# PN Junction Diodes and Applications - University of Massachusetts ...

Electronics - Devices

Oct 7, 2013 (4 years and 9 months ago)

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UNIVERSITY OF MASSACHUSETTS DARTMOUTH

DEPARTMENT OF ELECTRICAL AND COMPUTER
ENGINEERING

ECE 201 CIRCUIT THEORY 1

PN JUNCTION
DIODES AND APPLICATIONS

WHAT IS A
PN JUNCTION
DIODE?

A
PN Junction
diode is a 2
-
terminal
semiconductor
electronic On/Off swit
ch that allows current
flow in only one direction. The circuit symbol is shown below. The terminals are called the
Anode (A) and the Cathode (K). The state of the diode switch (open or closed) is determined by
the polarity of the voltage across it
.

C
ircuit Symbol and Typical Terminal Identification

Common types of Diode

Packages

Figure 1. Some common Diode Packages

2

CIRCUIT BEHAVIOR OF THE PN JUNCTION DIODE

The behavior of electro
nic devices, such as the pn junction diode, can be described by their Volt
-
Ampere, or (V
-
I) Characteristic. This is a plot of the current flowing through the device as a
function of the voltage across the device. Since the resulting plot of the diode’s V
-
I Characteristic
is not a straight line, the diode is referred to as a “non
-
linear” device. A typical V
-
I Characteristic
for a pn junction diode is shown here.

Figure 2. A typical
PN

J
unction diode V
-
I Characteristic

Thro
ugh experimentation,
the current through a diode as a function of the voltage across it

can be
mathematically modeled

by

the following

diode equation

w
here

i =
the
current through the diode in Amperes (A)

v = the voltage

across the diode from Anode (+) to Cathode (
-
)

in volts

V
T

= 25 mv (25 X 10
-
3
volts) at room temperature (25°C)

I
0

=
a constant with units of current

η = a constant ranging from 1 to 2 depending upon the diode material

This representation does not
account for the reverse breakdown region
!

3

APPLICATION AS A RECTIFIER

Consider the circuit shown below.

Figure 3. A PN Junction Diode as a Half
-
Wave Rectifier

When the applied sinusoidal voltage is on its positive half

cycle, conventional current w
ill want
to flow in the clockwise direction (the direction of the arrow in the diode symbol) and the voltage
polarity across the diode will be + on the Anode and

on the Cathode. This condition is known
as “forward bias”. The diode will act as a short c
ircuit and essentially all of the applied vol
ta
ge will
appear across the resistor R1.

The

applied voltage and the voltage across the resistor R1 are
shown on the oscilloscope display below. Note that the resistor voltage is the “positive half

cycle” of

the applied voltage, hence the circuit is called a

half

wave rectifier

.

Figure 4. Waveforms for the
H
alf

W
ave
R
ectifier circuit.

4

If the diode was installed in the opposite direction, we would observe the following.

Figure 5. A “nega
tive”
H
alf

W
ave
R
ectifier.

In each of the above circuits,
t
he input signal, having an average value equal to zero was
converted into a “unidirectional” signal having a non

zero average value. It can be shown that
the average value of a “half

wave r
ectified” sinusoid having a maximum value of V
m

has an
average
value (also known as the DC
value
)

equal to

5

ANOTHER RECTIFIER APPLICATION

The circuit shown below is a “full

wave” bridge rectifier.

Figure 6. A
F
ul
l

W
ave
B
ridge
R
ectifier with the output waveform.

In this circuit, the output waveform
consists of
both half

cycles of the input voltage
. The
average value of the voltage across R1 will be

This is twice the value of the
half

wave case.

6

AN IMPROVEMENT IN THE AVERAGE (DC) VALUE

The output voltages of the half

-
-

wave and full

wave rectifiers have average values other than
zero, but their instantaneous values, as seen on the waveforms, are not constant. One method to
“smooth

out” the voltage (make it look more like DC) is to install a capacitor in parallel with the
is

“filter”
capacitor will charge up to the maximum value of the output voltage
during the time that the diode conducts, and will dis
charge into the load when the diode is off. If
the time constant of the capacitor and load resistor is long when compared with the period of the
rectified sinusoid, the
capacitor
discharge time is longer and the output voltage is smoother. This
is

shown

in the circuits

below.

Figure 7. Half

Wave Rectifier with a 2.2 μF
F
ilter
C
apacitor.

DC Output Voltage

AC Input Voltage

7

Figure 7. A
H
alf

W
ave
Rectifier

with
a 10 μF
F
ilter

C
apacitor
.

The
DC value of the
output voltage
increases and the waveform get
s “smooth
er” when the
value
of the
filter capacitor
is increased.

DC Output Voltage

AC Input Voltage

8

THE EXPERIMENT

AND REQUIRED RESULTS

1.) Construct the Half

Wave
Rectifier

When
installing the diode, look for a “band” at one end of the p
ackage. That end is the Cathode (
-
).

2.)
Apply a 5 V 1 kHz sine wave
.

O
bserve
and record
the input and output voltage waveforms.
Use your Digital Multimeter to measure the rms value of the input voltage and the DC level of the
output voltage.
Compar
e the measured values with the expected (calculated) values.

3.)
Run a simulation of the circuit in MultiSim and c
. Comment on any
differences and suggest a reason (or reasons) for any difference.

4
.) Install a 2.2μF
capacitor ac
ross the resistor (watch the polarity signs). O
bserve
and record
the output voltage waveform. Measure
the DC level
of the output
voltage
using the Digital
Multimeter. Compare your results with a MultiSim simulation.

5
.) Re
peat Step 4 using a 10 μF

capacitor.

6
.)

Construct the Full
--

Wave

Bridge Rectifier

shown in Figure 6. Apply a 5 V 1 kHz sine wave
and observe and record the output voltage waveform. Measure the DC level of the output voltage
and compare with the expected
(calculated)
value.

7.) Compare your results with a MultiSim simulation.

Can you account for any differences?

8.) Install a 10 μF capacitor across the resistor (again observing the polarity). Observe and
record the output voltage waveform. Measure the DC level of the
output voltage using the Digital
Multimeter. Compare your results with the MultiSim simulation.

REQUIRED RESULTS
FOR EACH GROUP
TO BE HANDED IN

Step 2

Input and Output voltage waveforms

Measured
and calculated
values of rms input voltage and DC
o
u
tput voltage

Step 3

MultiSim simulation

Step 4

Output voltage waveform

Measured value of DC output voltage

MultiSim simulation

Step 5

Output voltage waveform

Measured value of DC output voltage

MultiSim simulation

Step 6

Output voltage
waveform

Measured and calculated value of DC output voltage

Step 7

MultiSim simulation

Step 8

Output voltage waveform

Measured value of DC output voltage

MultiSim simulation