G.P. Transistors in RF Amplifiers - Parallel Ouput Transistors- Success!

Having populated the board I previously blogged I applied power. Backwards of course. However, no harm done. Phew.

After noting the amplifier was working with no obvious issues I checked the temperature of the output transistors. Too hot for comfort so I added the thermal jumpers I had brought from Mouser for testing in just this kind of application. Never having used these before it was pleasing to see how they allowed a "hot" transistor tab to be thermally grounded but isolated for DC and AC voltages.

Now for a smoke test. With a 13.8volt supply I could only achieve a clean output of 16.6v peak to peak at 3.6MHz. Increasing the supply voltage to 15.9volts allowed a clean 1 watt output sine wave to appear on the CRO.

Tuning around and writing down measurements meant I noticed that the output of my signal generator is not as flat as you might expect from a HP product. Regardless, at a frequency of 20MHz I set the signal generator output to deliver 10v pp into the dummy load. I then tuned down and the output was quite flat until 10MHz when the output looked like some frequency doubling was happening. Changing the capacitor coupling the emitters together solved this. I had used a 15nF by mistake. But as I reduced the frequency still further the output resumed the appearance of a sine wave and started increasing until it reached 19v pp at 6MHz before declining as frequency was reduced.

I haven't time at present to investigate if that is the signal generator output level wandering around or something weird occurring in the amplifier. So I cranked the supply voltage up to 15.9 volts, set the output for 20v pp at 3.6MHz and walked away.  Later I returned and no smoke of any kind. I did note the output had dropped a few volts due to the temperature rise of the transistors. But no damage done and a few puffs dropped the temperature enough to show the output rise.

By now you  might be wondering what transistors I used. The very first packet I picked up were 2SD1007's. When I returned to the house I checked my LTSpice model and it was in loose agreement with my measurements. eg I had measured 1.4V on the emitter whereas LTSpice was suggesting is should be 1.18V. 

Some finessing is still required, but the proof of concept test board is actually a working amplifier.

 

Conclusion:

Cheap (A$0.10 each) SOT89 transistors can be made to work as HF amplifiers in the vicinity of 1watt when used in parallel.

However, you need a thermal jumper which costs 10 times more than the transistor. And my heat sinking using the ground plane could be larger. In practice that will require a wider board so I will abandon my plan to use the piece of extrusion I have on hand to house the amplifier.

So more thought needed regarding the thermal jumper.

73's

Richard