The Southwestern Historical Quarterly, Volume 97, July 1993 - April, 1994 Page: 71
754 p. : ill. (some col.), maps, ports. ; 23 cm.View a full description of this periodical.
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The Breakthrough Breadboard
nology of powdered iron cores in place of conventional air cores for the
windings (coils wound on plastic or cardboard forms). Although Roger
was acquainted with emerging component technology which would al-
low him to further reduce the IF transformers to the desired size, the
parts were not immediately available, so he decided on the next best
thing for the first feasibility breadboard radio. He decided to utilize
readily available coil and shield can components in his attempt to design
transformers which would match the transistor impedance require-
ments.
By Saturday night, Roger had arrived at a design, fabricated, and per-
formed preliminary tests of small (0.4 cubic inches) IF transformers in a
transistor amplifier circuit. Using a signal generator to feed the IF fre-
quency current into the amplifier, it appeared to approach the desired
gain of 25 dB a stage. However, it was difficult to measure and impossi-
ble to use because the dreaded parasitic oscillations showed up shortly
after any signal was applied to the input transistor.
What was he to do to stabilize the amplifier and eliminate the parasitic
oscillations? He simply designed compensating circuits to feed back
small, out-of-phase signals from the amplifier output to its input, much
the same as was done in the old triode tube circuits described previous-
ly. This solution sounds easy now, but it was far from easy at the time be-
cause of the radically different impedances involved and the relatively
small size of the components.
Once Roger had the parasitic oscillation under control in his bread-
board model IF amplifier, his next task was to work with Mark and me
on interfacing the output of our RF input/mixer circuit with the input
of the IF amplifier.
While Roger was designing and building the IF amplifier, Mark and I
had been designing the tuning circuit, or "front end," of the radio, i. e.,
the input/mixer stage. We first had to obtain a transistor from the semi-
conductor group for the mixer circuit, one that was capable of amplify-
ing signals (RF electrical currents) at frequencies up to 1610 kHz, the
top frequency of the AM radio band. (Actually, this transistor had to be
capable of operating at frequencies up to 1872 kHz, as we shall see lat-
er.) We were able to acquire such a transistor with the help of the tran-
sistor engineering and manufacturing people by selecting units which
had the smallest base layer in their three-layer (emitter-base-collector)
crystal structure, and then testing them in an RF amplifier test circuit,
which allowed us to find the ones with the highest gain. Actually, all the
time we were designing the radio circuits, semiconductor personnel, un-
der the direction of Dr. Adcock, were experimenting with various meth-
ods for "doping" the germanium crystal material and for achieving the
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Texas State Historical Association. The Southwestern Historical Quarterly, Volume 97, July 1993 - April, 1994, periodical, 1994; Austin, Texas. (https://texashistory.unt.edu/ark:/67531/metapth117154/m1/99/: accessed May 7, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting Texas State Historical Association.