Experiment 3
DIODE APPLICATIONS
OBJECTIVES. To analyse the diode as a part of electronic circuits. To handle libraries and
subcircuits in Pspice.
3.1.
Diode model with PSpice
OBJECTIVES. To extract a linear model for the diode from the experimental I-V curve. To
create a subcircuit in Pspice for this model. To load this subcircuit in a library.
Figure 3.1: Circuit with diode and resistance
1. PSpice work
NOTE: In Experiment 2, the curve tracer was used to measure I-V curves in
diodes. The aim of Experiment 3 is to explain how the instrument measured
the different I-V points of the curve. This will be done with the help of Pspice
and one of the diode models stored in its libraries. To do this, follow this
procedure.
Extraction of the model parameters.
Draw in OrCad Capture the circuit of Figure 3.1. Use a resistor with R = 1 kΩ and
the diode named D1N4002.
Obtain the I-V characteristic of the diode by sweeping the DC voltage of the power
supply. To do so, create a Simulation profile with the following specifications:
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Analysis Type: DC Sweep
Sweep Variable: Voltage Source
Sweep type: Linear
Start Value = 0 V
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• End Value = 5 V
• Increment = 0.1 V
The Name of the power supply to be swept must also be provided in the specifications.
Place a Current Marker at the diode anode and run the simulation.
In Pspice A/D Lite, the current I is a function of the input voltage V . The option
Plot followed by Axis Settings... allows to express I as a function of the voltage drop
in the diode. In the X-axis menu, click on Axis Variable and choose V(D1:1). Click
on OK in both windows.
Export the I − VD curve of the diode to a worksheet. To do this, select the curve and
use Ctrl+C.
Paste the data in the worksheet. Once in the worksheet, represent the curve I − VD
in a figure and superimpose a straight line
VD = Vγ + I ∗ Rd
(3.1)
over the high-current range of the I-V curve. Find the appropriate values of Vγ and
Rd for a good agreement.
Design of a subcircuit
Design a subcircuit that represents the previous model (3.1). Remember that this
model is valid for voltages larger than Vγ . For lower values, the diode behaves as an
open circuit. The design of subcircuits is made with hierarchical blocks; create one
with the option Place Hierarchical Block and fill in the following items in the menu:
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Reference: D1N
Primitive: No
Implementation Type: Schematic View
Implementation Name: Diode
Select the block. Introduce two terminals with the option Place H Pin and fill the
menu as:
• Name: IN; Type: Input; Width: Scalar
• Name: OUT; Type: Output; Width: Scalar
Click on the right button of the mouse over the hierarchical block and select Descend
Hierarchy. Specify a name for the new schematics: D1N.
A new window will appear with two ports corresponding to the terminals IN and
OUT defined previously. Draw the circuit of figure 3.2 providing the following values:
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R: the value of Rd in your model
V : the value of Vγ in your model
VON : Vγ + 0.01 1
VOFF : Vγ - 0.01
Once the model for the two conduction states of the diode has been drawn, it can
be loaded in a Pspice library. It will be a new library which can be named as
componentes.olb or whatever other name can be preferred. To do this, select Tools →
Generate Part... in the directory tree.
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VON and VOFF are parameters of the element S. This element switches between two conduction states of
the diode, depending on the value of the voltage drop at the two input terminals of this switch.
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Figure 3.2: Two conduction-state model for the diode
Netlist/source file: Design we are working in
Netlist/source file type: Capture Schematic/Design
Part name: Diode
Primitive: No
Destination part library: /COMPONENTES.olb
Source Schematic name: Name of the schematic that contains the circuit of the
diode model.
• Create New Part: Activate this option
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Accept Pin confirmation window: Ok → save
The library and part are generated and loaded in the “output” file. At this point it
is advisable to change the symbol of the new part by drawing new lines or inserting
text. To do this, go to the directory tree of the project, select the library, click on the
right button of the mouse and select Edit part.
Draw a symbol similar to the diode, as can be seen in Figure: 3.3. The dotted line
represents the symbol itself. Whatever you draw inside is not important. The red
lines are referred to the terminals (they must not be deleted).2
Using a subcircuit
In Pspice, draw back the circuit of figure 3.1 substituting the diode D1N4002 with
the new subcircuit. To extract this subcircuit from the library use the option Get
New Part → Libraries. Repeat the DC sweep previously defined, compare the I-V
curves in the same figure and highlight the differences.
NOTE: If you have any problem in creating or using the subcircuit, substitute the diode
D1N4002 with the equivalent circuit of Figure 3.4.
3.2.
The half-wave rectifier
OBJECTIVES. To obtain the transfer characteristic of the half-wave rectifier. To analyse the
response of the rectifier to a sine waveform. To check the capacitive effects at high frequencies.
1. Theoretical work
Consider the circuit of figure 3.5
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In the toolbar on the top right, Snap to Grid must be active. If it is red (inactive), problems can arise when
connecting the nodes in the schematics.
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Figure 3.3: Symbol of the diode
Figure 3.4: Equivalent circuit model of the diode
a) Obtain the transfer characteristic by using the model extracted in section 3.1 and
R = 1KΩ.
b) Obtain a new transfer characteristic by using Rd = 0.
c) If Vi is a sine wave of 5 V amplitude and period T = 100µs, represent in a figure Vi
and the response Vo as a function of the time from 0 to 200µs (use Rd = 0).
