,AMATEUR RADIO
BY GARRY CRATT, VK2YBX
A 5W power amplifier for the
6-metre FM transmitter
Last month, we described the construction of
the 6-metre summertime special - an FM exciter
for the 6-metre band. This month we follow up
with a matching power amplifier, designed to
provide up to 5 watts of RF power from a one
milliwatt source.
Designs for RF power amplifiers
have been published many times
before. Most rely on the use of special
power transistors, specifically produced for the low band (70-85MHz)
commercial 2-way radio market. Some
of these devices allow power levels
of up to 50 watts to be achieved but
all carry a significant price tag, often
around the $30 mark.
At such a high price, there certainly
is no margin for construction error.
The complete amplifier design presented here can be built for less than
the cost of one of these devices.
CB output transistors
Bearing in mind the high cost of
power transistors, we decided to opt
for a more economical line up. Considering the tens of thousands of
27MHz CB transceivers in service,
having an output of similar proportions but at around half our required
frequency, we thought it might be
possible to use the same configuration. It should simply be a matter of
scaling the values of L and C to suit
52MHz, and substituting devices rated
at that frequency.
Fig.1 shows our circuit. As can be
seen, we selected a fairly common
Japanese transistor, the 2SC710, as
the predriver (Ql). This particular
transistor is often found in CB transceivers.
The 2SC710 is readily available
from several sources as a spare part.
We used fixed biasing via 5.6kQ and
1.2kQ resistors to set the operating
conditions of this stage. The output is
then coupled from the collector of Ql
via an impedance "transformer" made
using a ferrite balun core and some
25 SWG enamelled copper wire.
This ensures a reasonable match to
the low impedance input of the second stage. The trimmer capacitor VCl
allows the primary winding of the
transformer to be resonated at the
operating frequency. This approach
improves both the efficiency and harmonic suppression compared with an
untuned stage.
The second stage, Q2, uses a Motorola device, the MRF629, which has
minimum gain of 8dB and can easily be driven to 1 watt output. In addition, this transistor, unlike several
others we tried (2N3866, 2N4427), is
supplied in a grounded emitter TO39 package, which greatly improves
grounding and heat dissipation. Also,
the price of the device in "one off"
quantities is quite reasonable.
The output stage (Q3) uses a
2SC1306, the same device used in the
a
The 6-metre power amplifier is designed to match the FM exciter described last
month. Because it works at RF, make sure that all component leads are kept as
short as possible. The completed board should be housed in a metal case.
104
SILICON CHIP
RFC5
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VIEWED FROM BELOW
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6-METRE POWER AMPLIFIER
Fig.1: three RF transistors are used in the circuit for the 6-metre power
amplifier. Q1 serves as the predriver & this drives Q2 via impedance matching
transformer T1. Q2 provides a minimum gain of 8dB & drives output stage Q3
via a tuned circuit consisting ofL1 & RFC4. L2 tunes the output stage for
maximum efficiency.
INPUT
PARTS LIST
1 PC board, code SC06101911,
117 x 80mm,
2 adjustable RF coils (L 1, L2);
available from Stewart
Electronic Components
5 RF chokes (see winding
details)
1 F14 balun core, DSE Cat. L1340 (T1)
2 BNC sockets
1 heatsink for 02, DSE Cat. H3412
1 heatsink for 03, DSE Cat. H3490
Semiconductors
Fig.2: the circuit is built on a PC board with a groundplane on the component
side to ensure stability. When installing the parts, be sure to solder the leads on
both sides of the board if the ground plane comes right up to the edge of the hole.
27MHz version of the power amplifier found in thousands of CB radios.
This device is rated to over 100MHz
and when driven with the MRF629,
provides a level of output power exceeding 4 watts.
This is quite sufficient to enable
local contacts during the winter
months and can be used to significant advantage during the 6-metre
season (about the time you read this).
Tuning and bypassing
Both the input and output circuits
of the final stage of amplification are
tuned, via Ll and L2, to provide a
further improvement in efficiency.
Bypassing is especially important in
a design such as this, and so exten-
sive use has been made of RF chokes
and decoupling capacitors in the DC
supply line to the three stages to ensure that they remain stable under all
conditions.
These RF chokes are made by winding a specified number of turns on a
2.2kQ or 2.7kQ resistor. This saves
the problem of having to obtain commercial prewound chokes.
