# Lab 3 ```Lab 3
Common Emitter I-V Characteristics of a Bipolar Junction Transistor
Purpose
In this lab, transistor current-voltage (I-V) characteristics of Common Emitter (CE)
circuit will be investigated. The characteristics will be derived both by I-V measurements
and using Elvis Three-Wire Current-Voltage Analyzer.
Material and Equipment
NI ELVIS
External Multimeter (for current measurements)
Regulated Power Supply
2N3904
Assorted Resistors(500 , 27k )
Theory
The bipolar junction transistor(BJT) can be modeled as a current controlled current
source. The circuit symbol and the pin out for the actual device can be seen in Figure 3-1.
Figure 3-1: The bipolar junction transistor.
As an aid in visualizing the current/voltage relationships and transistor operation,
a family of static characteristics plots of collector current versus collector-emitter
voltage for several values of base current are plotted using circuit in Fig. 3-2.
Figure 3-2: CE configuration circuit using npn transistor
9
Figure 3-3: Characteristic curves for the BJT transistor.(CE)
These curves in Fig. 3-3 can be used to calculate the large signal current gain
(or hFE) and the small signal current gain, AC(or hfe). These values are in general
calculated for a given bias point ICQ, VCEQ using the following equations:
DC
AC
DC
= ICQ / IBQ
= | ICQ - ICQ’ |/| IBQ- IBQ’|
From this, one can see that a large signal gain depends only on the Q point and the small
signal gain depends only on small deviations around the Q point.
Procedure
1) Transistor Common-Emitter Collector Characteristics using IV Measurement
a) Connect the circuit shown in Figure 3-4 below. Use Elvis Variable Power Supply,
VPS+ for VBB and use the External + Voltage Source Vext+ for VCC.
Hint: Use the External Multimeter provided to perform current measurements.
Figure 3-4 I-V Measurement Set-up
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b) Vary Vcc from 0-10V in steps of 1 and measure (record) the collector
current IC and the voltage across collector and emitter (VCE) .
c) Repeat the procedure for different values of IB (vary IB from 0-200&micro;A in
steps of 50&micro;A).
d) Plot the V-I characteristics with IC on Y-axis and VCE on X-axis.
e) Calculate DC and AC from the curves.
f) Calculate transconductance parameter gm.
2) Transistor Common-Emitter Collector Characteristics using Elvis Three-Wire
Current-Voltage Analyzer
a) Connect the 2N3904 transistor as shown in figure 3.5 below on your
LUEE Elvis Board
Figure 3.5: Transistor Connections for Three-Wire IV Analyzer
b) Run the Three-Wire Current-Voltage Analyzer. This should bring a
Graphical User Interface (GUI) for running the analysis. Vary Base
Current Ib from 30&micro;A to 60&micro;A in steps of 15&micro;A. Change Collector
Voltage VC from 0-2V insteps of 0.1V, set Collector Current limit to
11 Ma (max). Obtain the I-V plots and include in your lab report.
Questions for the Lab Report
Use all of the data obtained in Procedure 1 to answer the following questions:
1) What is the average value of
2) Give the expressions for
DC
DC,
and
AC
for the 2N3904 transistor?
AC.
3) Determine the collector resistance at VCE = 6V, and IB = 50
microamperes.
Use the following expression
11
```