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This describes the planning and construction of a 600W HF amplifier for Ham radio. This project is stalled. I completed the filter/diplexer board and did some low power testing of the main amplifier board. It is sitting on my work bench, but it may be another year before I finish it. I wanted more power (especially for voice on 75 meters) and I figured it would be a good learning experience. I have an electronics/engineering background, but am fairly new to RF. The "RF deck" was bought from Communications Concepts. You can buy the entire kit, or just the PCB or parts. This design is from Helge Granberg (SK) while he was at Motorola. It of course uses Motorola (MACOM) transistors (MRF150 MOSFETs). The kit will get you started, but you still need to add several sections to make it ready to get on the air. When I am all done, I should have an amplifier VERY similar to the Ameritron ALS-600, but with automatic band switching….. and the wealth of knowledge that I have hopefully learned in the process
Cooling: Most people use a large copper spreader (a thick sheet of copper) to get heat out of the four MOSFETs. Cooling has to be one of the most important parts of this project. If you don't get the heat away from the transistors, they are going to have a short life span. I agree that using a copper spreader is required. Copper conducts heat much better than aluminum and a thick sheet will get the heat spread out to the sides well. My first job out of school was at a custom power supply company. My mentor was an older Englishman who had been working in power well before I was born. One of his favorite things to say was "lets put a cooper spreadah on it" (my best spelling to try and give him an English accent). We use to harass him about that, but he was right. If there was a high concentration of heat, the best way to get that heat out to the large aluminum heat sink was through a copper spreader. The idea of the spreader is to pull the heat out of the almost point source heat producing transistors and get it distributed to the larger and cheaper aluminum heat sink. You can see the weight of the copper spreader is almost 6 pounds in the picture to the right.
uC: Protection will consist of both analog and digital control. It is hard to beat analog for fast acting (you don't have to take samples and do math before you change something). Also, analog is what I am most familiar with. I have played with microcontrollers both for hobby and work, but I am a beginner in the uC area. The uC will shut down the amplifier if the reflected power becomes too large (greater than 70W-100W) or if the VSWR becomes too great (4:1) for any power. My hope is to have good amplifier protection with the use of both analog and digital control. The uC also drives the LCD display. I decided that it would be nice to have amplifier information displayed on a LCD display. The display doesn't add much cost or complexity to the project. I really like the pretty vacuum florescent displays, but I decided on the much less costly LCD display. The Wavenode sensor gives a proportional voltage out (both forward and reverse) to the V on the coax. I used (Vforward + Vreflected) / (Vforward - Vreflected) to calculate SWR from the Wavenode data. The uC uses a 10bit A-D converter. I have not decided if I am going to add an analog SWR/FWD/REV meter to the amplifier. You can't beat an analog meter for displaying certain types of information.
Filter: I used the PCBs made by K6IF for the output filters. Although the PCBs were a little on the expensive side (compared to hand wiring), I thought it was worth it to have a circuit board with a nice ground plane and all ready to accept the inductors/capacitors. The PCB file provided on his (K6IF) website is in an ExpressPCB format. You can only have the boards made at ExpressPCB. The output filters are necessary to knock down the harmonics of the amplifier. The broadband output transformer of the amplifier lets harmonics come out the output. The low pass filter attenuates the harmonics. The push-pull nature of the amplifier means the even order harmonics are fairly low to start with (also true for push-pull audio amplifiers) I read a few articles on using diplexers instead of the more standard straight low pass filters. I found the diplexer to be just a fancy name for a crossover. Like the crossovers used in audio, the diplexer's job is to send the highs one way and the lows the other (as the crossover of a 2-way speaker sends highs to the tweeter and lows to the woofer). In this case, the crossover sends lows out to the antenna and highs to a 50 ohm power resistor. The highs are unwanted harmonics and the lows are the frequencies we want to get to our antenna. The reason for using a diplexer over a simple low pass filter is the high frequency information (the harmonics) can't get out of the amp (we don't want them on our antenna) so the energy can get reflected back to the output of the amplifier..... perhaps being mean to our MOSFETs (however the Ameritron ALS-600 that is almost identical to the EB104 uses just a low pass filter). So we take the energy and dump it into a resistor. The amplifier still sees a 50ohm load (assuming your antenna/antenna tuner is doing its job). Helge Granberg also noted that a diplexer can make the amplifier more stable by presenting it with a 50ohm load over a much wider frequency range.
