EE 482 — Electronics II
Lab #1: BJT Characterization
Overview
The objective of the lab is to study the DC current vs. voltage (I–V) characteristics and the smallsignal equivalent model of a bipolar junction transistor (BJT).
Theory
A BJT is inherently a non-linear device, but it can be modeled in a limited region of operation as a
combination of linear components to simplify analysis. The small-signal hybrid- model shown in
Figure 1 is the one that is most commonly used. The dependent current source can also be shown as
 ib, where ib is the small-signal current entering the base node.
Figure 1. Hybrid- BJT model
The capacitors in the model are important only at high frequencies and will be ignored (considered
to be open circuits) in this exercise. Note that the capacitances between base and collector and
between base and emitter correspond to the two pn junctions in the BJT. There is no device
capacitance from collector to emitter. The resistance r accounts for leakage current flowing from
base to collector, which is a reverse-biased pn junction. This leakage current is of the same order of
magnitude as I s in a diode (~ 10 12 A ), and current flowing in that branch can be ignored (r  ∞).
The parameters r, gm, , and ro of the BJT hybrid- small-signal model — as well as a related
parameter, the Early voltage VA — can be extracted from careful experimental measurements. The
required experimental setups are shown in Figures 2 and 3.
Electronics II – EE 482 — Lab #1: BJT Characterization — Rev 1.1 (4/22/09)
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Two sets of curves are needed:
(1) The setup of Figure 2 can be used to make measurements of collector current iC as a function
of collector-to-emitter voltage vCE with base-to-emitter voltage vBE held constant.
Figure 5.19 of Sedra & Smith, 5th ed., shows examples of the expected iC – vCE behavior (the
various curves correspond to different values of vBE); and,
+
100 
–
vR_C
iC
iB
VBB
vR_B
VCC
–
+
100  v+BE –
–
+
vCE
Figure 2. Experimental setup for measuring common-emitter (iC – vCE) characteristics,
with vBE varying parametrically
(2) The setup of Figure 3 can be used to make measurements of collector current iC as a function
of collector-to-emitter voltage vCE with base current iB held constant. Figure 5.21 of Sedra &
Smith, 5th ed., shows examples of the expected iC – vCE behavior (the various curves
correspond to different values of iB.
100 
+
–
iB
vR_C
iC
VCC
+
+
vBE –
–
vCE
Figure 3. Experimental setup for measuring common-emitter (iC – vCE) characteristics,
with iB varying parametrically
Electronics II – EE 482 — Lab #1: BJT Characterization — Rev 1.1 (4/22/09)
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Determination of ro:
First-order BJT theory assumes that the collector current curves flatten out in the active region.
This is implied in the relationship: I C  I B , according to which the collector current is
independent of VCE . This implies that the transistor has an infinite output resistance. The actual
characteristics, however, have a finite slope, indicating that the collector current does depend on
VCE (the Early effect). This is caused by the transistor having a finite output resistance, which is
equal to the reciprocal of the slope of the collector current characteristic. Note that the value of ro
 i
changes as a function of the collector current. Formally, ro   C
 vCE



1
VBE
 i
  C
 vCE



1
. To find
VBE
ro, choose two points on a collector current curve with fixed vBE. The ratio of vCE to iC gives ro.
Determination of VA:
If you extend all of the iC vs. vCE curves (vBE varying parametrically) back to the (horizontal) vCEaxis, you will notice that they all intersect at roughly the same point. (See Figure 5.19 in Sedra &
Smith, 5th ed.) The magnitude of this voltage is the Early voltage, VA (the intercept is –VA).
Determination of :
 i 
 i 
The common-emitter current gain,    C 
  C  . To find , choose two collector
 i B  VCE  i B  VCE
current curves corresponding to slightly different fixed values of base current iB. At a fixed value of
vCE, take the ratio of (a) the difference between the collector currents ( iC ) on these curves to (b)
the corresponding difference in the base currents ( i B ).
Determination of gm:
Transconductance is
 i
voltage: g m   C
 v BE
the change in collector current produced by a given change in base-emitter
vBE

 i 
I
 . Using iC  I S e VT , we have g m   C   C , where VT is the
 IC
 v BE  I C VT
"thermal voltage," with a value of 0.0259 V at room temperature. Experimentally, gm can be found
 i 
 i 
as g m   C    C  . To find gm, choose two collector current curves corresponding to
 v BE  I
 v BE  v
C
CE
slightly different fixed values of base-to-emitter voltage vBE. At a fixed value of vCE, take the ratio
of (a) the difference between the collector currents ( iC ) on these curves to (b) the corresponding
difference in the base-to-emitter voltage ( v BE ).
Electronics II – EE 482 — Lab #1: BJT Characterization — Rev 1.1 (4/22/09)
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Determination of r :
 v
The element r in the hybrid- model is defined as r   BE
 i B
 v
get r   BE
 i B
 v

