Download Transistors Current Feedback 10-23-2012

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OBJECTIVE
1. To measure the stability of a current
feedback circuit.
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INTRODUCTION
• A substantial improvement in bias
current can be obtained using the
emitter-feedback resistor shown in the
following schematic diagram.
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EXP. 2 SCHEMATIC DIAGRAM
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• Any change in collector current causes a
change in emitter current as long as the
base is held at a constant voltage.
• As collector current rises with
temperature, emitter current rises as
well
• Thus increasing the voltage drop across
the emitter resistor R2.
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• This decreases the base-emitter voltage
drop, and tends to reduce the base
current.
• The reduction in base current tends to
reduce the collector current as well,
thus offsetting the increase in collector
current caused by the heat.
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• These two opposing factors reach
equilibrium, and the original increase in
collector current is opposed by the
decrease in base current.
• This is known as current feedback.
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In this experiment we will again set the
collector current to 5mA and heat the
transistor as in Experiment 1.
However, this time we will see less current
drift than observed in Experiment 1.
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PARTS REQUIRED
1
1
1
1
1
220W, 2 Watt resistor (red, red,
brown)
470W, ½ Watt resistor (yellow, violet, brown)
1kW, ½ Watt resistor (brown, black, red)
100kW, ½ Watt resistor (brown, black, yellow)
2N2431 Germanium Transistor
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PROCEDURE
1. Construct the previous circuit.
2. Bend the leads of the transistor so that
the case is away from the 220W
resistor
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EXPERIMENT 2 SCHEMATIC
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EXP. 2 PICTORIAL DIAGRAM
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2. Identify the leads of the transistor
using the diagram below.
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3. Turn the trainer on.
a) Adjust the positive power supply to 15V
b) And the negative power supply to -10V
4. Adjust the 1kW potentiometer so you
have 5V across R1
5. Bend the leads so the case now touches
the resistor.
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a) Allow the transistor to heat for 5
minutes
5. Measure the voltage across R1, and
calculate the current through R1.
a) Record the current measurement in
the Experiment book
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CIE RESULTS
CIE measured 5.9mA in step 6.
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FINAL DISCUSSION
The emitter-feedback resistor improved the
stability of the transistor circuit
considerably. The percentage of drift
was only 28%, compared to 96% change
we observed without the feedback.
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5mA – 6.4mA
X 100
28% =
5mA
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QUESTIONS?
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RESOURCES
Casebeer, J.L., Cunningham, J.E. (2001).
Lesson 1430: Transistors, Part 1.
Cleveland: Cleveland Institute of
Electronics.
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THE END
Developed and Produced by the
Instructors in the CIE Instruction
Department.
© Copyright 09/2012
All Rights Reserved / Sept. 2012
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