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Bipolar Transistors I
The first test of a transistor
This lab uses an NPN transistor, the 2N2219. Like any NPN transistor, it consists
of two PN junctions, as shown in Fig. 1 (b)
You can make a crude test of a transistor by using the diode test option on a
multimeter. This option sets the terminals of the multimeter so as to forward bias the
junction and then to read the voltage across it. For a silicon transistor like the
2N2219 you expect to find a forward voltage of 0.6 or 0.7 volts. Test both the base-
collector and the base-emitter junctions of your transistor. (Note: With the
multimeter in diode-test mode, you can determine which meter lead is positive by
connecting the leads to the oscilloscope.)
2. Common emitter characteristics
The goal of this section is to make a plot showing the collector current I C
vs. the
collector-emitter voltage V CE
for several different values of the base current I B
To eliminate some of the repetitious work we will do this using the LabView
program Tran_param.vi. Figure 2 below shows the circuit that is used to do the
measurement. The voltage on the collector is swept through a range of values chosen
by the student for several values of the base voltage. The collector current is
measured by measuring the voltage drop across the (nominally) 1k resistor and the
base current is obtained similarly by measuring the voltage drop across the 1M
resistor. The resistors should be measured with the Multimeter and their exact values
used in the program.
e
b
c
e
c
b
e
c
a)
b
b) c)
Figure 1 : Views of NPN Transistor: a) schematic, b) functional, c)
physical
Follow these steps:
(1) Enter the values of the resistors.
(2) Choose a range of values for the base voltages and the number of points to
measure.
(3) Similarly select a range of collector voltages and number of points.
The program will return plots similar to the one below with the base currents
corresponding to each line given in a register.
Calculating β :
The current gain of the transistor, β, is the change in collector current divided by the
change in base current. From the data above, choosing V CE
=6 volts as typical, one
can calculate β as follows:
" 3
" 6
1M
1k
b
e
c
0 - 10V
0 - 10V
Figure 2 : Circuit for 2N2219.
I
C
(mA)
V
CE
(Volts)
1.3 1.36 1.
For IB=7.2μA
For I B
=4.4μA
Figure 3 : Collector Current vs. Collector-Emitter Voltage.
(c) Measure the voltage gain of the amplifier by comparing the input and output AC
signals on a scope. Is the amplifier an inverting amplifier?
An emitter bypass
Add a 10 μF capacitor, observing the correct polarity, to bypass the emitter
resistor as shown below and re-measure the gain of the amplifier. It should have
increased substantially.
7. The emitter follower
A few simple changes to the common emitter circuit above completely change the
character of the amplifier.
10 μF
R
1
=100k!
R C
=10k!
R E
=1.0k!
R 2
=12k!
2N
10 V
5 V pp
Vout
10 μF
10 μF
R 1
=100k!
R E
=10k!
R 2
=100k!
2N
10 V
5 V pp
Vout
Figure 6 : Emitter Follower.
Figure 5 : Bypass capacitor added.
(a) Construct the emitter follower circuit in Figure 6 below:
(b) Explain why resistors R 1
and R 2
are approximately the same for an emitter
follower, given that we would like the quiescent value of V out
to be about 5 volts.
(c) Measure the gain and show that it is close to unity.
(d) Show that the amplifier is non-inverting.