   BE
 I C  iC
  iC
 
 I C  i B

 i

  C
 VCE  v BE



1
IC
 iC

 i B

 i
 . Since    C
 IC
 i B

 , we
 VCE



. Thus, one way of obtaining

 VCE g m
r is from experimentally determined values of and gm. Alternatively, r can be found directly
 v 
 v 
from experimental measurements:
In order to compare the
r   BE    BE  .
 i B  I C  i B 

experimentally determined value of r to a calculated value of r 
, it is crucial to determine r
gm
and gm at the same DC condition — i.e., the same IC.
Pre-Lab
Review the planned experiments and record relevant background information and equations that
will be used for determination of BJT characteristics. No simulation is required for this laboratory.
Before coming to the laboratory, study the Theory section in this lab write-up (also covered in
Sections 5.1, 5.2, and 5.6 in Sedra and Smith,5th ed.).
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Lab — Measurement and Calculation of Transistor Parameters
Part 1
Refer to Figure 2. Note that both VBB and VCC are variable. Further, note that VBB is not the same as
vBE, and VCC is not the same as vCE because, in each case, some voltage will be dropped across a 100
 resistor. The resistors are placed in the circuit to provide a means of determining the base and
collector currents. (By measuring the voltage across the 100  resistor, Ohm’s Law can be used to
determine the current that is flowing.) However, you cannot assume that the resistors are precisely
100 . Their values must be measured and recorded before placing them in the circuit.
(1) Measure the actual resistance of the “100 ” resistors, then construct the circuit of Figure 2.
Use a 2N3904 npn bipolar junction transistor.
(2) Set VCC ≈ 1 V initially, and adjust VBB until vBE = 0.70 V.
(3) Keeping vBE fixed at 0.70 V (adjust VBB if necessary), vary VCC over a range of voltages so as to
produce five pairs of measured vCE and iC values. In order to make later extraction of smallsignal parameters valid, you should adjust VCC so as to produce targeted values of vCE — e.g.,
vCE = 1.0 V, 1.5 V, 2.0 V, 2.5 V, 3.0V. For each of the five settings, record the following:
▪ vBE (keeping this constant is crucial)
▪ VBB
▪ VR_B (allows you to calculate iB)
▪ vCE
▪ VCC
▪ VR_C (allows you to calculate iC)
(4) Reset VCC ≈ 1 V, and adjust VBB until vBE = 0.68 V.
(5) Repeat step (3), except keeping vBE constant at 0.68 V. For each of the five measurements,
adjust VCC so as to produce the same vCE values as those in step (3).
(6) Reset VCC ≈ 1 V, and adjust VBB until vBE = 0.72 V.
(7) Repeat step (3), except keeping vBE constant at 0.72 V. For each of the five measurements,
adjust VCC so as to produce the same vCE values as those in step (3).
Altogether, you should have taken 15 (fifteen) sets of measurements. You should now be able to
plot three iC – vCE curves (one each for vBE = 0.68, 0.70, and 0.72 V). These data will allow you to
extract ro, VA, and gm, as described in the Theory section and the text. (Note: Your curves will not
show saturation roll-off at low values of vCE because the target vCE values were chosen to be above
vCEsat.)
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Part 2
Refer to Figure 3. Note that both iB and VCC are variable. Further, note that VCC is not the same as
vCE because some voltage will be dropped across the 100  resistor. The resistor is placed in the
circuit to provide a means of determining the collector current. (By measuring the voltage across
the 100  resistor, Ohm’s Law can be used to determine the current that is flowing.) However, you
cannot assume that the resistor is precisely 100 . Its value must be measured and recorded before
placing it in the circuit.
(1) Measure the actual resistance of the “100 ” resistor, then construct the circuit of Figure 3. Use
a 2N3904 npn bipolar junction transistor.
(2) Set VCC ≈ 1 V initially, and adjust iB until iB = 30 A*.
(3) Keeping iB fixed at 30 A, vary VCC over a range of voltages so as to produce five pairs of
measured vCE and iC values. In order to make later extraction of small-signal parameters valid,
you should adjust VCC so as to produce targeted values of vCE — e.g., vCE = 1.0 V, 1.5 V, 2.0 V,
2.5 V, 3.0V. For each of the five settings, record the following:
▪ iB (keeping this constant is crucial)
▪ vBE
▪ vCE
▪ VCC
▪ VR_C (allows you to calculate iC)