2. PSpice work
a) Draw the circuit of figure 3.5 and substitute the diode with the model built in section
3.1. Use R = 1kΩ and a voltage source VSIN with offset voltage VOFF = 0 V, an
amplitude VAMPL = 5 V and frequency FREQ = 10kHz.
b) Define a transient analysis in the simulation profile. End the analysis at 200 µs. Place
the voltage markers at the input and output of the circuit and run PSpice. Represent
the response Vo as a function of the time from 0 to 200µs and compare it with the
theoretical results.
3. Laboratory practice
a) Assemble the circuit of figure 3.5 with R = 1 KΩ and a 1N4004 diode.
b) Introduce a sine wave at the input, consider two different frequencies (10 KHz and
100 KHz) and measure the output voltage in these two cases. Explain the results and
the differences between the two cases.
c) Use the XY mode of the oscilloscope to measure the transfer characteristic.
d) Compare the measurements with the theoretical and PSpice results.
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Figure 3.5: The half-wave rectifier
3.3.
A basic limiting circuit
OBJECTIVES. To study the response of a basic limiting circuit.
Figure 3.6: Basic limiting circuit
1. Theoretical work
Consider the circuit of figure 3.6
a) Obtain the transfer characteristic using the model found in section 3.1.
b) Obtain the transfer characteristic using the same model but with Rd = 0.
c) If Vi is a sine wave of 5 V amplitude and period T = 100µs, represent in a figure Vi
and the response Vo as a function of the time from 0 to 200µs (use Rd = 0).
2. PSpice work
a) Draw the circuit of figure 3.6 and substitute the diode with the model built in section
3.1. Use R = 1kΩ and a voltage source VSIN with offset voltage VOFF = 0 V, an
amplitude VAMPL = 5 V and frequency FREQ = 10kHz.
b) Define a transient analysis in the simulation profile. End the analysis at 200 µs. Place
the voltage markers at the input and output of the circuit and run PSpice. Represent
the response Vo as a function of the time from 0 to 200µs and compare it with the
theoretical results.
3. Laboratory practice
a)
b)
c)
d)
Assemble the circuit of figure 3.6 with R = 1 KΩ and a 1N4004 diode.
Introduce a sine wave at the input (f = 10 KHz) and measure the output voltage.
Use the XY mode of the oscilloscope to measure the transfer characteristic.
Compare the measurements with the theoretical and PSpice results.
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3.4.
The clamped capacitor (dc-restorer circuit)
OBJECTIVES. To study the response of a clamping circuit.
Figure 3.7: Clamped capacitor
1. Theoretical work3
Consider the circuit of figure 3.7.
a) If Vi is a sine wave of 5 V amplitude, represent in a figure Vi and the response Vo .
b) If Vi is a square wave of 5 V amplitude, represent in a figure Vi and the response Vo .
2. PSpice work
a) Draw the circuit of figure 3.7 and substitute the diode with the model built in section
3.1. Use a capacitor C = 1 nF and a voltage source VPULSE with the following
parameters: low value V1 =-5 V, high value V2 =5 V, delay time TD = 0 s, rise
time TR = 0.1 ns, fall time TF = 0.1 ns, pulse width P W = 50µs and time period
P ER = 100µs.
b) Define a Transient analysis in the simulation profile. End the analysis at 200 µs.
Place the voltage markers at the input and output of the circuit and run PSpice.
c) Draw in the same figure the theoretical and PSpice results. A disagreement will be
observed when the diode is in its OFF state. A small discharge of the capacitor is
observed in the PSpice result. The origin of this discharge is found in the model of
the voltage controlled commuter. The resistance between its output terminals is not
infinite. Thus, a leakage current flows between its terminals. Estimate the average
leakage current as:
C∆V
Qleakage
=
ileakage =
∆t
∆t
where ∆t is the pulse width where the discharge is observed and ∆V is the variation
of the voltage during the discharge. Estimate the value of the resistance between the
terminals as:
<V >
Rleakage =
Ileakage
where < V > is the average voltage during the discharge. Click on the model of
the voltage controlled commuter and compare the value used in the model for the
resistance of the OFF state with your results (fill the gaps in Table 3.1).
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It makes no sense to represent the transfer characteristic of the circuit of figure 3.7 because it depends on the
initial charge of the capacitor.
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spice
Ileakage
ROF F (calculated)
ROF F (commuter model)
Rprobe
origin of the leakage current
laboratory
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Table 3.1: Table to be filled with the results of the study of the circuit of Figure 3.7.
3. Laboratory practice
a) Assemble the circuit of figure 3.7 with C = 1 nF and a 1N4004 diode.
b) Introduce a sine wave of 5 V amplitude at the input (f = 10 KHz) and measure the
output voltage.
c) Introduce a square wave of 5 V amplitude at the input (time period T = 10−4 s) and
measure the output voltage. Observe that the output waveform has a finite average
value or DC component. Which is this value? 4
d) Compare the measurements with the theoretical and PSpice results. Do you observe
any leakage current in the experimental data? In an affirmative case, determine the
average leakage current Ileakage and the equivalent resistance Rleakage (fill the gaps in
Table 3.1).
Figure 3.8: Figures that must be filled with the results
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An application of this circuit is the following. Consider a pulse signal being transmitted through a capacitively
or ac-coupled system. The capacitive coupling will cause the pulse train to lose whatever dc component it originally
had. Feeding the resulting waveform to a clamping circuit provides it with a dc-component. Actually, the clamped
circuit restores the dc-component. This is why the circuit is also called dc restorer.
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Figure 3.9: Figures that must be filled with the results
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