A small power amplifier (part 2)

After building the amplifier we get to two critical point:

1. interfacing the amplifier to the radio
2. setting the bias correctly

Step 1

In my case the first one was not a big deal. I took a look at the FT 817 Operating Manual, it showed that the pin labeled "TX GND" is put to ground every time you key the PTT. 

Now all I had to do was to buy a 8-pin mini-DIN jack and solder two wires, one to "TX GND" and the other to "GND", and then connect them as shown in the following picture.

Remember to use a decent length of wire, in my case I used ~ 1 m, to move the amp at the wanted distance from the rig.

Step 2

For this step it is better to have a dummy load capable of sustaining about ~ 80 W of power. If you do not own one probably you can find another ham at the local club who will borrow his own. Remember that it is not considered good ham radio practice tuning your amplifier on the air (HI). Then you will need a power supply, in my case I used a switching one rated for 20 A @ 13.8 V, a voltmeter (about 5 V fullscale) and an ammeter (about 200 mA full scale). Remember to power radio and amplifier from the same power supply in order to avoid ground loops.

Now connect the voltmeter to the pad marked with the red arrow in the following picture and to ground. Connect the amp to the radio only with the ACC connector, do not connect the radio to the input, and then connect the dummy load to the output of the amplifier.

Turn on the power supply, you should see no voltage applied. Now when you key the PTT you should see a voltage applied. Rotate the potentiometer to the minimum voltage. At this point connect the ammeter in series with the VCC connection to the power supply. When you key the PTT you should see a current of about 50 mA due to the relay coil. Slowly increase the voltage by rotating the potentiometer. This current should be stable until you reach a point, the FETs turn-on voltage, were more current starts to flow. I decided to use a quiescent current of about 50 mA trough the FETs, so you should I read about 100 mA on the ammeter. In this way the amplifier is now operating in class AB mode. 

Step 3

Now you can try to see how much power the amplifier outputs. Deliver minimum power from the radio to the amp and measure the power flowing into the dummy load. I found useful to write a small table of the power needed to reach the full power output on the various HF bands. Remember that the gain of the MOSFETs is uneven among various frequencies. Try not to exceed the 70 W declared maximum power.

So now you end up with your working amplifier. I hope you found this post useful. If I find someone with a spectrum analyzer I will post some spectral purity analysis to determine if a filter is needed (the answer is probably yes).

73s de IU3IDF