Using the Agilent 54621A Digital Oscilloscope as a
Curve Tracer for a BJT (Bipolar Junction Transistor)
EL 121 – Solid-State Devices
By:
Walter Banzhaf
University of Hartford
Ward College of Technology
USA
Procedure:
Using your own equipment and BJT, repeat the steps that follow, mastering the new oscilloscope
procedures and options in the process.
Equipment Required
• Agilent 54621A Digital Oscilloscope with two 1X cables (BNC - alligator clips)
• Agilent E3631A DC Power Supply to provide DC base current for the bipolar junction transistor
• Agilent 34401A Digital Multimeter to measure DC base current
• Agilent 33250A or 33120A Function Generator to create sine waves
• Components: 10 W ½-watt Resistor, 100 kW ½-watt Resistor, 2N2222 (or similar) NPN Bipolar Junction Transistor,
floppy disk
Introduction:
We normally think of the oscilloscope as a tool to give us a picture of voltage vs. time (for example, a
sine wave). It can also create a graph of voltage vs. voltage (using x-y mode). With a little clever
circuit design, it can be made to produce a graph of current vs. voltage, which is called a volt-amp
characteristic.
A graph of current vs. voltage (the volt-amp characteristic) can show us how a device responds to an
applied voltage. An ordinary resistor has a very linear relationship between the applied voltage and
the resulting current: I = V/R = V(1/R), and R is constant. In other words, current through a fixed
resistor is directly proportional to the voltage across it. This is true unless we exceed the safe current
(and voltage) for the resistor, in which case it can turn into a “SER” (smoke-emitting resistor☺) and
(5 V, 5 mA)
R = V/I
R = 5 V/5 mA =
1kΩ
(0 V, 0 mA)
(8 V, 1 mA)
R = V/I
R = 8 V/1 mA =
8kΩ
be damaged permanently. The graph below shows current (in mA) vs. voltage (in volts) for four
different resistors.
The graphs of current vs. voltage for a short circuit, and an open circuit, are interesting:
Lots of current
and NO
voltage
Short Circuit (R = 0 Ω)
Lots of voltage
and NO
current
Open Circuit (R = ∞ Ω)
Valuable information (forward junction voltage, dynamic resistance) can be obtained from a V-A
characteristic of a diode or other nonlinear device. In this experiment you will use your oscilloscope
as a curve tracer to look at the V-A characteristics of an LED, silicon diode and Zener diode.
I. The Basic Settings of Your Oscilloscope:
1) Turn the oscilloscope ON by pressing the white button at the lower right corner of the CRT screen
(the graticule of the cathode-ray tube). At this time, don’t connect anything to Channel 1 or
Channel 2 vertical inputs, or to the Ext Trigger input.
2) You probably weren’t the last person to use the oscilloscope, and it’s probably NOT set up to do
the measurements you want to make. A good way to start is to return the oscilloscope to its
“Default” condition by pressing the Save/Recall Hardkey in the File area of the front panel, then
pressing the Default Setup softkey. Do this now.
You will see a horizontal line in the middle of the display (CRT graticule), and at the top right is a
blinking “Level”. This area of the display is telling you that the oscilloscope is configured to trigger
its sweep on Channel 1, using positive edge triggering in Auto Level mode, and it’s NOT finding
a signal at the trigger Level of 0.00 V. This makes sense, as there is no input signal at this time.
The default Probe Factor for both channels is 1:1. This is ideal, since in this experiment we will
be using non-attenuating cables with BNC plug on one end, and alligator clips on the other end.
REMEMBER: Pressing and holding ANY key (hardkey or softkey) will bring up a help screen on the
display.
II. Setting the Function Generator to Sweep VCE from 0 V to 10 V:
Connect the output of the function generator directly to the input of the oscilloscope, Channel 1, using
a BNC-to-BNC cable. Set the frequency of the generator to produce a 50 Hz sinusoid, 10
Vpp, with a 5 VDC offset. This sinusoid will go from 0 V to + 10 V. See waveform below:
Shortly we will connect this voltage to the collector of your 2N2222 transistor, and make it sweep the collectoremitter voltage from 0 V to + 10 V.
