After building my homebrew DSB transceiver and making some good contacts it was sometimes very frustrating with only 1 watt of power. I was recommended this excellent circuit by Andy Small (M0DRN) to use as an afterburner. As you can see from the diagram it is very easy to build and the amp circuit itself is between the two dotted lines. If your QRP transmitter is more than 1watt then you will have to build the attenuator circuit to reduce the power to the required input level. The amplifier requires 0.5watt to 1 watt of power to operate so it has made a perfect matching QRP amp for my DSB rig. I decided to fit a bypass switch so I could use the unit as a separate attenuator and check the power on other homebrew rigs before putting any signal through the amp.
Following the amp on the right side of the dotted line is a low pass filter which I would highly recommend as this will greatly reduce any harmonics making for a much cleaner transmission. The filter components above are for the 40M band but if you want to operate the amp on another band you only need to change the filter component values.
I have built my amplifier on a piece of single sided copper PCB and cut out small insulated islands to solder the components on. T1 was made by cutting two pieces of enamelled copper wire about 300mm long. I placed the wires side by side and clamped one end in a vice. The wires were then twisted together keeping the tension on all the time making around 3 turns per cm. The two wires are then treated as one and I made ten turns through the T 50-43 core. The enamel was scraped off the edge of the wires and tined ready to be soldered. To avoid confusion each of the two wires was checked with a multimeter, but another way is to mark the wire ends with coloured ink. The polarity of these wires are very important and must be correct.
The setting up procedure is simple. Once you have attenuated your signal to the correct drive level switch the power on and with a multimeter set the initial gate voltage to 3volts. Do this procedure while the amp is idling and not in transmit. This will give around 5watts output ideal for QRP operating. Adjusting the voltage to 3.5 volts will increase the power to 8watts. I have had 10watts output from the amp but only operate it at 5watts. The circuit uses the popular IRF510 transistor and will need a heatsink with a smear of petroleum jelly or heat compound to dissipate the heat. The drain leg of the transistor is electrically connected to the metal tag inside, so the drain leg was lifted clear of the PCB and a solder tag was fitted to the fixing nut for the drain connection to the T1 bifilar transformer. The drain connection must be insulated from ground so I cut out an island on the PCB to mound the transistor and heatsink which also helps with the heat dissipation. The PCB was then bolted onto four stand offs inside an aluminium enclosure and when all the sockets, dials and switches had been fitted it looked a nice finished project ready for testing.