EET-221
Exp 6: Intro to the BJT and the Base-Biased Transistor
Introduction
BJT Transistors:
A BJT (or any transistor) can be used either as a switch with positions of on or off, or an
amplifier that controls its output at all levels in between the extreme on or off positions. And for
either a switching or an amplification application, either a PNP or NPN BJT can be used. This
lab will focus primarily on the NPN BJT.
Never
Points IN
Points In
Proudly
C
B
IC
VBC
C
B
B
IB
VCE
B
VCE
IE
VBE
IE
E
E
E
Figure 1: BJT Schematic Symbols
IC
VBC
IB
VBE
E
PNP
C
NPN
C
Figure 2: BJT Voltages and Currents
Whether used as a switch or in an amplifying circuit, the main idea of the transistor is to control
the transistor’s relatively large output with a small amount of input. More specifically, the BJT
allows us to control the large amount of current flowing through the collector-emitter (ICE)
junction with a small amount of base current (IB).
Often, it is useful to model an important part of these transistors’ behavior by imagining a diode
between the base-emitter (BE) junction in the direction of the schematic symbols’ arrows. The
forward-biasing of this junction is the first thing that needs to happen in order to turn the
transistor on. Specifically, this means that for a NPN, VBE = +0.7V and for a PNP, VBE = -0.7V.
Figure 3 shows an example of a BJT circuit.
IC
RC
VRB
IB
CE
junction
VBB
RB
VCC
Figure 3
VBE
IE = IC + IB ≈ IC
base loop
collector loop
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Author: Dan Kruger
EET-221
Exp 6: Intro to the BJT and the Base-Biased Transistor
Since the two circuit loops share the emitter terminal, this is an example of a specific common
emitter circuit, and so the loops are named after the terminals that are unique to each loop.
IB (very small) is being used to control the current flowing in the collector loop (IE ≈ IC). The
base terminal is essentially a valve controlling the current in the collector loop. Its only role is to
control IE via its input, the much smaller current IB.
BJT as an Amplifier:
When a BJT is used as an amplifier, for every amp that goes into the base terminal (IB), there will
be βDC times that current flowing out of the collector terminal. βDC (or simply β) is known as the
DC current gain of the BJT. Unfortunately, β is difficult to precisely control and varies from BJT
to BJT, within those with the same part number and even the same production lot. It is typically
between 100 and 300. This unpredictable current gain can be troublesome for an amplifier, but
can be mitigated with methods discussed later in this course.
Also, the voltage drop experienced as the collector current flows through the CE junction (VCE)
is greater than 0.3V. When these things are happening, the BJT is said to be in the active region.
Active BJT (amplifier):
 IC   DC I B

VCE  0.3V
BJT as a Switch:
As you know, a switch has two positions: off and on. To turn the transistor off, all that needs to
be done on the base terminal is to cut its current off, then the CE junction stops conducting.
Turning the transistor to the on position means letting nearly all current possible flow through the
CE junction.
A BJT is a passive device. This means that its output, IC, can only be as large as the circuitry in
the collector loop allows. Again, the base terminal is a like a valve controlling current through
the CE junction. The only control the transistor has over this current comes from varying how
well its CE junction conducts current (or, thus, varying the CE junction’s resistance).
See Figure 4 for an analogous circuit representing the collector loop in Figure 3.
IE ≈ IC
RC
VCC
RCE
Figure 4
CE Junction
Resistance
collector loop
Author: Dan Kruger
2
EET-221
Exp 6: Intro to the BJT and the Base-Biased Transistor
To turn this device on, we want to allow the most current possible through the CE junction. All
the transistor can do is to turn its resistance between the C and E terminals down as low as it can
go, until there is less than 0.3V across the CE junction.
Looking at the circuit above, the maximum possible emitter current is going to be determined
entirely by VCC and RC, things external to the transistor.
I C max 
VCC
RC
To make this fully on position happen, the transistor’s input (base) must have enough current
flowing into it so the transistor is “demanding” more current than the collector can supply. When
this happens, the transistor is operating in the saturated region.
Saturated BJT (switch, on):
  DC I B  IC

