting Diodes and Displays
Light-Emitting Diodes and Displays
• The cathode leg of the LED is the shorter of
the two
• LEDs are used in lots of applications and come
in a variety of packages
• LED symbols are the same as a regular diode
with little arrows to suggest light
Light-Emitting Diodes and Displays
• LED displays include multiple LEDs arranged in a fixed pattern
with shaped lenses, the most familiar pattern being the 7segment displays for showing numbers (digits 0-9)
Zener Diode
• A Zener Diode is a special kind of diode which permits current
to flow in the forward direction as normal
• The current also flows in the reverse direction when the
voltage is above a certain value - the breakdown voltage
known as the Zener voltage
Zener Diode
• Typical Zener voltage, Vz, is
2.4V
• The voltage drop across the
Zener diode is equal to the
Zener voltage of that diode
no matter how high the
reverse bias voltage is above
the Zener voltage.
Zener Diode
• Zener diodes are typically used to regulate the voltage in
electric circuits
– Since the voltage dropped across a Zener Diode is a known
and fixed value
• Using a resistor to ensure that the current passing through
the Zener diode is at least 5mA (0.005 Amps), the circuit
designer knows that the voltage drop across the diode is
exactly equal to the Zener voltage of the diode
DIODE LOGIC – AND GATE
• The AND gate performs logical multiplication, commonly
known as AND function. The AND gate has two or more inputs
and single output. The output of AND gate is HIGH only when
all its inputs are HIGH (i.e. even if one input is LOW, Output
will be LOW
Truth Table
Symbol
X
Y
F(X,Y)
0
0
0
0
1
0
1
0
0
1
1
1
DIODE LOGIC – AND GATE
• If X = 0 and Y = 0, then both diodes D1 and D2
are forward biased and thus both diodes
conduct and pull F low
• If X = 0 and Y = 1, D2 is reverse biased, thus
does not conduct. But D1 is forward biased,
thus conducts and thus pulls F low
• If X = 1 and Y = 0, D1 is reverse biased, thus
does not conduct. But D2 is forward biased,
thus conducts and thus pulls F low
• If X = 1 and Y = 1, then both diodes D1 and D2
are reverse biased and thus both the diodes are
in cut-off and thus there is no drop in voltage at
F. Thus F is HIGH
DIODE LOGIC – AND GATE
• In the figure below, X and Y are two switches which
have been connected in series (or just cascaded) with
the load LED and source battery. When both switches
are closed, current flows to LED
Switch Representation of AND Gate
DIODE LOGIC – OR GATE
• The OR gate performs logical addition, commonly known as
OR function. The OR gate has two or more inputs and single
output. The output of OR gate is HIGH only when any one
of its inputs are HIGH (i.e. even if one input is HIGH, Output
will be HIGH)
• If X and Y are two inputs, then output F can be represented
mathematically as F = X+Y. Here plus sign (+) denotes the
OR operation. Truth table and symbol of the OR gate is
shown in the figure below
Truth Table
Symbol
X
Y
F(X,Y)
0
0
0
0
1
1
1
0
1
1
1
1
DIODE LOGIC – OR GATE
• If X = 0 and Y = 0, then both diodes D1 and
D2 are reverse biased and thus both the
diodes are in cut-off and thus F is low
• If X = 0 and Y = 1, D1 is reverse biased, thus
does not conduct. But D2 is forward biased,
thus conducts and thus pulling F to HIGH
• If X = 1 and Y = 0, D2 is reverse biased, thus
does not conduct. But D1 is forward biased,
thus conducts and thus pulling F to HIGH
• If X = 1 and Y = 1, then both diodes D1 and
D2 are forward biased and thus both the
diodes conduct and thus F is HIGH
DIODE LOGIC – OR GATE
• In the figure, X and Y are two switches which have
been connected in parallel, and this is connected in
series with the load LED and source battery. When
both switches are open, current does not flow to LED,
but when any switch is closed then current flows
Switch Representation of OR Gate
Source: http://www.asic-world.com/digital/gates1.html
AC to DC Power Supply
• http://www.youtube.com/watch?v=cyhzpFqX
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