Construction
Construction of the power amplifier is made easy by the use of a
double-sided PC board. The top side
(component side) of the board forms
the groundplane, as with the exciter
described last month. Components
connected to ground are soldered on
1 2SC710 NPN RF transistor
(01)
1 MRF629 NPN RF transistor
(02)
1 2SC1306 NPN RF transistor
(03)
Capacitors
1 10µF 16VW electrolytic
2 .01 µF disc ceramic
1 680pF disc ceramic
1 1SOpF disc ceramic
1 120pF disc ceramic
2 100pF silvered mica
1 82pF disc ceramic
1 5-70pF ceramic trimmer
Resistors (0.25W, 5%)
1 5.6kQ
1 1.2kQ
1 68Q
2 47Q
1 22Q
Miscellaneous
25 & 30 B&S enamelled copper
wire for winding RF chokes
DECEMBER 1990
105
ELECTRONICS
WORLD
DECEMBER
SPECIALS
USED 'AA' 700mAH NICAD
BATTERIES
$
0.50
TEMPERATURE CONTROLLED
$135.00
SOLDERING STATION
UNIVERSAL NICAD BATTERY
$ 29.95
CHARGER
Can do 4 at a time (AA, C, D, or 9V
Nicads)
p-101911
1{
+
ae•
0
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~
0
ao'Co
,o-a
o
L
ao
00
v~
p
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ocl»
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40 CHANNEL AM CB RADIO $ 79.00
SMOKE DETECTOR
$ 49.95
PIEZO SIREN WITH BACKUP
BATTERY
$ 59.95
Ideal for Car Alarms
DIGITAL DISPLAY AM/FM STEREO
CAR RADIO
$ 49.95
PORTASOL GAS SOLDERING IRON
$ 35.00
ARLEC SUPER TOOL KIT
$ 69.50
LOGIC PROBE
$ 35.00
SCANNER FANATICS FREQUENCY
REGISTER OF VIC.
$ 24.95
PHILIPS INFRARED REMOTE
CONTROL
$ 35.00
COMMON CATHODE 7-SEGMENT
DISPLAY
$ 1.00
$ 15.95
VIDEO DUBBING KIT
ARLEC 2 SPEED CORDLESS DRILL
AND SCREWDRIVER
$ 69.95
2-CHANNEL FM WIRELESS
INTERCOM
$ 89.95
SINGLE CHANNEL UHF
TRANSMITTER KIT
$ 18.00
SINGLE CHANNEL UHF RECEIVER KIT
$ 34.90
VULTURE CAR ALARM KIT $ 39.90
INFRA RED NIGHT VIEWER KIT
$239.00
TALKING ELECTRONICS ULTIMA
FM BUG KIT
$ 12.50
Mail Orders Welcome
30 Lacey St, Croydon
VIC, 3136.
Telephone:
Fax:
106
(03) 723 3860
(03) 723 3094
(03) 725 9443
SILICON CHIP
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SC06101911
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Fig.3: here are the two full-size artworks for the PC board.
the top side of the board, while the
remaining circuitry is interconnected
by the pattern on the underside of the
PC board.
This arrangement ensures a good
low impedance earth, which is most
important for stable operation, especially as some of the components carry
significant RF currents. The entire
board assembly should be mounted
in a metal enclosure and fitted with
BNC connectors for input and output
terminations. Be sure to use silicone
heatsink compound between Q2 ,Q3
and their respective heatsinks, to
ensure efficient thermal transfer from
the devices. It is quite normal for Q3's
heatsink to become hot after extended
transmit operation.
No doubt larger heatsinking arrangements can be made for the output transistor by constructors with
access to metalworking equipment.
The current drain of the power
amplifier, when fed from a 13.8 volt
DC supply, is around 1 amp. Make
sure that you have a power supply of
sufficient current rating.
Apart from the transistors, which
are freely available from the sources
quoted in this article, most components are standard types and should
be available from parts stockists.
The two 100pF capacitors in the
output (associated with L2) should
be silvered mica or other high voltage
types, as the RF voltage developed
across these components ·can exceed
the nominal 50 volt rating of the ceramic capacitors used in the lowerpower stages of the amplifier.
Fig.2 shows the component layout
of the amplifier. Keep all component
leads as short as possible. This is easily achieved as most of them are horizontally mounted on the circuit board.
