OH2NLT 50W PA for Cheap DSP transceiver




While ago I managed to get some used Philips UHF power MOS FETís. UHF/VHF fets are excellent devices for HF amplifiers because of their very low capacitance. This amplifier is done with BLF548 dual FET. Design target was PA for 3,5 and 7 MHz operating at 14V and fit into Cheap DSP (modem) enclosure. I choosed dual output transformer topology with 1:5 winding ratio. This configuration gives little less than 50W @ 14V. Driver was done with two BLF521 FET transistors. PA total gain is about 35dB, which is enough to get full output power with Cheap DSP board drive capability. Drive comes directly from multiplexer mixer. Driver transformers are TDK common mode filters used as transformer. Amplifierís linearity and signal quality is excellent. You could almost use output signal directly. Band filters for 3,5 and 7MHz are however implemented in the design. FET bias voltages are generated with 7805 regulator and temperature correction diode. Extra LM317T regulator for Cheap DSP 10V feed is installed on the PA board/heat sink.


Schematic diagram

PCB layout

PCB jumper wires

Component placement

Mechanical preparation of self made PCB for installation.

The BLF548 transistor This dual MOS transistor is capable for much more than taken out in this design. See data sheet.

Final transformers

Driver / driver transformers


Data sheets:



.Note Beryllium Oxide hazard when handling these devices. BeO is toxic material.