0V  VCE  0.3V
If there was 1μA flowing into the base terminal, and β was 200, then this would “demand” that
200μA flow in the collector loop. If VCC was 5V, and RC was 10kΩ, then ICmax would be 50μA.
The transistor would be saturated, letting nearly all of the 50 possible μA flow through its CE
junction.
When it is desired that a transistor be put in saturation under all possible varying conditions, this
is called hard saturation. There needs to be enough IB so that  I B  IC max for all possible values
of β and all possible values of VCE while saturated (0-0.3V).
CE Junction Conduction:
In order for the CE junction to conduct at all, the BE diode must be forward biased. That is, for
an NPN BJT, VBE must be +0.7V.
The conductivity of the CE junction is controlled by IB as described earlier. To control IB in the
circuit in Figure 3, we can manipulate both VBB and RB. IB can be found by applying Ohm’s law
to RB, since, assuming the transistor is conducting, the voltage on either side of RB is known.
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Author: Dan Kruger
EET-221
Exp 6: Intro to the BJT and the Base-Biased Transistor
Collector Curves:
Collector curves give us visual of how much collector current (y-axis) we can pass through a
diode as the diode’s collector-emitter voltage varies (x-axis).
Collector Curves
(IC vs |VCE|)
IC (mA)
saturation
Figure 5
4
active
IB = 40μA
3
IB = 30μA
breakdown
IB = 20μA
2
IB = 10μA
1
cutoff
1V
|VCE|
breakdown
(~40V)
Each curve is for a set input, or base current (IB). As we can see from the plot, base current has a
definite effect on the transistor’s output (IC).
In order to more directly see the effect the transistor (IB) has on its output (IC), we will set up a
base biased circuit, and then change its input (IB). We will record values for the output (IC) and
then plot IC versus IB, or output versus input. We will then be able to see the cutoff, active, and
saturated regions in a different way. These will not be collector curves, but, simply, a plot of
output current versus input current.
A base-biased circuit is one that establishes a fixed base current. Because the current is fixed, we
can change the circuit to have controllable values of IB. Since DC current gain varies from
transistor to transistor, we will first measure βDC for three different transistors. In the last part of
the lab, we will take our highest and lowest values of βDC and plot their IC versus their IB and see
how DC current gain affects the circuit’s operating regions.
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Author: Dan Kruger
EET-221
Exp 6: Intro to the BJT and the Base-Biased Transistor
Pre-Lab
Unless your instructor says otherwise, it is recommended that you do this lab in a spreadsheet.
There are repetitive calculations required and plots will be made from the data.
1) Devise a way to obtain a measured value for DC current gain using for the circuit in
Figure 6 using only your voltmeter and ohmmeter (no ammeter). State clearly what you
intend to measure directly, and what calculation(s) you will do to obtain a measured value
for DC current gain.
VCC
15V
RC
Figure 6
1kΩ
VBB
5V
C1
C2
10µF
RB
10µF
The capacitors are only there as a precaution against noise.
2) Create Excel tables for your Part A, B, and C measurements and calculations.
3) Create Excel tables for your Part C predictions for IB (in µA) and IC (in mA) for each of
4)
the base resistor RB1 through RB12. Here, you assume that βDC is 200. If you use Excel
properly, you can do this by copying and pasting your work from the previous step, and
making a few small changes. Remember, IC has an upper limit that it reaches when
saturation occurs. You can have Excel check for saturation using the IF function.
Plot your results for IC versus IB from steps 3 and 4 on the same plot.
Required Equipment:







Resistors (Ω): RC = 1k, RB1 = 1M, RB2 = 560k, RB3 = 180k, RB4 = 100k, RB5 =
82k, RB6 = 68k, RB7 = 56k, RB8 = 47k, RB9 = 39k, RB10 = 33k, RB11 = 18k, RB12 =
10k
Capacitors (F): C1 = C2 = 10µ (or larger)
three 2N3904 transistors
one 2N3904 transistor (bad, get from instructor)
one 2N3906 transistor
DMM
Power supply
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Author: Dan Kruger
EET-221
Exp 6: Intro to the BJT and the Base-Biased Transistor
Part A: Transistor Measurements
1) Look up the datasheet for your 2N3904 and 2N3906 transistors and sketch their pinouts
2)
in your lab book.
Use your DMM’s diode test mode to obtain the barrier voltages for all possible junctions
(pairs of transistor pins) for the 2N3904 and 2N3906 transistors. Record your DMM’s
results in a table like the one in Figure 7.
DMM Measurements of Vbarrier w/ Diode Tester
Diode
BC
CB
BE
EB
CE
Good NPN
Bad NPN
Good PNP
EC
Figure 7
3) Obtain a bad NPN transistor from your instructor and complete the same tests for this
4)
transistor, recording your results in the same table.
Using your results for the good NPN transistor, sketch a rough model of what is going on
inside the transistor. Label the pin names on your transistor model.
Part B: Measuring DC Current Gain
1) Assemble the circuit in Figure 6 using RB4 and one of your working 2N3904 transistors.
2) Use the method you developed in the Prelab to obtain a measured value for DC current
3)
gain.
Repeat steps 1 and 2 for your other two good 2N3904 transistors. Be sure to keep track
of which physical transistor had what value of gain.
Part C: Plotting IC vs. IB
1) Select the transistor with the lowest value for DC current gain from Part B.
2) You should still have your circuit built from Figure 6 with RB1. Directly measure VCE.
Perform the measurements necessary to obtain measured values IB and IC without using
an ammeter.
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Author: Dan Kruger
EET-221
Exp 6: Intro to the BJT and the Base-Biased Transistor
3) Repeat step 2 using each of the remaining base resistors (RB1 through RB12) as your base
4)
5)
6)
7)
8)
resistor.
Select the transistor with the highest value for DC current gain from Part B and repeat
Part C steps 1-3.
Create a plot of IC versus IB for both of these transistors. Plot both of these data sets on
the same graph as your prelab’s graph (right click the graph and click “Select Data…” to
get started). Indicate which plots are the prelab prediction, and which are your higher
and lower beta-valued transistors.
Where is each transistor in its active and saturated regions? From your plot, determine
the precise location (coordinates) of this border between the two regions.
Which of the two transistors saturates more easily? Why?
If you needed a higher maximum collector current out of this circuit, what things could
you change to make that happen?
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Author: Dan Kruger