Saturday, October 30, 2010

adding a 70 watt PA to my homebrew SSB transceiver




I found a MaComm DU2880V transistor at a local hamfest for $10. I've decided to use that as the final output PA. This transistors is a dual mosfet, capable of providing 80 watts at 2-175 Mc. I've built up a pa from this by swamping the input gate impedance and using broad band transformers on the input/output. Getting about 70 watts out with 1 watt of drive.

In order to maintain a constant bias over the operating temperature i recently heated up the amplifier with a heat gun while monitoring its temperature, i then used two silicon diodes and a potentiometer to pull down the bias voltage as the transistor heats up. This is a method used to help compensate for increased bias voltage with temperature that RF mosfets typically experience (see 2010 ARRL handbook mostfet PA project).

Planning to incorporate this into my radio sometime next week, stay tuned for the results....