University of Ottawa Faculty of Engineering
Experiment No. 3 - Bipolar Junction Transistor Characteristics
Name/ID: Date: Mark:
Name/ID: Date: Mark:
1. Objectives
In this lab, you will study the DC characteristics of a Bipolar Junction
Transistor (BJT).
2. Overview
You need to first identify the physical structure and orientation of BJT based
on visual observation. Then, you measure the IC – VCE characteristics of the BJT in
forward active mode. You need to determine base-to-collector DC current gain (hFE),
Early voltage (VA) and common-emitter breakdown voltage (BVCE0).
Information essential to your understanding of this lab:
1. Theoretical background of the BJT (Streetman 7.1, 7.2, 7.4, 7.5, 7.7.2, 7.7.3)
Materials necessary for this Experiment:
1. Standard testing station
2. One BJT (Part: 2N4400)
3. 1kΩ resistor
3. Background Information
Bipolar junction transistors (BJTs) are three terminal devices that make up one
of the fundamental building blocks of the silicon transistor technology. Three terminals
are emitter (E), collector (C) and base (B). Figure 1 shows the transistor symbol for
the npn transistor, pnp transistor and a schematic of TO-92 package transistor, with
the pin connections identified for the BJT 2N4400. 2N4400 is a general purpose
NPN amplifier transistor.
(c)
Figure 1. (a) NPN transistor symbol, (b) PNP transistor symbol and (c) TO-92
package 2N4400 BJT pin configuration.
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BJTs are used to amplify current, using a small base current to control a large
current between the collector and the emitter. This amplification is so important
that one of the most noted parameters of transistors is the dc current gain, β (or
hFE), which is the ratio of collector current to base current: IC = β*IB. In designing
an amplifier circuit using BJTs, there are several important and sometimes
conflicting factors to be considered in the selection of the DC bias point. These
include gain, linearity, and dynamic range.
Several BJT bias configurations are possible, three of which are shown in Fig. 2.
The circuit in Fig. 2a is called a common-base configuration which is typically used as
a current buffer. In this configuration, the emitter of the BJT serves as the input,
the collector is the output, and the base is common to both input and output.
The circuit in Fig. 2b is called common- emitter configuration which is typically
used as an amplifier. In this circuit, the base of the BJT serves as the input, the
collector is the output, and the emitter is common to both input and output. The
circuit in Fig. 2c is called common-collector configuration which is typically used as a
voltage buffer. In this circuit, the base of the BJT serves as the input, the emitter is
the output, and the collector is common to both input and output.
Figure 2. (a) Common base, (b) Common emitter and (c) Common collector
configuration of BJT.
The DC characteristics of BJTs can be presented in a variety of ways. The
most useful and the one which contains the most information is the output
characteristic, IC versus VCB and IC versus VCE shown in Fig. 3.
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Figure 3. Typical I-V characteristics of BJT for (a) common base and (b) common
emitter configuration.
4. Experimental Part
Step 1 Identify the leads of the BJT 2N4400 using Figure 1 and
construct a circuit shown in Figure 4.
Figure 4. A circuit for obtaining the IC-VCE characterisotics.
V1 is used to supply VBE and V2 is used to supply VCE.
Step 2 Obtain IC-VCE characteristic curves using the following
settings:
VCE = 0 V to 4 V in 0.2 V steps with 0.1 A supply current
limiting.
IB = 10 μA to 60 μA in 10 μA.
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Step 3 Use the measured data and calculate hFE. Note that IC is likely in the mA
range while IB is in the μA range. The common-emitter DC gain (base-to-
collector current gain, hFE) is calculated by hFE = IC/IB with VCE at a
constant voltage. hFE is also called βF, the forward DC current gain. It is
often simply written as β, and is usually in the range of 10 to 500 (most
often near 100). hFE is affected by temperature and current.
Table 1. IC-VCE characteristic of the BJT 2N4400.
IB 10 20 30 40 50 60
[μA]
VCE IC hFE IC hFE IC hFE IC hFE IC hFE IC hFE
1V
2V
3V
4V
Step 4 Discuss about variations of the DC current gain with different values of IB
and VCE.
Step 5
Now, using the same set of data that you got for the DC current gain
measurement, estimate the small-signal current gain h fe and fill out the
Table 2 below. The small-signal current gain is calculated by hfe = ΔIC/ΔIB
with the VCE at a constant voltage.
Table 2. Small-signal current gain, h . Subscripts 1 denotes I = 10 μA, 2 denotes I = 20 μA,
fe B B
3 denotes I = 30 μA, and so on.
B
VCE hfe (IB2, IB1) hfe (IB3, IB2) hfe (IB4, IB3) hfe (IB5, IB4) hfe (IB6, IB5) hfe (IB5, IB2)
1V
2V
3V
4V
Step 6 Discuss about variations of the small-signal current gain with different
values of IB and VCE.
Step 7
Again, using the same set of data, estimate the output conductance hoe
and fill out the Table 3 below. The output conductance is calculated by hoe
= ΔIC/ΔVCE with the IB at a constant current.
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Table 3. Output conductance, hoe. Subscripts denote VCE values.
VCE3, VCE1 VCE4, VCE2
IB [μA] IC hoe IC hoe
10
20
30
40
50
60
Step 8
Discuss about variations of the output conductance with different values
of IB and VCE.
Step 9
Use the IV plot to extrapolate your data set as shown below using lines
and find the Early Voltage (VA). The Early voltage is typically in the range
of 15 V to 200 V.
Figure 5. IC-VCE characteristics of a BJT and the Early voltage (VA).
Step 10 explain why you have the Early voltage.
Step 11 Verify your results using Multisim simulation tool for steps 2,3, 5, 7 and 9
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Write a laboratory report including introduction, your
results and conclusions of the conducted experiments!
Do not forget to answer the questions raised in the
laboratory sheets in your laboratory report!!
