Multi-stage amplifier
PARTS AND MATERIALS
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Three NPN transistors -- 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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Three 100 kΩ resistors
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Three 10 kΩ resistors
CROSS-REFERENCES
Lessons In Electric Circuits, Volume
3, chapter 4: "Bipolar Junction Transistors"
LEARNING OBJECTIVES
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Design of a multi-stage, direct-coupled
common-emitter amplifier circuit
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Effect of negative feedback in an
amplifier circuit
SCHEMATIC DIAGRAM
ILLUSTRATION
INSTRUCTIONS
By connecting three common-emitter amplifier
circuit together -- the collector terminal of the previous
transistor to the base (resistor) of the next transistor --
the voltage gains of each stage compound to give a very high
overall voltage gain. I recommend building this circuit
without the 1 MΩ feedback resistor to begin with, to see
for yourself just how high the unrestricted voltage gain is.
You may find it impossible to adjust the potentiometer for a
stable output voltage (that isn't saturated at full supply
voltage or zero), the gain being so high.
Even if you can't adjust the input voltage
fine enough to stabilize the output voltage in the active
range of the last transistor, you should be able to tell
that the output-to-input relationship is inverting; that is,
the output tends to drive to a high voltage when the input
goes low, and visa-versa. Since any one of the
common-emitter "stages" is inverting in itself, an even
number of staged common-emitter amplifiers gives
noninverting response, while an odd number of stages gives
inverting. You may experience these relationships by
measuring the collector-to-ground voltage at each
transistor while adjusting the input voltage
potentiometer, noting whether or not the output voltage
increases or decreases with an increase in input voltage.
Connect the 1 MΩ feedback resistor into the
circuit, coupling the collector of the last transistor to
the base of the first. Since the overall response of this
three-stage amplifier is inverting, the feedback signal
provided through the 1 MΩ resistor from the output of the
last transistor to the input of the first should be
negative in nature. As such, it will act to stabilize
the amplifier's response and minimize the voltage gain. You
should notice the reduction in gain immediately by the
decreased sensitivity of the output signal on input signal
changes (changes in potentiometer position). Simply put, the
amplifier isn't nearly as "touchy" as it was without the
feedback resistor in place.
As with the simple common-emitter amplifier
discussed in an earlier experiment, it is a good idea here
to make a table of input versus output voltage figures with
which you may calculate voltage gain.
Experiment with different values of feedback
resistance. What effect do you think a decrease in
feedback resistance have on voltage gain? What about an
increase in feedback resistance? Try it and find out!
An advantage of using negative feedback to
"tame" a high-gain amplifier circuit is that the resulting
voltage gain becomes more dependent upon the resistor values
and less dependent upon the characteristics of the
constituent transistors. This is good, because it is far
easier to manufacture consistent resistors than consistent
transistors. Thus, it is easier to design an amplifier with
predictable gain by building a staged network of transistors
with an arbitrarily high voltage gain, then mitigate that
gain precisely through negative feedback. It is this same
principle that is used to make operational amplifier
circuits behave so predictably.
This amplifier circuit is a bit simplified
from what you will normally encounter in practical
multi-stage circuits. Rarely is a pure common-emitter
configuration (i.e. with no emitter-to-ground resistor)
used, and if the amplifier's service is for AC signals, the
inter-stage coupling is often capacitive with voltage
divider networks connected to each transistor base for
proper biasing of each stage. Radio-frequency amplifier
circuits are often transformer-coupled, with capacitors
connected in parallel with the transformer windings for
resonant tuning.
COMPUTER SIMULATION
Schematic with SPICE node numbers:
Netlist (make a text file containing the
following text, verbatim):
Multi-stage amplifier
vsupply 1 0 dc 12
vin 2 0
r1 2 3 100k
r2 1 4 10k
q1 4 3 0 mod1
r3 4 7 100k
r4 1 5 10k
q2 5 7 0 mod1
r5 5 8 100k
r6 1 6 10k
q3 6 8 0 mod1
rf 3 6 1meg
.model mod1 npn bf=200
.dc vin 0 2.5 0.1
.plot dc v(6,0) v(2,0)
.end
This simulation plots output voltage against
input voltage, and allows comparison between those variables
in numerical form: a list of voltage figures printed to the
left of the plot. You may calculate voltage gain by taking
any two analysis points and dividing the difference in
output voltages by the difference in input voltages, just
like you do for the real circuit.
Experiment with different feedback
resistance values (rf) and see the impact on
overall voltage gain. Do you notice a pattern? Here's a
hint: the overall voltage gain may be closely approximated
by using the resistance figures of r1 and rf,
without reference to any other circuit component!
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