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Electronic Instrumentation
Experiment 6: Diodes
* Part A: Diode I-V Characteristics
* Part B: Rectifiers
•Part C: PN Junction Voltage Limitation
•Part D: Zener Diode Voltage Regulator
Diodes
D1
ANODE
CATHODE
DIODE


A diode can be considered to be an
electrical one-way valve.
They are made from a large variety of
materials including silicon, germanium,
gallium arsenide, silicon carbide …
Diodes

In effect, diodes act like a flapper valve
• Note: this is the simplest possible model of a
diode
Diodes

For the flapper valve, a small positive pressure
is required to open.
 Likewise, for a diode, a small positive voltage
is required to turn it on. This voltage is like the
voltage required to power some electrical
device. It is used up turning the device on so
the voltages at the two ends of the diode will
differ.
• The voltage required to turn on a diode is typically
around 0.6-0.8 volt for a standard silicon diode and
a few volts for a light emitting diode (LED)
Diodes
D1
D1N4002
VAMPL = 10V
V1
R1
FREQ = 1k
1k
0

10 volt sinusoidal voltage source

Connect to a resistive load through a diode
• This combination is called a half-wave rectifier
Diodes
VAMPL = 10V

V1
Sinusoidal Voltage
FREQ = 1k
10V
5V
0V
-5V
-10V
0s
0.5ms
1.0ms
1.5ms
V(D1:1)
Time
2.0ms
2.5ms
3.0ms
Diodes
VAMPL = 10V
D1
V
V1
D1N4002
V
R1
FREQ = 1k
1k

Half-wave
rectifier
0
10V
5V
0V
-5V
-10V
0s
0.5ms
V(D1:1)
1.0ms
1.5ms
V(D1:2)
Time
2.0ms
2.5ms
3.0ms
At the junction, free electrons from the
N-type material fill holes from the Ptype material. This creates an insulating
layer in the middle of the diode called
the depletion zone.
Part A: Diode i-v Characteristics
Diode V-I Characteristic

For ideal diode, current flows only one way
 Real diode is close to ideal
Ideal Diode
Diode Characteristics

A very large current can flow when the diode is
forward biased. For power diodes, currents of a
few amps can flow with bias voltages of 0.6 to
1.5V. Note that the textbook generally uses 0.6V
as the standard value, but 0.7V is more typical for
the devices we will use in class.
 Reverse breakdown voltages can be as low as 50V
and as large as 1000V.
 Reverse saturation currents Is are around 1nA.
Diode Characteristics

The iD-vD relationship (excluding
breakdown) can be written simply as:

 vD  
iD  I s  exp
  1
 nVT  



Note that for vD less than zero, the
exponential term vanishes and the current iD
is roughly equal to the saturation current.
For vD greater than zero, the current
increases exponentially.
Diode Characteristics


Recall that the i-v relationship for a resistor
is given by Ohm’s Law: v=Ri
If we plot this expression, we obtain
i
v
The slope of the
straight line is
given by the
resistance R
i-v Characteristics

PSpice can be used to obtain such plots
10mA
5mA
V-I Characteristic of a 500 Ohm Resistor
0A
-5mA
-10mA
-6.0V
I(R1)
-4.0V
-2.0V
0V
2.0V
4.0V
V(R1:1) - V(R1:2)
R1
500
V1
15V
R2
1k
0
6.0V
R1
V
5Vdc
V
1k
V2
D2
D1N4148
i-v Characteristics
0
19m
16m
12m

 vD  
iD  I s  exp
  1
 nVT  

8m
4m
iD
0
-16V
-14V
I(D2)

(7e-9)*(exp(
-12V
-10V
-8V
-6V
-4V
-2V
0V
V(D2:1)/(.05107))-1)
V(D2:1)
Both the simulated current vs. voltage and the
characteristic equation for the diode are plotted
2V
i-v Characteristics

In this experiment, you are asked to find the
parameters for the equation

 vD  
iD  I s  exp
  1
 nVT  


That is, you need to find the constants in
this equation so that it matches what PSpice
determines. Note that VT=25mV, so you
need to find n and Is
i-v Characteristics


The i-v characteristic can be checked by
building the circuit and measuring the same
two voltages shown on the diode circuit.
From these voltages and the value of the
resistance, both the current through the
diode and the voltage across the diode can
be determined.
Part B: Rectifiers
Rectifiers

As noted above, the main purpose of diodes is to
limit the flow of current to one direction.
 Since current will flow in only one direction, even
for a sinusoidal voltage source, all voltages across
resistors will have the same sign.
 Thus, a voltage which alternately takes positive
and negative values is converted into a voltage
that is either just positive or just negative.
Rectifiers


If a time-varying voltage is only positive or
only negative all of the time, then it will
have a DC offset, even if the original
voltage had no offset.
Thus, by rectifying a sinusoidal signal and
then filtering out the remaining timevarying signal, we obtain a DC voltage
from an AC source.
Diodes – Recall from Previous Slide
D1
D1N4002
VAMPL = 10V
V1
R1
FREQ = 1k
1k
0

10 volt sinusoidal voltage source

Connect to a resistive load through a diode
• This combination is called a half-wave rectifier
Diodes
VAMPL = 10V

V1
Sinusoidal Voltage
FREQ = 1k
10V
5V
0V
-5V
-10V
0s
0.5ms
1.0ms
1.5ms
V(D1:1)
Time
2.0ms
2.5ms
3.0ms
Diodes
VAMPL = 10V
D1
V
V1
D1N4002

V
R1
FREQ = 1k
1k
Half-wave
rectifier
0
10V
5V
0V
-5V
-10V
0s
0.5ms
V(D1:1)
1.0ms
1.5ms
V(D1:2)
Time
2.0ms
2.5ms
3.0ms
Diodes

