ELEC 225L
Circuit Theory I Laboratory
Fall 2010
Lab #1: Application of PSpice to a Transistor Circuit
Introduction
Nodal analysis is a powerful circuit analysis technique. Its primary advantage is that it provides
a systematic approach for finding the voltages and currents in a circuit. If the method is applied
properly, then a solution will be obtained with a predictable amount of effort. Nodal analysis
also lends itself well to implementation in software. Researchers at the University of California
in the late 1960s and early 1970s were among the first to do this, and the result of their efforts is
now known as SPICE – Simulation Program with Integrated Circuit Emphasis. SPICE has been
modified and improved over the decades, and a version for Windows-based PCs, called PSpice,
has been available for many years. In this lab exercise you will use PSpice to analyze a simple
circuit built around a bipolar junction transistor (BJT) and compare the results to those you
obtain via “pencil-and-paper” analysis and to measurements from the actual circuit.
Furthermore, you will gain practice writing professional-style reports.
Theoretical Background
Before exploring the capabilities of PSpice, it will be helpful to have some information on BJTs.
The bipolar junction transistor (BJT), sometimes simply called the transistor, is one of the most
widely used electronic devices. BJTs are employed as amplifying devices, control devices, and
sensors, to list just a few of their uses. They also serve as the basic building block of many kinds
of integrated circuits, including the popular LM741 operational amplifier. BJTs come in two
basic varieties, known as npn and pnp. The designation indicates the overall structure of the
device. At the most basic level a BJT is fabricated using three layers of silicon, each of which
has had added to it a tiny amount of some other chemical element (such as boron, phosphorus, or
antimony) that either frees up electrons in the crystal lattice or creates gaps in the lattice into
which electrons can become trapped. The former type of modified silicon is called n-type
because of the availability of negatively charged free electrons; the latter is called p-type because
of the positively charged “holes” in the lattice. Because of various performance differences
between the two types, npn BJTs are more widely used, and we will focus on npn BJTs in this
lab exercise.
The circuit symbol for the npn BJT is shown in Figure 1a. The characteristic that makes the BJT
such a useful device is that, under the proper operating conditions, the current that flows into
terminal C (the collector terminal) is directly proportional to the current that flows into terminal
B (the base terminal); that is, IC = FIB, where the parameter F is a constant called the DC
current gain; it typically has a value in the range of 50-300. This property is what makes
possible the use of the BJT as an amplifier; a small base current can control a large collector
current. The terminal labeled E in Figure 1 is called the emitter terminal. The electrical
behavior of BJTs can be modeled in many cases by the simple equivalent circuit shown in
Figure 1b. The linear relationship between IC and IB is represented by a current-dependent
current source. Between the base and emitter terminals there is an almost constant voltage drop
of 0.7 V, and it is represented in the model by an ideal voltage source.
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B
C
IB
C
IC
B
VBE ≈ 0.7
V
IB
FIB
+
−
E
E
(a)
(b)
Figure 1. a) Circuit symbol for npn BJT, and b) a simple circuit model.
Although manufacturing methods have improved steadily since the first BJT was invented in
1947, there is still a surprising amount of performance variation among individual samples of
specific types of BJTs. Most importantly, the value of the parameter F can vary widely, even
among units from the same production run. When designing circuits with BJTs it is often
necessary to establish stable voltages at various points in the circuit that stay at their intended
values regardless of changes in F (and other parameters). One simple way to do this is to place
a resistor of an appropriate value in series with the emitter terminal of the BJT. The resistor
introduces electronic feedback that counteracts the voltage or current changes that result from a
drift in the value of F. A typical example is shown in Figure 2a, and the equivalent circuit
model is shown in Figure 2b. This design technique, which you will study in detail in ELEC 350
and/or 351, ensures that the node voltages VB, VC and VE stay relatively constant. The process of
designing a circuit to achieve specified DC voltage and current values is called biasing. It is one
of the key steps in almost all kinds of electronic circuit design. We will use PSpice to analyze
the biasing circuit and verify that the node voltages are relatively insensitive to the value of F.
VCC
IC
RC
RC
VC
R1
R1
IB
VC
IC
IB
VB
+
VBE −
R2
VB
FIB
VBE ≈
0.7 V
VCC
+
−
VE
+ −
VE
R2
RE
(a)
RE
(b)
Figure 2. a) A BJT biasing circuit, and b) its equivalent circuit model. The small
triangles in diagram (a) represent connections to the reference (“ground”) node.
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Assignment
Complete the following tasks with your lab group:

Follow along on your lab station computer as the instructor guides you through a brief
introduction to PSpice. The tutorial available via the “Supplemental Readings” button at
the Blackboard site should be of considerable help as you work on this part of the lab
exercise.