To successfully tin enamelled copper wire, in order to secure the ends
of the conductors, first scrape away
the enamel with a scalpel or "Stanley"
knife. This done, wind a few turns of
this bared enamel wire around one
lead of the resistor, then apply solder
and a reasonably hot iron to the resistor lead. After several seconds, the
solder will flow onto the copper wire,
and the lead will be secured.
It is now a simple matter of wind-
ing the prescribed number of turns
onto the resistor and terminating the
other end in the same manner.
The windings of the RF chokes can
be held in place by coating them with
clear nail polish, prior to insertion
into the PC board.
Alignment
To align the power amplifier, connect a power meter to the output and
a current meter in series with the DC
supply. Set the ferrite cores of both
Ll and L2 so that they are flush with
the tops of the formers.
It's then just a case of tweaking both
cores for maximum RF output power.
You will have to repeat this procedure several times to get the best alignment.
Neither of our prototypes could be
maladjusted to cause deliberate high
harmonic content in the output. In
fact, all transmitter harmonics were
suppressed by 40dB minimum. It
should be noted that, due to the high
gain of the power amplifier, as little
as -10dBm input will generate some
output (around 1 watt in fact). However, compression does not occur
Winding Details for the Inductors
RFC-1
RFC-2
RFC-3
RFC-4
RFC-5
10 turns of 30
35 turns of 30
10 turns of 25
20 turns of 30
20 turns of 30
B&S
B&S
B&S
B&S
B&S
ECW wound
ECW wound
ECW wound
ECW wound
ECW wound
on
on
on
on
on
a 2.2kQ 0.5W
a 2.7kQ 0.5W
a 2.2kQ 0.5W
a 2.2kQ 0.5W
a 2.7kQ 0.5W
resistor.
resistor.
resistor.
resistor.
resistor.
Transformer T1 : Primary - 3 turns 25 B&S ECW; Secondary - 2 turns 25
B&S ECW. Balun core is F14 material (DSE Cat. L-1340).
Note 1: all resistors must be carbon or composition types.
Note 2: ECW stands for "enamelled copper wire".
until a level of +3dBm is applied. If
using a higher input level, a suitable
attenuator should be used to limit the
input power to lmW.
The RF output can be measured
using an inexpensive CB power meter. These meters have a reasonable
frequency response up to the region
of 70-B0MHz.
Parts availability
The 2SC710 transistor can be obtained from Benelec Pty Ltd, phone
ANTRIM
TOROIDAL TRANSFORMERS
QUALITY TOROIDAL POWER TRANSFORMERS,
MANUFACTURED IN U.K. NOW AVAILABLE
EX-STOCK AT REALISTIC PRICES.
General
Construction
UV IIM
1/./SULA ll{lr.l
(02) 693 5111. The MRF629 transistor can be obtained from Benelec, VSI
Electronics and Dick Smith Electronics. Heatsinks can also be obtained
from DSE, Cat H-3412 (Q2), and H3490 (Q3). The adjustable RF coils
(Ll, L2) can be purchased from Stewart Electronic Components (phone
03 543 3733), part number CG 105.
The output transistor can be obtained
from DSE or Benelec, and the toroidal
former can be purchased from Dick
Smith DSE.
E-1 LAMINATED POWER
TRANSFORMERS
PCB MOUNT
STOCK RANGE
PCB mount ex-stock in sizes
from 2.5V A to 25V A with secondary voltages from 2 x 6V to
2 x 20V. Triple output models
also available for logic circuits.
Chassis mount manufactured to
order in sizes from 2.5VA to
1 KVA in E-1 and C core.
CHASSIS MOUNT
TO ORDER
w,~o•~G
,,.su,,..1,0~
We are the largest ex-stock supplier of toroidal power transformers
in Australia. Our standard range consists of the U.K. manufactured
ANTRIM range in 10 VA sizes from 15VA to 625VA. All have a
single 240V primary and dual secondary windings ranging from
2 x 6V to 2 x 55V. Our local manufacturing facilities supplement
this range by manufacturing specials to order. Models are available
from stock to suit most project kits. Comprehensive data sheets
are available on request. Enquiries from resellers and manufacturers are welcomed. Prices are extremely competitive and generous
trade discounts apply for quantity.
HARBUCH ELECTRONICS PTY LTD
90 George St .. HORNSBY NSW 2077 Phone (02)476-5854
DECEMBER1990
107