Note that the resulting voltage is only
positive and a little smaller than the original
voltage, since a small voltage (around 0.7V)
is required to turn on the diode. 0.7V
10V
5V
0V
-5V
-10V
0s
0.5ms
V(D1:1)
1.0ms
1.5ms
V(D1:2)
Time
2.0ms
2.5ms
3.0ms
Diodes
 Filtering can be performed by adding a
capacitor across the load resistor
D1
D1N4148
V2
R1
1k


0
C1
47uF
Do you recognize this RC combination as a
low pass filter?
You will see how this looks both with
PSpice and experimentally
Diodes


The rectifier we have just seen is called a
half-wave rectifier since it only uses half of
the sinusoidal voltage
A full-wave rectifier uses both the positive
and negative half cycles of the sinusoid
Full-Wave Rectifier
Shown are the
original voltage, the
rectified voltage and
the smoothed voltage
Capacitor
Discharging
Full Wave Rectifier
R3
50
V
D1N4148
D5
D6
R4
V2
VOFF = 0
VAMPL = 10
FREQ = 1k
10k
V-
D7
V+
D8
D1N4148
0

Note path of current when source is positive
Full Wave Rectifier
10V
1.4V (2 diodes)
5V
0V
-5V
-10V
110.0ms
V(D5:2)
110.5ms
111.0ms
111.5ms
V(R4:2,D7:1)
Time
112.0ms
112.5ms
113.0ms
Full Wave Rectifier With
Smoothing
R1
50
D1N4148
D1
D3
R2
V1
VOFF = 0
VAMPL = 10
FREQ = 1k
10k
C1
D4
0.1uF
D1N4148
0
D2
Full Wave Rectifier With
Smoothing
Smoothed Voltage
10V
5V
0V
-5V
-10V
110.0ms
V(R1:2)
110.5ms
V(R2:2,D1:1)
111.0ms
111.5ms
V(R4:2,D7:1)
Time
112.0ms
112.5ms
113.0ms
Part C: PN Junction
Voltage Limitation
Voltage Limitation


In many applications, we need to protect
our circuits so that large voltages are not
applied to their inputs
We can keep voltages below 0.7V by
placing two diodes across the load
R1
A
B
1k
V1
D1
D1N4148
0
D2
D1N4148
Voltage Limitation

When the source voltage is smaller than 0.7V, the
voltage across the diodes will be equal to the
source
 When the source voltage is larger than 0.7V, the
voltage across the diodes will be 0.7V
 The sinusoidal source will be badly distorted into
almost a square wave, but the voltage will not be
allowed to exceed 0.7 V
 You will observe this with both PSpice and
experimentally
Voltage Limitation
R1
A
B
1k
V1
D1
D1N4148
D2
D1N4148
0

Case 1: The magnitude of the diode voltage
is less than 0.7 V (turn on voltage)
R1
1k
100mVdc
V1
0
Diodes act
like open
circuits
Voltage Limitation
R1
A
B
1k
V1
D1
D1N4148
D2
D1N4148
0

Case 2: The magnitude of the diode voltage
is greater than 0.7 V (turn on voltage)
Diodes act
like voltage
sources
Voltage Limitation
R1
1k
10Vdc
V1
V2
0.7Vdc

0
Case 2: The current drawn by the diode is
given by the resistor current
V 10  0.7
I 
 9.3mA
R
1000
Voltage Limitation
R1
1k
V
V3
V
VOFF = 0
VAMPL = 10
FREQ = 1k
D1
D2
D1N4148
D1N4148
0
10V
5V
(1.2420m,718.277m)
0V
-5V
-10V
0s
0.5ms
V(R1:1)
1.0ms
1.5ms
V(R1:2)
Time
2.0ms
2.5ms
3.0ms
Input Protection Circuits

More than one diode can be connected in
series to increase the range of permitted
voltages
Part D: Zener Diodes
Zener Diodes

Up to this point, we have not taken full
advantage of the reverse biased part of the
diode characteristic.
Ideal Zener Diode
I
-VZ
V
Zener Diodes

For the 1N4148 diode, the breakdown
voltage is very large. If we can build a
different type of diode with this voltage in a
useful range (a few volts to a few hundred
volts), we can use such devices to regulate
voltages. This type of diode is called a
Zener diode because of how the device is
made.
Zener Diodes
R1
A
B
1k
V1
D1
1V
D1N750
0


You will again find the i-v characteristic
with PSpice and experimentally.
Such circuits can be used in combination
with the rectifier and filtering to obtain a
well regulated DC voltage.
Zener Diodes
Knee
Current

Note that, for a real Zener diode, a finite current
(called the knee current) is required to get into the
region of voltage regulation
 VZ is the Zener Voltage

Zener Diodes
R1
1k
V
V
D1
V1
VOFF = 0
VAMPL = 10
FREQ = 1k
D1N750
Note the voltage
limitation for both
positive and
negative source
voltages
0
10V
5V
0V
-5V
-10V
0s
0.5ms
V(R1:1)
1.0ms
1.5ms
V(D1:2)
Time
2.0ms
2.5ms
3.0ms
Zener Diode Voltage Regulation
8.0V
R5
50
V
D1N4148
D9
D10
D1N4148
V3
VOFF = 0
VAMPL = 10
FREQ = 1k
4.0V
D12
D1N4148
D1N4148
0V
0
C4
Note stable
voltage
1mF
R7
10
-4.0V
D14
D1N750
R8
-8.0V
110.0ms
10k
V-
V+
V(D10:1)
110.5ms
111.0ms
111.5ms
V(R8:1,R8:2)
Time

Full Wave Rectifier