Use PSpice to find the node voltages VB, VC, and VE and the collector current IC for the
circuit shown in Figure 2b with R1 = 91 k, R2 = 27 k, RC = 5.1 k, RE = 2 k, VCC =
12 V, and F = 150. Record the values of the node voltages and IC for this case; you will
need to include them in a data table later.

Print out the PSpice circuit diagram with the node voltages, branch currents, and power
dissipated/delivered for each part labeled. Make sure that the diagram is readable
(especially the component values) and that all quantities are clearly matched with their
appropriate nodes, branches, and/or components. Give the printed diagram an
appropriate title, and use the text toolbar button to add your group members’ names to the
diagram. Also, be sure to indicate the value of F somewhere on the diagram.

Re-run the circuit simulation with F changed from 150 to 250, and verify that the node
voltages do not change significantly. Print out the PSpice circuit diagram with the new
value of F clearly indicated.

Apply nodal analysis by hand (the “pencil-and-paper” approach) to the BJT circuit model
using the values given in the second bullet above. Produce a clear and concise version of
your solution that you can read later, since it will serve as part of your written report for
this lab. Find the values of node voltages VB, VC, and VE and the collector current IC, and
verify that they closely match the values obtained using PSpice. Solve the system of
equations using Matlab, and save a copy of the relevant parts of the Matlab command
window to use later in your report.

Construct the BJT biasing circuit using the component and voltage values given in the
second bullet above. Use a 2N3904 for the BJT. Measure and record the node voltages
VB, VC, and VE. Determine the value of IC indirectly from the measured node voltages
(i.e., without an ammeter), and record its value as well.

Replace the 2N3904 BJT with a 2N2222 BJT, and verify that the node voltages have not
changed significantly. Record the new measured values of VB, VC, VE, and IC that you
obtain. Once you have verified that the measured node voltages agree relatively closely
with the values you predicted using nodal analysis, demonstrate the circuit to the
instructor or TA.

After the lab session, write a clear, concise, and professional-style report describing in
detail the steps you followed to apply nodal analysis to the circuit. (Imagine that your
supervisor has asked you to explain how you analyzed the circuit.) You should provide
enough details and background information so that a technically competent but
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uninvolved reader could understand the results you are presenting, but do not “pad” the
report with extraneous information. There is no minimum or maximum page limit; make
the report long enough to transmit the important information and no longer. Remember
to include details that are helpful to readers, such as an introduction and conclusion,
diagrams, units on values, well-defined variables, etc. The organization should be that of
a progress report, so your calculations, results, procedures, and other details should be
presented in a chronological, integrated format, not in separate sections. You are strongly
urged to consult the “Lab Report Guidelines” available at the lab web site. Integrate the
data table (described in the next bullet) and the PSpice diagrams into your report as well.

Organize your calculated and measured voltage and current data into a table, and
incorporate it into your report. You should include the values of VB, VC, VE, and IC
obtained using PSpice for both F = 150 and F = 250; the values obtained using manual
(“pencil-and-paper”) nodal analysis for F = 150; the measured values for the 2N3904;
and the measured values for the 2N2222. You may produce the table by hand or using
software. Remember to label the columns properly and include units. Add a row or
column that shows the percentage difference between the PSpice results obtained using
F = 150 and those obtained using F = 250 (use 150 as the “reference” value), using
manual analysis, and by measurement. Make sure that the data table is labeled and
organized so that an engineer only vaguely familiar with the lab exercise could figure out
what all of the data means. You might want to add a descriptive caption to the table.
Table captions should be placed above tables.
Grading
Each group’s written work (nodal analysis solution, circuit diagram, and data table) is due in
class the following Wednesday. General guidelines for writing professional-style reports are
available at the course web site. Cover sheets are not required for reports, but the names of all
lab partners, the lab section (8 am or 1 pm), the course number (ELEC 225), and the lab date (not
the report due date) should appear on the first page. Do not forget to cite other authors’
intellectual property, if necessary. Each group member will receive the same grade, which will
be based on the following distribution:
15%
15%
25%
20%
25%
Nodal analysis “hand” solution – Technical content
Nodal analysis “hand” solution – Organization, neatness, spelling, and grammar
PSpice circuit diagrams, with voltages, currents, and power levels clearly labeled
Data table, with rows and columns properly labeled and organized
Demonstration of working BJT biasing circuit with proper node voltages
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