Download Transistors Reduced Base Biased Resistance 2-26-2013

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OBJECTIVE
To measure the improvement in bias
stability when the base-bias
resistance is decreased.
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INTRODUCTION
Experiment 2 illustrated the base must be
held at a constant value in order for the
emitter to work properly.
In experiment 2, the base was biased
through a 100kW resistor and a 1kW
potentiometer.
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This was a simple arrangement, but did not
provide a very stable voltage to the base of
the transistor.
This experiment used an alternate biasing
method, with a lower resistance as seen
from the base.
• The 100kW base current-limiting resistor
has been reduced to 10kW.
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• A 1.2kW resister was also placed in series
with the potentiometer and the -10V
supply.
• The action of the emitter feedback will be
improved because the base is held to a
steadier voltage.
• The following slide illustrates the
Experiment 3 schematic diagram.
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EXP. 3 SCHEMATIC DIAGRAM
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REQUIRED PARTS
1 220W, 2W resistor (red-red-brown)
1 470W, ½ W resistor (yellow-violetbrown)
1 1200W, ½ W resistor (brown-red-red)
1 10kW, ½ W resistor (brown-blackorange)
1 2N2431 Germanium transistor (PNP)
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PROCEDURE
1. Construct the circuit for Experiment 3.
2. Turn the trainer on.
a) Adjust the positive power supply to 15V
b) Adjust the negative power supply to -10V.
3. Bend the leads of the transistor so the
case is away from the 220W resistor.
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EXP. 3 SCHEMATIC DIAGRAM
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PICTORIAL DIAGRAM OF EXP. 3 CKT.
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4. Adjust the 1kW potentiometer so you
have 5V across R1.
5. Bend the leads of the transistor so that
the case now touches the resistor.
a)
Allow the transistor to heat for 5 minutes
6. Measure the voltage across R1, and
calculate the current through R1.
a) Record calculation in lab book.
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CIE RESULTS
6.
5.9mA
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FINAL DISCUSSION
We saw a reduction in base-bias current
drift. The improved stability was:
• Experiment 3; 18%
• Experiment 2; 28%
• Experiment 1; 96%
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For best stability with an emitter-feedback
resistor, then, the resistance of the basebias network should be as low as possible.
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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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