The K6IF filter PCB was laid out to include relays that can be controlled to switch the filters in/out. That means that if your transceiver has a way of communicating it's band to the amplifier you can use that information to automatically select the filter. Many Icoms use a band status output whose DC output changes with the band. Since I already have a uC in the amp, I plan on using a single A-D input to monitor this band info. Several uC outputs will switch the relays. An analog method could be used for switching, but the digital method makes more sense. There is not a who is better (analog or digital), just a who is better for the job at hand.
Power Supplies: Power supplies are 85% of what I do for a living, so at first I thought about using a nice active PFC (power factor corrected) switching power supply that could take 90VAC-264VAC (Universal input). I also looked at some switchers made for the telecom industry (48VDC) as that is about the perfect voltage. Making my own switcher even using a previous design was just too much work… a project in its own. Ebay is certainly a good alternative as several thousand watt 48V supplies can be found for $75 - $200. However those supplies need 220VAC. That isn't too hard for me since my electrical box is on one side of the ham shack, but my partner in crime didn't have easy access to 220VAC and I really wanted to make all three amplifiers the same. So I decided to go with a linear. That means big, but easy and quick to make. Also, linear power supplies get the reputation of being inefficient. That is true in many conditions…. mostly when you are trying to use a linear regulator and drop a lot of voltage. Since this supply is not regulated (there are no active devices trying to keep the output voltage constant), the efficiency won't be too bad. The amount the DC output voltage varies is dependant on the input voltage staying steady (we have great power in the U.S.), the amount of output capacitance, and the load (how much power you put out and for how long). Regulating is a fine thing, but it takes a lot more work and heat for a small return (how small of a return is certainly up for discussion). I looked at some transformers from Toroid of Maryland and Plitron, but decided to go with the transformer used in the Ameritron ALS-600. The part number is 406-1246 and I was charged $59.64 and $41 shipping. That is about $100 to the door. The toroid option was going to be at least $50 more. Although a toroid is pretty and has low magnetic leakage, price ultimately made me go with the Ameritron transformer. It has a small winding that can be put in or out of the primary to bump the output voltage up or down. Adjustable output voltage is one of the big advantages of a toroid since you can simply add a turn (or several) in the "right" or "wrong" directions to bump up/down the output voltage. That can be very hard to do with a standard transformer.
MRF150: These are the output MOSFETs used in the amplifier. Of course MOSFETs really shine when they used in switching power supplies since they are being used as switches (full on or off). The MOSFETs in our amplifier are working in their linear region (we hope). Helge Granberg identifies a few reasons to use MOSFETs in linear amplifiers. At first I had a hard time locating the MOSFETs at a "reasonable" price, but eventually I found them at Future Electronics for $35.22 each. Communication Concepts wants $256 for four matched MOSFETs ($64 each). They are matched (meaning their RDS-on should be fairly close between devices for a given gate to source voltage), but since each MOSFET is individually biased using a trimpot matching is not nearly as important. I plan to use voltage clamping devices across the transistors (drain to source) to help protect against voltage surges caused by high SWR. It is hard to know if the devices can act fast enough to help MOSFET survivability.
Analog protection: The Wavenode outputs two voltages that represent the forward and reflected power. The forward power information can be used to operate an ALC. The ALC control voltage of the amplifier will connect to the ALC input of the transceiver and tell the transceiver lower the transceivers power when the output approaches maximum output (probably make ALC kick in at about 500W). A negative voltage is sent to the transceiver. From what I have read, there is not an ALC standard so the output level will be set with each radio. If the reverse power becomes too high, the amplifier can be shut down automatically.
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