Download Lab Assignment 5: Diode Circuits

Lab Assignment 5: Diode Circuits
Revision: April 16, 2015
Overview
In this lab assignment, we will be exploring characteristics of diodes and their use in several simple
circuits. Diodes are our first formal exposure to circuit elements with a nonlinear voltage-current
relationship.
In Parts I, II, and III of this lab assignment, we measure the i-v characteristics of various diodes. In
Part I, we measure forward i-v characteristics for various diodes and use these data to estimate the
diode parameters I s and n. In Part II, we will measure the reverse i-v characteristics for several
diodes. In Part III, we will use the TEK 575 transistor curve tracer to measure diode characteristics.
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In Part IV of this assignment, we will be performing a small design project using diodes. Design of
circuits containing nonlinear elements can be extremely difficult. For that reason, preliminary designs
are generally based on linearized models of the nonlinear elements; later stages of the design
process generally involve more rigorous simulation and validation of the design, incorporating
nonlinear models of the appropriate elements. Sedra and Smith provide several linearization
approaches toward diode modeling. In Part IV, we will design a simple but practical diode circuit (a
voltage regulator). In our design process, we will use the constant voltage drop diode model and the
small-signal approaches toward linearizing the diode characteristic. We will also generate a nonlinear
model of our voltage regulator to simulate the nonlinear circuit response. We will then construct our
circuit, acquire response data, and compare the experimental data with the predicted response from
both the linear and nonlinear models.
In Part V of this assignment, we will investigate several simple diode circuits.
Before beginning this lab, you should be
able to:
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Sketch the diode voltage-current
relationship (Sedra and Smith, section
3.2)
State the nonlinear forward voltagecurrent relationships for diodes (Module
5.1)
Sketch the following linear models for
diode forward characteristics:
(a) piecewise-linear model (Sedra and
Smith, section 3.3.5)
(b) constant-voltage-drop model (Sedra
and Smith, section 3.3.6)
(c) ideal-diode model (Sedra and Smith,
section 3.3.7)
Linearize diode voltage-current
characteristics (Module 5.2 and Sedra
and Smith, section 3.3.8)
Perform least-squares straight line
curve fits to experimental data (Module
5.3)
Contains material © Digilent, Inc.
After completing this lab, you should be able to:
•
•
•
•
•
Measure diode forward and reverse voltagecurrent characteristics
Estimate diode parameters from measured
data
Use PSPICE to model diodes
Use linearized diode characteristics to
perform circuit design
Sketch diode circuits to perform the
following functions:
(a) Clipping
(b) Voltage doubling
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Lab Assignment 5: Diode Circuits
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This lab exercise requires:
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EE 352 Analog Parts Kit. Check the numbers on your diodes as follows.
o 1N3064, switching diode; 1N4002 or 1N4001, rectifier diode; 1N4735, Zener diode
Breadboard
Function Generator, oscilloscope
Symbol Key:
Demonstrate circuit operation to teaching assistant; teaching assistant should initial lab
notebook and grade sheet, indicating that circuit operation is acceptable.
Analysis; include principle results of analysis in laboratory report.
Numerical simulation (using PSPICE or MATLAB as indicated); include results of Matlab
numerical analysis and/or simulation in laboratory report.
Record data in your lab notebook.
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Diode Forward Characteristics
In this part of the assignment, we will be measuring the current through a diode while varying the
voltage drop across the diode. We will maintain a positive voltage polarity relative to that shown in
Figure 1; this will allow us to directly measure the diode forward i-v characteristics. These measured
characteristics will be used to identify unknown parameters in the voltage-current characteristic
(equation 2 in Module 5.1). The voltage-current characteristic can then be used to mathematically
model the diode for later analysis of circuits incorporating the diode.
