Download Lecture 9

Lab 9
AM Transistor Radio
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Full Lab Report Due 2 PM Same Day Next Week
Next week: Final Exam Practice.
and, for 0301 (Thurs): Oral Presentations
In two weeks (6, 7 May): in class Final Exam
Load Impedances
Piezoelectric crystal earpiece
>1 MW resistor in parallel with a 25 nF capacitor
High
Earbuds
~10 W resistor
Low
Your speaker
~10 W resistor in series with ~ 0.2 mH inductor
Transistor Radio (Part A)
A transistor amplifier circuit to drive a high impedance load
Crystal
earpiece
A small base current is amplified in to a large collector current
(Part B)
A transistor amplifier circuit to drive a low impedance load
Earbuds
or Speaker
Resistor
Box
Darlington
Transistor
Transistor Radio With Earbuds (Part C)
Trick: Start with the radio tuned to a strong signal. Then slowly
increase the voltage of the DC power supply from zero and
listen carefully for the signal in the earbud.
Some Issues with Transistor Amplifiers
The base input requires a minimum DC bias to ‘turn on’
the base-emitter pn junction
A large voltage swing on the base can either
Turn off the base-emitter pn junction,
Saturate the collector-emitter current,
Resulting in “clipping” and distortion of the signal
input
input
output
output
Hints
To start: One partner re-builds the radio circuit (lab 7)
while the other builds the amplifier
Limit the current from the DC power supply (upper
screen) to 0.2 Amps
In part B, keep the resistor box at 30 W and above to
limit the current going through the transistor.
Don’t accidentally set it to 0 W!
In part B, step 2, measure and make sure that you are
applying a positive DC voltage to the base
I suggest using only inexpensive earbuds in this experiment
Crystal Radio Resources
http://en.wikipedia.org/wiki/Crystal_radio