The modular approach to the si570 (DDS) MEPT has meant I can try out different methods. I used a Class A PA strip to get a good clean 250mW on 50MHz for multimodes.
I thought I would try the same 2N3866 Class A strip idea for a Broad Band HF PA, to get a bit more power on 24 and 28 MHz bands. I needed a little more drive so I added a 2 transistor BC184 Driver Amplifier was added too.
Right click and select View Image for full view.
Compared to the 74HC240 PA version total efficiency is low, only 12.5%. Total consumption including si570 being 4Watts, not my idea of true QRP but once you throw that Xtal away you are on a slippery slope. I quake in fear at some of the temperatures that silicon gets run at so I added a 'Smoke Stack' to the output transistor heat sink. Here is another view.
To the right of the 'Smoke Stack' is the band specific LPF at the left hand side is the band specific input filter. Both modules are easily changed over, just 3 solder links on each for band swapping.
Here we see the si570 Synthesizer, mixer and PIC WSPR, Hell and QRSS modulator.
I have run on air tests on 24MHz and 28MHz, unfortunately with no reports as yet. Future band improvements should change that soon.
