December 1957 Popular Electronics
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular
Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged. See all articles from
announced his invention of the transistor 10 years to
the month before this article was written in Popular Electronics.
A decade later, engineers and scientists had revolutionized electronics
for commercial, military, and hobby applications. It seems strange
to read here about applying transistors at high frequencies, where
the definition of "high" is in the hundred of MHz.
We've come a long way, baby
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By LOU GARNER
a Merry Christmas to you . . . together with the sincere wish that
your Christmas Stocking will be brimming full with exactly what
you would like old St. Nick to bring-be it a new car, new test equipment,
or a whole cartload of transistors and miniature components.
In the past few weeks our mail has included a lot of letters
asking about high-frequency operation of transistors. There's nothing
especially mysterious about transistors at high frequencies. The
circuits themselves are quite similar to those used at, say, AM
broadcast-band frequencies (550 to 1500 kc.) except, of course,
for the values of components in the tuned circuits. But the important
thing is to have the proper type of transistor.
Tetrode transistors (see our
May 1957 column) may be used up into the hundreds of megacycles.
The highest frequency tetrode available at this writing is Texas
Instruments' Type 3N25 - this is a p-n-p unit with a cutoff frequency
of 250 mc. General Electric, too, offers a whole series of tetrode
transistors the highest frequency unit is the 3N30, an n-p-n transistor
with a cutoff frequency of 120 mc. RCA is currently offering the
highest frequency triode transistor, the 2N384; this is a p-n-p
unit with cutoff frequency of 100 mc. which may be used as an oscillator
up to 250 mc.
In general, r.f. transistors will serve as
amplifiers up to, or slightly past, their nominal "cutoff frequency"
(at which their gain is approximately 70% of that achieved at low
frequencies), and as oscillators well above cutoff frequency. Take
the RCA transistor as an example. With a cutoff frequency of 100
mc., this unit will provide 15 db gain at 50 mc. in a common base
amplifier circuit. It provides unity gain at 250 mc.
lowest priced high-frequency transistor as far as the home experimenter
is concerned is probably Philco's Type AO-1 surface barrier transistor.
With a rated cutoff frequency of 30 mc., the AO-1 is offered at
slightly under $2.00 by leading mail order supply houses.
All of these transistors are suitable for receivers and low-power
transmitters. Unfortunately, no manufacturer is currently offering
medium or "hi" power transistors for use at high r.f. values.
One- and two-transistor
receiver circuits are extremely popular with home builders. Almost
all such circuits featured in past columns have used either detector-amplifier
or simple regenerative-detector arrangements. These circuits, at
best, have limited sensitivity and, usually, just fair selectivity.
This month, however, we have a most interesting "maximum
performance" two-transistor receiver circuit. Submitted by Gerald
Le Fevre, of York Rd., Pavilion, N. Y., as his "favorite," this
receiver has a superheterodyne circuit. In addition, it uses a reflex
arrangement, permitting one transistor to serve both as an i.f.
stage and as the audio amplifier. Coils L1 and L2 are Miller Types
2000 and 2020 respectively.
Fig. 1. Reader Gerald Le Fevre's two-transistor superheterodyne
Referring to Fig. 1, you will see that a Sylvania Type 2N94
r.f. transistor serves as a combination mixer-local oscillator.
The i.f. output signal developed across the primary of transformer
T1 is coupled to the i.f. amplifier stage, a second 2N94. T1 is
a Miller 2041.
The amplified signal then goes from i.f.
transformer T2 (Miller 2042) to a 1N34A diode, which serves as the
second detector. The resulting audio signal, appearing across volume
control R8, is "reflexed" back to the i.f. stage through audio coupling
transformer T3. Finally, the amplified audio output signal is applied
across a pair of headphones. (T3 is an Argonne AR-100.)
Fig. 2. Two simple uses for a transistorized high-voltage "generator,"
reader Charles Rakes: (A) night light; (B)
bias for the first stage is supplied by voltage divider R1-R2, operating
in conjunction with emitter resistor R4, bypassed by C5. Base bias
for the second stage is supplied by voltage divider R5-R6 and emitter
resistor R7, bypassed by C7. R6 is bypassed by C6; other r.f. bypass
capacitors are C4, C8, and C9. Operating power is supplied by a
standard 6-volt battery, B1 (such as a Burgess Type Z4).
Construction should pose no problem for the experimenter who
has assembled other simple transistor receivers. Gerald assembled
his unit on a 4" x 4" chassis, but suggests that a small Masonite
board will serve as well. A Miller #2112 two-gang unit serves as
tuning capacitor (C1a/C1b). All resistors are "1/2 watt units, while
all fixed capacitors may be small disc ceramics or paper tubulars.
Transistorized code practice oscillators are currently available
in kit form from, left to right, Knight (Allied Radio), WRL Electronics
and Lafayette Radio. See page 122 for details on all of these units.
If you assemble one of these receivers, be sure to keep your
layout as "clean" as possible, with all signal leads short and direct.