Pre-lab:
(a) There are essentially two unknown parameters in the diode current-voltage characteristic – I s and
n. (V T is a function of known constants including the diode temperature; the appropriate
relationship is provided in Sedra and Smith.) We will, therefore, need at least two data points to
identify the two constants. Re-write the diode voltage-current characteristic to solve the equation
for I s and n, given two measured (i, v) points. (Hint: taking the natural logarithm of equation (2) in
Module 5.1 will result in a linear relation between ln(i), ln(I s ), and v/nV T .)
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(b) Develop a PSPICE input deck for the diode, which will accept your measured values of I s and n.
To check out your input deck, assume Is=100pA and n=1.679 (later you will adjust these values
to match the values you determine experimentally). An example input deck is shown below.
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* Spice As a Curve Tracer
VD
1
0
DC 700mV
Dtest 1
0
1mA_diode
.model 1mA_diode D (Is=100pA n=1.679)
.DC VD 0V 800mV 10mV
.plot DC I(Dtest)
.probe
.end
Plot a diode i-v curve using this model.
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Lab Assignment 5: Diode Circuits
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(c) Derive an expression for the diode resistance,
rD =
dv
di
(3)
Plot the diode resistance. Where is the maximum diode resistance? Is this result as you would
expect from the plot of the diode characteristic curve?
Lab Procedures:
(a) Connect the equipment shown in Figure 2 using a switching diode. Adjust the power supply
voltage from 0 to 15 volts and record the V FWD and current. Record at least ten points and plot the
i – v characteristics in your lab notebook. Demonstrate your circuit to the TA and have them initial
your lab notebook and the lab checklist.
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(b) Repeat part (a) using a rectifier diode.
+
DMM
A
R = 200
D.C.
Power
Supply
V
DMM
FWD
Figure 2. Experimental set up to measure diode forward characteristics.
(c) Use the TEK 571 curve tracer to obtain a hard copy output of the forward characteristics of both
the switching and rectifier diodes. Overlay a sketch of the TEK 571 plots on the diode data you
generated in parts (a) and (b). Make sure you plot your points from parts (a) and (b) on the TEK
571 printouts.
Post-Lab exercise:
Using the data acquired in parts (a) and (b) above,
(i) Use the relationship you developed in part (a) of the pre-lab to estimate I s and n for the diode.
(Note: using reasonably widely separated data points will result in less sensitivity to small
errors in your data.) Assume the junction temperature to be room temperature (300°K) when
you calculate the thermal voltage, V T .
(ii) Determine I s and n using a least-squares fit of a curve to the measured data. Use whatever
software you wish to perform the curve fit. As in (i) above, assume the junction temperature to
be room temperature (300°K) when you calculate the thermal voltage, V T .
(iii) Plot the analytical diode voltage-current curves obtained in parts (i) and (ii) against the
measured data. Comment on the results you obtained in part (i) vs. those obtained in part (ii)
and the confidence you have in each estimate.
(iv) Use PSPICE to generate voltage-current curves for both switching and rectifier diodes, using
the values of I s and n determined in part (ii) above.
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Lab Assignment 5: Diode Circuits
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II. Diode Reverse Characteristics
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In this part of the assignment, we will be measuring the current through a diode while varying the
voltage drop across the diode. We will maintain a negative voltage polarity relative to that shown in
Figure 1; this will allow us to directly measure the diode reverse i-v characteristics. Diode currents in
the reverse region are very small – on the order of pico-amperes. The resolution available for current
measurement using the Fluke 45 is not sufficient to measure currents of this magnitude. (The most
sensitive range of the Fluke 45 for DC current is 10mA with resolution of 0.1µA.) We will therefore
measure the voltage drop across the diode using the Fluke 45 and use the internal resistance of the
voltage measurement device to infer the diode current. The experimental setup is shown in Figure
3. The Fluke 45’s nominal internal resistance is 10MΩ. With the DC voltage range set to mV we
have a full-scale range of 100 mV; dividing these values by the 10MΩ internal resistance results in an
approximate resolution of 1pA and a full-scale range of 10nA.
