Differential amplifier
PARTS AND MATERIALS
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Two 6-volt batteries
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Two NPN transistors -- models 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 10 kΩ potentiometers, single-turn,
linear taper (Radio Shack catalog # 271-1715)
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Two 22 kΩ resistors
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Two 10 kΩ resistors
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One 100 kΩ resistor
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One 1.5 kΩ resistor
Resistor values are not especially critical
in this experiment, but have been chosen to provide high
voltage gain for a "comparator-like" differential amplifier
behavior.
CROSS-REFERENCES
Lessons In Electric Circuits, Volume
3, chapter 4: "Bipolar Junction Transistors"
Lessons In Electric Circuits, Volume
3, chapter 8: "Operational Amplifiers"
LEARNING OBJECTIVES
SCHEMATIC DIAGRAM
ILLUSTRATION
INSTRUCTIONS
This circuit forms the heart of most
operational amplifier circuits: the differential pair.
In the form shown here, it is a rather crude differential
amplifier, quite nonlinear and unsymmetrical with regard to
output voltage versus input voltage(s). With a high voltage
gain created by a large collector/emitter resistor ratio
(100 kΩ/1.5 kΩ), though, it acts primarily as a comparator:
the output voltage rapidly changing value as the two input
voltage signals approach equality.
Measure the output voltage (voltage at the
collector of Q2 with respect to ground) as the
input voltages are varied. Note how the two potentiometers
have different effects on the output voltage: one input
tends to drive the output voltage in the same direction (noninverting),
while the other tends to drive the output voltage in the
opposite direction (inverting). This is the essential nature
of a differential amplifier: two complementary
inputs, with contrary effects on the output signal. Ideally,
the output voltage of such an amplifier is strictly a
function of the difference between the two input
signals. This circuit falls considerably short of the ideal,
as even a cursory test will reveal.
An ideal differential amplifier ignores all
common-mode voltage, which is whatever level of
voltage common to both inputs. For example, if the inverting
input is at 3 volts and the noninverting input at 2.5 volts,
the differential voltage will be 0.5 volts (3 - 2.5) but the
common-mode voltage will be 2.5 volts, since that is the
lowest input signal level. Ideally, this condition should
produce the same output signal voltage as if the inputs were
set at 3.5 and 3 volts, respectively (0.5 volts
differential, with a 3 volt common-mode voltage). However,
this circuit does not give the same result for the
two different input signal scenarios. In other words, its
output voltage depends on both the differential voltage
and the common-mode voltage.
As imperfect as this differential amplifier
is, its behavior could be worse. Note how the input signal
potentiometers have been limited by 22 kΩ resistors to an
adjustable range of approximately 0 to 4 volts, given a
power supply voltage of 12 volts. If you'd like to see how
this circuit behaves without any input signal limiting, just
bypass the 22 kΩ resistors with jumper wires, allowing full
0 to 12 volt adjustment range from each potentiometer.
Do not worry about building up excessive
heat while adjusting potentiometers in this circuit! Unlike
the current mirror circuit, this circuit is protected from
thermal runaway by the emitter resistor (1.5 kΩ), which
doesn't allow enough transistor current to cause any
problem.
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