Watch the battery polarity ... remember that the 2N94 is an n-p-n
transistor! Moderate-impedance (2000 to 4000 ohm) magnetic headphones
should be used.
You shouldn't need an external antenna for
stronger local stations. However, for additional pickup, try connecting
an antenna to the "hot" side of the ferrite antenna coil (L1) through
a 200-µµfd. ceramic or mica capacitor.
To align the unit
for best operation, use a standard r.f. signal generator. With a
modulated r.f. signal, peak the i.f. transformers for m aximum output
at 455 kc. Adjust the oscillator trimmer capacitor (across C1b)
for tracking at the high-frequency end of the dial (around 1500
kc.) and the padder capacitor (C2) and oscillator "slug" (in L2)
for tracking at the low-frequency end of the band (about 600 kc.),
"rocking" the tuning capacitor as you do so. If an antenna trimmer
capacitor is provided (across C1a), adjust this for maximum output
above 1550 kilocycles. High-Voltage "Generator."
Reader Charles Rakes, of 4419 Harrison St., Kansas City, Mo., has
submitted a pair of interesting applications for a high-voltage
"generator" designed around a low-cost power transistor. Referring
to Fig. 2, a CBS-Hytron Type 2N256 power transistor is connected
as a modified "Hartley- type" oscillator. Base bias is determined
In operation, the a.c. voltage developed across the
"primary" winding by the oscillator is stepped up by the secondary
and used to operate a small (15-watt) fluorescent lamp (A) as a
soft "night light," or power an "electric fence" (B) to keep livestock
Construction of the basic circuit is relatively
simple and the wiring is completely non-critical. R1 is a 22-ohm,
2-watt carbon resistor. B1 can be any standard medium or large-sized
6-volt battery (such as a Burgess F4P1 or 2F4).
oscillator/step-up transformer (T1) is made from a discarded vacuum-tube
audio output transformer. The secondary (low-impedance) winding
is partially unwound and center-tapped. It is then rewound and becomes
the "primary" winding in the oscillator circuit. The original primary
winding (high-impedance) then becomes the high-voltage secondary
winding, providing the necessary boost. Code Practice
Back in April, we reviewed currently available
transistorized superhet receiver kits. As a result, we received
quite a number of requests that such "reviews" be a regular feature
of the column . . . so here goes with a review of transistorized
code practice oscillators.
Of the various code practice
oscillator (CPO) kits now available, the KT-72, offered by Lafayette
Radio (165-08 Liberty Ave., Jamaica 33, N. Y.), is the least expensive.
Catalog price is $2.99 (plus postage), including both batteries
and a standard hand key. This kit assembles on a small piece of
perforated Masonite. The oscillator is designed for headphone operation
Allied Radio Corporation (100 N. Western Ave., Chicago
80, Ill.), offers a nice CPO which assembles into an attractive
black plastic box. It operates from a single penlite cell (included).
The kit number is 83Y239 and the catalog price is $3.95 (plus postage),
less hand key and 'phones. This unit is also designed for headphone
WRL Electronics (Council Bluffs, Iowa) offers
a CPO with an easy-to-wire etched-circuit wiring board which assembles
in a sturdy aluminum case. Two penlite cells (included) power the
circuit. WRL's kit sells for $4.95 (plus postage), and is for headphone
operation. The hand key and headphones are available as optional
Lafayette Radio is the only firm currently
offering a CPO kit which provides loudspeaker operation. The KT-118
kit assembles into a small plastic case not much larger than a package
of cigarettes but which includes the loudspeaker, transistor, battery,
and entire circuit. Catalog price of the KT-118, less hand key,
is $795 (plus postage).
Lafayette Radio's two-transistor
"Sunflex" receiver. Sun Batteries.
While a "sun battery" is not, strictly speaking, a transistor item,
it is often used in conjunction with transistorized equipment and,
therefore, is of interest to Transistor Topics' readers.
The International Rectifier Corporation (1521 East Grand Ave.,
El Segundo, Calif.) , will shortly release a new silicon solar cell
with a lower cost per watt than previously available units. For
full specifications, price and availability information, write directly
to the manufacturer.
Lafayette Radio's latest catalog offers
a "Heliodyne" silicon battery at $665 (plus postage). A multicell
unit, this solar battery will supply up to 2.0 ma. at 3.2 volts
in full sunlight. This is ample power for most small one- and two-transistor
radio receivers or amplifiers, and is considerably more than that
supplied by standard selenium cells of comparable size and cost.
In addition, Lafayette has announced a new two-transistor
radio receiver kit, the "Sunflex," designed especially for solar
battery operation, It uses a highly efficient reflexed, complementary
symmetry circuit which permits two transistors to give the performance
of three. Able to operate on standard penlite cells as well as a
solar battery, the KT-132 Sunflex receiver kit sells, less earphone
and solar battery, for $11.95 (plus postage).
for now, fellows. See you next year.