Pre-lab: None
Lab Procedures:
(a) The manufacturing specification for the Fluke 45 lists the input resistance for all voltage ranges to
be 10MΩ, as indicated in Figure 3. Do not trust the 10MΩ specification. Measure and record in
your lab notebook the DMM resistance using another ohmmeter, making sure to turn on the DMM
with the correct setting.
(b) Connect the equipment shown in Figure 3 with a switching diode. Measure the reverse diode
voltage and the diode current for a range of applied voltages. Compute the reverse diode voltage
as the voltage across DMM1 minus the voltage across DMM2. Plot the reverse characteristics of
the diode and compare the reverse current with I S as determined from the diode equation.
Explain any discrepancy. Demonstrate your circuits to the TA and have them initial your lab
notebook and the lab checklist.
(c) Repeat the experiment of part (b) for a Zener diode.
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D.C.
Power
Supply
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DMM2
+
V
DMM1
10 MEG
-
Figure 3. Experimental set up to measure diode reverse characteristics.
III. TEK 575 Transistor-Curve Tracer
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Measuring diode characteristics by hand is tedious and the TEK 571 curve tracer has several
limitations. Therefore, it is useful to understand the operation of the other transistor curve tracer
available to us – the Tektronix 575 Transistor Curve Tracer. In this part of the lab assignment we will
use the TEK 575 curve tracer to determine both the forward and reverse characteristics of a switching
diode.
Pre-lab: None
Lab Assignment 5: Diode Circuits
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Lab Procedures:
(a) Using the Tektronix 575 Transistor-Curve Tracer connect a switching diode between C and E
terminals with the toggle switch in the center position. Set the collector voltage range to 0-20 volts
and its dial to zero. Set the collector resistance large enough to limit current to ≈ 400mA, the
diode's maximum rating, R C > 20/I max . The display scales (upper right) can be on 1mA/div
(vertical) and 0.2 volts/div (horizontal) with display centered. Set the toggle switch to the diode
and gradually increase the collector voltage, looking for non-zero I. If current remains zero, you
should reverse the diode connections. Record the forward I-V characteristic curve. Be sure to
record the diode’s turn-on voltage. Compute the dynamic resistances, r D , at 1mA and 10mA;
you may change the scales on your plots to increase the accuracy of your dynamic resistance
estimate. Reverse the voltage and increase R C to 100KΩ. Expect very little current with the
voltage between 0 to 20 volts; select the current to 0.01m A/div.
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To find the reverse breakdown voltage, you can switch to the 0-200 volt collector selection. You
must CAUTIOUSLY increase voltage from zero. When you reach a sharp break in current STOP INCREASING VOLTAGE.
Provide in your lab notebook a comparison between the results obtained using the TEK 575 with
your previous results. Show your results to the TA and have them initial your lab notebook.
IV. Circuit Design/Linearization
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In this part of the lab assignment, we will design a circuit for voltage regulation. The basic circuit we
will use is shown in Figure 4 – by connecting the load across the diode terminals, the voltage supplied
to the load will be somewhat isolated from fluctuations in the power supply voltage. Our design
problem is to choose a resistor (R) and diode which will provide 0.7V to a load connected across the
diode, with minimal variations from the nominal value. Your choice of diodes is limited to either the
switching or rectifier diode for which you generated characteristic curves in part I of this lab.
Imperfect
DC power supply
with
“ripple”
R
+
+
AC
vd
vs
-
-
DC
Figure 4. Voltage regulator
Pre-lab:
Note: Completion of the pre-lab for this portion of the lab assignment requires knowledge of the
voltage-current forward characteristics of the diodes. These characteristics are measured in Part I of
this lab assignment. Thus, Part I of this lab assignment should be completed prior to completion of
this pre-lab.
Lab Assignment 5: Diode Circuits
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(a) Basic circuit equation : Application of KVL around the circuit of Figure 4 results in:
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ID =
Vs − V D
R
where I D is the current through the diode. We will use this expression in the following prelab
exercises.
