Common-emitter amplifier
PARTS AND MATERIALS
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One NPN transistor -- model 2N2222 or
2N3403 recommended (Radio Shack catalog # 276-1617 is a
package of fifteen NPN transistors ideal for this and
other experiments)
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Two 6-volt batteries
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One 10 kΩ potentiometer, single-turn,
linear taper (Radio Shack catalog # 271-1715)
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One 1 MΩ resistor
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One 100 kΩ resistor
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One 10 kΩ resistor
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One 1.5 kΩ resistor
CROSS-REFERENCES
Lessons In Electric Circuits, Volume
3, chapter 4: "Bipolar Junction Transistors"
LEARNING OBJECTIVES
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Design of a simple common-emitter
amplifier circuit
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How to measure amplifier voltage gain
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The difference between an inverting and a
noninverting amplifier
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Ways to introduce negative feedback in an
amplifier circuit
SCHEMATIC DIAGRAM
ILLUSTRATION
INSTRUCTIONS
Build this circuit and measure output
voltage (voltage measured between the transistor's collector
terminal and ground) and input voltage (voltage measured
between the potentiometer's wiper terminal and ground) for
several position settings of the potentiometer. I recommend
determining the output voltage range as the potentiometer is
adjusted through its entire range of motion, then choosing
several voltages spanning that output range to take
measurements at. For example, if full rotation on the
potentiometer drives the amplifier circuit's output voltage
from 0.1 volts (low) to 11.7 volts (high), choose several
voltage levels between those limits (1 volt, 3 volts, 5
volts, 7 volts, 9 volts, and 11 volts). Measuring the output
voltage with a meter, adjust the potentiometer to obtain
each of these predetermined voltages at the output, noting
the exact figure for later reference. Then, measure the
exact input voltage producing that output voltage, and
record that voltage figure as well.
In the end, you should have a table of
numbers representing several different output voltages along
with their corresponding input voltages. Take any two pairs
of voltage figures and calculate voltage gain by dividing
the difference in output voltages by the difference in input
voltages. For example, if an input voltage of 1.5 volts
gives me an output voltage of 7.0 volts and an input voltage
of 1.66 volts gives me an output voltage of 1.0 volt, the
amplifier's voltage gain is (7.0 - 1.0)/(1.66 - 1.5), or 6
divided by 0.16: a gain ratio of 37.50.
You should immediately notice two
characteristics while taking these voltage measurements:
first, that the input-to-output effect is "reversed;" that
is, an increasing input voltage results in a
decreasing output voltage. This effect is known as
signal inversion, and this kind of amplifier as an
inverting amplifier. Secondly, this amplifier exhibits a
very strong voltage gain: a small change in input voltage
results in a large change in output voltage. This should
stand in stark contrast to the "voltage follower" amplifier
circuit discussed earlier, which had a voltage gain of about
1.
Common-emitter amplifiers are widely used
due to their high voltage gain, but they are rarely used in
as crude a form as this. Although this amplifier circuit
works to demonstrate the basic concept, it is very
susceptible to changes in temperature. Try leaving the
potentiometer in one position and heating the transistor by
grasping it firmly with your hand or heating it with some
other source of heat such as an electric hair dryer (WARNING:
be careful not to get it so hot that your plastic breadboard
melts!). You may also explore temperature effects by cooling
the transistor: touch an ice cube to its surface and note
the change in output voltage.
When the transistor's temperature changes,
its base-emitter diode characteristics change, resulting in
different amounts of base current for the same input
voltage. This in turn alters the controlled current through
the collector terminal, thus affecting output voltage. Such
changes may be minimized through the use of signal
feedback, whereby a portion of the output voltage is
"fed back" to the amplifier's input so as to have a
negative, or canceling, effect on voltage gain. Stability is
improved at the expense of voltage gain, a compromise
solution, but practical nonetheless.
Perhaps the simplest way to add negative
feedback to a common-emitter amplifier is to add some
resistance between the emitter terminal and ground, so that
the input voltage becomes divided between the base-emitter
PN junction and the voltage drop across the new resistance:
Repeat the same voltage measurement and
recording exercise with the 1.5 kΩ resistor installed,
calculating the new (reduced) voltage gain. Try altering the
transistor's temperature again and noting the output voltage
for a steady input voltage. Does it change more or less than
without the 1.5 kΩ resistor?
Another method of introducing negative
feedback to this amplifier circuit is to "couple" the output
to the input through a high-value resistor. Connecting a 1
MΩ resistor between the transistor's collector and base
terminals works well:
Although this different method of feedback
accomplishes the same goal of increased stability by
diminishing gain, the two feedback circuits will not behave
identically. Note the range of possible output voltages with
each feedback scheme (the low and high voltage values
obtained with a full sweep of the input voltage
potentiometer), and how this differs between the two
circuits.
COMPUTER SIMULATION
Schematic with SPICE node numbers:
Netlist (make a text file containing the
following text, verbatim):
Common-emitter amplifier
vsupply 1 0 dc 12
vin 3 0
rc 1 2 10k
rb 3 4 100k
q1 2 4 0 mod1
.model mod1 npn bf=200
.dc vin 0 2 0.05
.plot dc v(2,0) v(3,0)
.end
This SPICE simulation sets up a circuit with
a variable DC voltage source (vin) as the input
signal, and measures the corresponding output voltage
between nodes 2 and 0. The input voltage is varied, or
"swept," from 0 to 2 volts in 0.05 volt increments. Results
are shown on a plot, with the input voltage appearing as a
straight line and the output voltage as a "step" figure
where the voltage begins and ends level, with a steep change
in the middle where the transistor is in its active mode of
operation.
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