(4) Reset VCC ≈ 1 V, and adjust iB until iB = 40 A.
(5) Repeat step (3), except keeping iB constant at 40 A. For each of the five measurements, adjust
VCC so as to produce the same vCE values as those in step (3).
(6) Reset VCC ≈ 1 V, and adjust iB until iB = 50 A.
(7) Repeat step (3), except keeping iB constant at 50 A. For each of the five measurements, adjust
VCC so as to produce the same vCE values as those in step (3).
Altogether, you should have taken 15 (fifteen) sets of measurements. You should now be able to
plot three iC – vCE curves (one each for iB = 30, 40, and 50 A). These data will allow you to
extract  as described in the Theory section and the text. (Note: Your curves will not show
saturation roll-off at low values of vCE because the target vCE values were chosen to be above vCEsat.)
___________
* If a suitably low level of current cannot be reliably delivered directly from the available power
supply, construct a current divider using two resistors, measuring the actual value of the resistors, of
course.
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Analysis of Data in Tech Memo
 Summarize the measurements procedures followed in Parts 1 and 2 of the laboratory exercise,
and tabulate the data collected.
 Plot the data collected in Parts 1 and 2 of the laboratory exercise.
 Use the data collected in Part 1 of the laboratory exercise to extract small-signal parameters ro,
VA, and gm.
 Use the data collected in Part 2 of the laboratory exercise to extract small-signal parameters 
and r .
 Briefly discuss the results obtained — i.e., did you obtain reasonable, typical values of the smallsignal parameters — and discuss/explain problems encountered, if any.
Your report must be professional in appearance and content. Some common editing problems are
summarized on the next page. Before submitting your paper for grading, take the time to ensure
that it is free of these problems.
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Checklist of Common Editing Problems
 Paginate! Every page should have a page number.
 All figures and tables must be referred to in the body of the text. It is not up to the reader to
determine when to refer to a particular figure or table.
 When a figure or table is referred to in the text, it should be capitalized.
RIGHT: “A comparison of model predictions to experimental results is shown in Figure 5.”
RIGHT: “A comparison of model predictions to experimental results is shown in Fig. 5.”
WRONG: “A comparison of model predictions to experimental results is shown in figure 5.”
 Figures and tables should have captions. Refer to any IEEE journal for examples. The captions
should generally cite the reference from which the information/graph is drawn.
 Subscripts should really be subscripts, not just lowercase and/or smaller point size characters. A
subscript is both (1) smaller in point size, and (2) lower in position relative to the main character.
RIGHT: IDS , vsat
WRONG: IDS, IDS, IDS, vsat, vsat, vsat
 Superscripts should really be superscripts, not just lowercase and/or smaller point size characters.
A superscript is both (1) smaller in point size, and (2) raised in position relative to the main
character.
-3
3/2
RIGHT: cm , V
WRONG: cm-3, cm^-3, cm**-3, Ve3/2, V^3/2
 Variables in the text should be italicized.
RIGHT: VT
WRONG: VT, VT
 Units should be separated by a single space from their number.
RIGHT: 5 nm, 100 V
WRONG: 5nm, 100V
 A number and its units should not be split between two lines. In MS WORD,
CTRL+SHIFT+SPACEBAR gives a non-separating space.
 Decimal fractions should be preceded by a 0.
RIGHT: 0.25 m
WRONG: .25 m
 Don’t confuse “its” and “it’s.” The word its means that something belongs to it. The contraction
it’s means “it is.”
 Latin derivatives should be italicized (or underlined, as a less-preferred alternative).
RIGHT: e.g., i.e., et al., e.g.
WRONG: e.g., i.e., et al., et al
 References, if any, must be complete. The reader must be able to find the paper or book that you
are referencing based solely on the information that you provide. If you can’t provide the
complete reference, don’t include it. Use IEEE format. See any paper from an IEEE journal for
examples.
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Check-Off Sheet
TA Signature: ____________________________ Date: ___________________________
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