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(b) Design R to bias the circuit to the proper operating point : For both the switching and rectifier
diodes that you tested in Part I of this assignment, use constant voltage drop diode models in part
(a) above to design R. Recall that we wish to bias the circuit to provide a 0.7 V voltage drop
across the diode. Note that, in general, you will obtain different values of R to appropriately bias
the circuit for each diode. Does the constant voltage drop model allow you to choose a particular
diode to minimize the fluctuations in the diode voltage? Why is this model useless for predicting
fluctuations in the voltage across the diode?
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(c) Small signal linearization : Linearize the governing equation for the diode voltage-current
characteristic equation, for variations in the diode voltage, v d , around some nominal operating
point. Small signal linearization is discussed in section 3.3.8 of Sedra and Smith and in Module
5.2. Substitute the linearized equation for the diode current into the circuit equation from part (a)
to obtain a linearized model of the circuit. Note: this linear equation will relate the fluctuating part
of the voltage drop across the diode to the power supply voltage, v s (t). Note that this fluctuating
component must be superimposed upon the bias value to obtain the overall diode voltage. Your
linearized model will be a function of I s and n at this stage. Use the linear model to predict the
diode voltage fluctuations as a function of the power supply voltage fluctuations.
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(d) Perform a system design (e.g. choose R and a particular diode to provide the desired bias point
and minimize voltage fluctuations across the diode). To do this, you will use your linear equation
from part (c) above and the diode characteristics you measured in Part I of this assignment to
choose a diode (either the switching or rectifier diode for which you have characteristic curves
from part I) and determine R such that the nominal diode voltage drop is 0.7V and the excursions
from this operating point are minimized.
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Lab Procedures:
Construct the circuit of Figure 4, using the resistor and diode selected above. Apply the following
input voltage signal to the circuit:
v s (t)=8+sin(200πt) V
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Note: Figure 4 provides a conceptual approach toward generating this signal; you may create the
desired signal any way you wish. (Note: it is strongly recommended that you create the
sinusoidal signal plus offset by using the DC offset feature of your function generator.)
Measure the voltage response across the diode. Sketch the response wave forms and determine the
maximum variation in diode voltage. Demonstrate your circuits to the TA and have them initial your
lab notebook and the lab checklist.
In your final report, discuss the magnitude of the fluctuations in the diode voltage relative to the
magnitude of the fluctuations of the supply voltage. Also provide the expected diode voltage
Lab Assignment 5: Diode Circuits
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fluctuations as predicted by your small signal linearized model. Compare the experimental and
analytical estimates and discuss your results.
V. Diode Circuit Applications
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In this section of the lab assignment, we will implement some simple but useful diode circuits. Note:
when discussing your results in your final report, you may assume that the diodes behave ideally.
Pre-lab: None
Lab Procedures:
(a) Diode clipping circuit :
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Construct the circuit shown in Figure 5. Measure and sketch V out (t) in your lab notebook for DC
offset values of 1 volt and 3 volts. Discuss the operation of the circuit in your lab notebook.
Demonstrate your circuits to the TA and have them initial your lab notebook and the lab checklist.
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R = 1KΩ
Switching
Diode
5 V 0-P
Triangular
Wave
V out
DC 1V/3V
Figure 5. Diode clipping circuit.
(b) Voltage Doubler Circui t:
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Construct the circuit shown in Figure 6. You may use either switching or rectifying diodes, but use
the same type of diode throughout the circuit. Measure and sketch v out (t) in your lab notebook.
Discuss the operation of the circuit in your lab notebook. What should the voltage v out (t) look like?
Demonstrate your circuits to the TA and have them initial your lab notebook and the lab checklist.
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Figure 6. Voltage doubler.
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Lab Assignment 5: Diode Circuits
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Lab Report:
In your lab report, provide a summary of the results of this lab assignment. You should include, at a
minimum, all items indicated on the lab checklist. Append the lab checklist sheet with teaching
assistant initials indicating completed lab demos to your report.
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