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,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 0.65uH 10 + 16VWJ RFC2 5.tik PAIi ff'~•· .,. VC1 5-70pF 1.2k C ELJe ; VIEWED FROM BELOW ECB 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 0 ~ 0 ao'Co ,o-a o L ao 00 v~ p 00-0 ocl» ~ 0 0 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 ,' 0 ~00 dli c&o 0 8 019 . oo ao SC06101911 0 acmog ·8 0 •ao 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