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Lab Assignment 8: BJT Characteristics
Revision: October 27, 2014
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Overview
In this lab assignment, we will experimentally determine the characteristic curves for both n and p
channel BJTs. We will use these curves to estimate BJT model parameters.
Before beginning this lab, you should be
able to:
•
•
•
•
Sketch circuit symbols for npn and pnp
BJTs; labeling collector, emitter and
base terminals (Module 8.1)
Sketch typical characteristic curves for
npn BJTs, labeling collector-to-emitter
voltage, base current, and collector
current (Sedra and Smith, section 5.2)
Identify active and saturation regions on
npn BJT characteristic curves (Sedra
and Smith, section 5.2)
State voltage-current relationships for
npn BJTs (Module 8.1).
After completing this lab, you should be able to:
•
•
Experimentally determine the characteristic
curves for both npn and pnp BJTs
Use experimental characteristic curves to
determine the BJT model parameters β, ro,
VA, & rπ.
This lab exercise requires:
•
•
•
•
EE 352 Analog Parts Kit
Breadboard
Mechanical subsystem (DC motor, rotational inertia
Function Generator, oscilloscope, DC power supplies
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.
Contains material © Digilent, Inc.
4 pages
®
Page 2 of 4
Lab Assignment 8: BJT Characteristics
I. BJT Characteristics
In this lab assignment, we will display the first quadrant collector characteristics of an npn (2N3904)
transistor by the use of Tektronix 575 and 571 curve tracers. From these characteristics, you will
determine:
(a) common-emitter current gain, β0
(b) collector output resistance,
(c) Early voltage, VA, and
(d) the small signal input resistance, rπ.
Figure 1 provides pin-outs and nominal operating characteristics for the 2N3904 BJT.
C (Collector)
B
(Base)
E
B
C
E (Emitter)
2N3904 - IC < 400mA; VCE < 40 volts; PC = 1W; β > 100
Figure 1. Transistor Terminals and nominal operating characteristics.
Pre-lab: None
Lab Procedures:
1. Initially connect the 2N3904 transistor (npn) to the appropriate terminals of the TEK 575 curve
tracer with the toggle switch at the middle position. For a metal capped transistor the emitter is
tagged, with the base in the middle.
2. Use the TEK 575 to generate curves similar to those shown in the EE 311 text (Sedra and Smith)
Fig 5.21 (replicated as Figure 2 below). The conceptual circuit that is being created is shown in
Figure 2(a). The base current IB is set to some value, the collector-emitter voltage vCE is varied
and the collector current IC is measured and plotted as a function of vCE. The IB is then changed to
a different value and the process is repeated. This process is automated by the TEK 575;
suggested settings are as follows:
In the selector sweep section (lower left) select the 0-20V, npn voltage settings, initially set VC =
0V, and select an appropriate load resistor > 20v/400mA. In the base step generator section
(lower right) select repetitive, 4 - 12 steps, + polarity, 240 steps/sec and IB steps ≈ 10µA. Display
IC on the vertical, 2mA/div, VCE on the horizontal, and 2 volts/div. Draw the curves as data similar
to Figure 2(b). Compute current gain β, collector resistance ro = ∆VCE/∆IC and Early voltage VA.
You may cautiously increase VCE, looking for non-linearities as you approach rated limits. Put
another 2N3904 into the right-hand socket; visually compare to the first transistor.
Page 3 of 4
Lab Assignment 8: BJT Characteristics
3. Now observe the INPUT characteristics on a graph similar to the EE 311 text (Sedra and Smith)
Fig 5.16 (replicated as Figure 3 below). Simply switch the display section (upper right) to base
current (vertical) vs. base to emitter voltage (horizontal), both on blue scales. The vertical scale
corresponds to the IB steps selected (lower right). You may set the horizontal on 0.2 volts/div.
Record curves and compute rπ.
rπ =
∆ v BE
∆ iB
4. Determine the reverse characteristics of the npn BJT. This process is identical to the process of
procedure 2 above, but with the C and E connections reversed. To do this:
Return to the display of the output characteristics for the 2N3904. Bring VC to zero; reverse the C
and E connections of your transistor; gradually increase VCE. The curves somewhat resemble
those of part 1, but much less current IC. Keep IC small to avoid damaging the transistor. Sketch
the curves in your lab notebook; compute the reverse β.
5. Using the Tektronix 571 Curve Tracer, make a hard-copy plot of the 2N3904 I-V characteristics
near a quiescent point, Q, of 1mA and 6 Volts. Compare them with your TEK 575 calculations.
Figure 2. npn iC-vCB BJT characteristics. (From Sedra and Smith, Figure 5.21)
Lab Assignment 8: BJT Characteristics
Page 4 of 4
Figure 3. npn iC-vBE characteristics. (From Sedra and Smith, Figure 5.16)
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.