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FOUNDATION EXP 8 – BIPOLAR TRANSISTOR
EXPERIMENT 8
BIPOLAR TRANSISTOR
1.0 INTRODUCTION
A transistor is basically a three terminal semiconductor device that may be
used to provide a number of functions. Its chief applications are use as a
switch or as an amplifier. The three terminals of a bipolar transistor are
labelled base, emitter and collector (see Figure 1.1). A small change of
current in the base can give rise to a large change in the current flowing
between the collector and emitter.
Figure 1.1
The circuit diagram for a transistor is shown in Figure 1.2. This is of an NPN
transistor, which will be used in this experiment. The diagram for a PNP
transistor is similar, but the arrow on the emitter is pointing in the opposite
direction.
Collector
Base
Emitter
Figure 1.2
2.0 TESTS
2.1 OPERATION AS A SWITCH
A transistor’s function as a switch is the simplest mode of operation and
involves turning the transistor from a fully on to a fully off state, or vice-versa.
A simple switch is shown in Figure 2.1. When a sufficient positive voltage is
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FOUNDATION EXP 8 – BIPOLAR TRANSISTOR
applied to the base (Vin) the transistor switches on. This allows a current to
flow from the collector and emitter and hence Vout falls to a low value.
+5V
1kΩ
B
Vout
A
Vin
10kΩ
Figure 2.1
Connect up the circuit shown in Figure 2.1 on your breadboard. Take care to
identify the base, collector and emitter correctly (see Figure 1.1). Connect
point A to 0V and measure the value of Vout (voltage between point B and 0V).
Connect point A to 5V and measure the value of Vout. Record your results in
tabular form. Measurements should be taken with respect to ground. Also vary
Vin and measure the value that makes Vout = 2.5V (i.e. half way between ON
and OFF).
2.2 OPERATION AS AN AMPLIFIER
VCC = +10v
A
RA 4k7Ω
10kΩ
RC
-10μF+
+10μF-
1kΩ
INPUT
10kΩ
POT
RB
B
OUTPUT
RE
120Ω
C
+
CE = 47μF
-
0V
Figure 2.2
In this mode of operation, the transistor is set so that its output is halfway
between the fully on and fully off values when no input signal is applied. This
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FOUNDATION EXP 8 – BIPOLAR TRANSISTOR
is known as biasing. A small bipolar signal may then be added to the bias
voltage on the base, to produce a larger (or amplified) output on the collector.
A typical transistor amplifier is shown in Figure 2.2.
Note the use of capacitors on the input and output to block the DC bias voltage
which allows the varying (alternating) signal to pass (refer to Experiment 4).
Connect up the circuit shown in Figure 2.2 on your breadboard. Choose RC to
be 1k. Take care to identify the base, collector and emitter correctly and the
polarity of the capacitors. Set the variable power supply to 10V DC and adjust
the potentiometer (RB) to obtain a 5V DC level at the collector. Do not confuse
the 10V power supply with the input voltage. The power supply is there to
make the circuit work, whereas the input voltage, Vin, is the signal to be
applied to the circuit.
You will need to measure the voltage at the collector at the end of RC as the
capacitor will block this bias voltage from reaching the output.
If you are unclear on the connection or operation of the potentiometer, ask for
help. A potentiometer has three legs. The leg in the middle represents the line
with the arrow in the symbol for a potentiometer (or variable resistor) in the
circuit diagram.
Apply a 300mV peak-to-peak, 1kHz sinusoidal input signal (from the function
generator) to the input of the amplifier. Use Channel 1 of the oscilloscope to
set and display this signal. Connect Channel 2 of the oscilloscope across the
output to display and measure the output signal.
It is important to check that the output signal is nearly sinusoidal with no
flattening around its peaks, so ask staff to double check your waveform. Make
careful, scaled drawings of the Vin and Vout waveforms using the same time
axis. From your measurements, calculate the voltage gain (AV) of your
amplifier, using the following formula:
AV = Vout / Vin
Disconnect the power supply and carefully remove the lead connecting the
wiper (middle leg) of the potentiometer to the base of the transistor. Use the
DMM, set to measure Ohms, to measure the resistance RA between points A
and B and RB, between points B and C. Record your results in your lab book in
tabular form.
Change the value of RC from 1k to 470 and repeat the above experiment,
recording your results (Vin, Vout, AV, RA and RB) in your lab book in tabular
form.
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FOUNDATION EXP 8 – BIPOLAR TRANSISTOR
3.0 COMMENTS AND CONCLUSIONS
Comment on the effect of changing RC, and how you would go about
designing an amplifier to achieve a certain gain AV.
Dr. Daniel Nankoo
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