III. Turning Your Oscilloscope into a Curve Tracer:
1) Put your oscilloscope into X-Y Mode by pressing the Main/Delayed hardkey in the Horizontal section of
the front panel, then press the XY softkey. Channel 1 now becomes the X-axis input, and Channel 2
becomes the Y-axis input. Channel 1 (the X-axis) will soon display the transistor’s collector-emitter
voltage VCE), and Channel 2 (the Y axis) will soon display the emitter current (IE).
2) Construct the circuit below (start with VBB adjusted so that IB = 20 µA). Use 1X BNC-alligator cables to
connect the oscilloscope Channel 1 to the emitter of the BJT, and Channel 2 to the collector of the BJT.
3) Adjust the oscilloscope volts/division and position controls for both Channel 1 and Channel 2 to obtain a
display like the one shown below. If your trace is noisy, you can turn on Averaging and have a much
cleaner disply.
IE
5 mA/Division
IB = 20 µ A
VCE
1 Volt/Division
Origin of this graph
(x = 0 and y = 0) is
shown on the display;
the ground symbols
for channel 1 and
channel 2 show where
the origin is
The X-axis, VCE, is 1volt per division.
The Y-axis, IE, says 50 mV/division. But, since the Y-axis
input is the voltage across the 10 Ω resistor between the
emitter and ground, 50 mV/10Ω = 5 mA/division. And IC ≈ IE.
4) Have your instructor look at your display now, before moving on.
5) Notice in the display above that a single horizontal line (IC vs. VCE) is present: it starts one division above
the origin, and runs horizontally for ten divisions (0 volts to +10 volts) on the VCE axis. Since the IC is
caused by the IB, the DC current gain can be calculated. One division vertically = 5 mA, so the DC current
gain beta (β) = IC/IB = 5 mA/20 µA = 250 A/A.
6) Determine the DC current gain beta (β) for your transistor. Show all work.
7) We need now to use the oscilloscope’s digital memory to “freeze” this single horizontal line (IC vs. VCE)
caused by IB = 20 µA. Then, we’ll increase IB to 40 µA, “freeze” that display, and keep increasing IB (and
“freezing”)until we have a complete “family” of IC vs. VCE curves.
8) To “freeze the single horizontal line (IC vs. VCE) for IB = 20 µA, press the Display hardkey and then press
the ∞ Persistence softkey TWICE. (The first press “freezes” what’s on the screen, and the second press
turns off the ∞ Persistence.
9) Now, increase VBB until the base current increases to IB = 40 µA. Repeat step 8).
10) Now, increase VBB until the base current increases to IB = 60 µA. Repeat step 8).
11) Now, increase VBB until the base current increases to IB = 80 µA. Repeat step 8).
12) Now, increase VBB until the base current increases to IB = 100 µA. Repeat step 8).
13) Now, increase VBB until the base current increases to IB = 120 µA. Repeat step 8).
14) When you’re done, your display should look like the one below. Have your instructor check your display.
IB = 120 µ A
IB = 100 µ A
IB = 80 µ A
IB = 60 µ A
IB = 40 µ A
IB = 20 µ A
IB = 0 µ A
15) Use QuickPrint hardkey to store the display on your floppy disk.
16) Now we’re going to determine the DC current gain, beta, at different values of IB and IC. This will be made
easy by using the Cursors.
17) Turn on the Cursors, and position them as follows:
X1 = 0
X2 = 5.000V Y1 = 0
Y2 = positioned on the IC line labeled IB = 100 µA. Record the Y2
voltage in the table below. Shown below, the Y2 voltage is 253.1 mV.
X-Axis: VCE = 5 V
Y-Axis: V = 253.1 mV, so
IC = 253.1 mV//10Ω = 25.3 mA
IB = 100 µ A
18) Repeat step 17) for IB = 20 µA, 40 µA, 60 µA, 80 µA and 120 µA, and write results in the table below.
IB
20 µA
40 µA
60 µA
80 µA
100 µA
120 µA
V2
IC = V2/10Ω
β = IC/IB