10W Class AB with parallel transistors - Not yet, but 5W is OK

It appears a cheap, stable and effective 5W HF amp can be realised with parallel SOT89's and thermal bridges. 

 

A variety of transistors have been tested. Some could deliver 10W continuously for extended periods thanks to the thermal bridges and heatsink. However, none appeared capable of delivering 10W with low distortion.

 

Gain and power handling varied between transistor types. Some test boards could deliver well over 10W into the dummy load with no mishap. Harmonic distortion was present so a good low pass filter is essential. No issues with thermal runaway were noted.

 

I learned the hard way that a 1206 resistor had an insufficient power rating as an emitter ballast resistor when testing for extended periods. After replacing several sets of resistors I switched to a larger SMD resistor.

Having destroyed many sets of SOT-89 transistors I have concluded the SOT-89 package is not going to deliver 10W of RF continuously.  Using 10 parallel transistors the per package dissipation is around 2W (1W of Rf requires at least 2W of electrical energy). At times I saw 5 Amps of total collector current on the "guess" meter fitted to the power supply which was just too much for many of the devices. 

While several transistor types could deliver 5W with low distortion that wasn't my goal. So it's time to move on to a SOT223 package.

73's