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

Seems I can't help myself but try different transistors. In the course of testing my resistor as a thermal bridge concept I needed to populate a test board for measurements. After that I finished populating the board and tested numerous transistors with it. 

Initially I had some problems, both a soldering fault and poor biasing due to an error in my spreadsheet. But I learned a lot about debugging this amplifier in the process. I have now tested a large assortment of transistors, both SMD and through hole. Most worked very well at 40m and 80m, some well beyond that. And I never encountered any instabilities.

Even on my narrow board I never let the smoke out of any transistor despite relentlessly abusing them. The only casualty was a 2.2ohm decoupling resistor burning out.

1 watt is about the limit with a 13.8 volt supply. The output signal is 20Vpp. After decoupling and loss in the collector choke you have about 13.2V at the collector. You need a bit over 1V on the emitter to set the current with a 5Ω emitter resistor. So you are left with a margin of say 2V to deal with saturation considerations. Even with everything going well the best you might achieve is an extra 1dB of output. Probably not worth the grief chasing it.

Too much standing current and the 2V margin is reduced. Too little standing current and distortion is present. What I do is calculate the bias component values based on a standing current of say 220mA since the bare minimum needed is 200mA for 1W of output into 50Ω. The base resistor to ground is then rounded up to a convenient value. Once built, check the emitter voltage. If higher than desired put a suitable value resistor in parallel with the base resistor to ground to reduce the emitter voltage to the desired value.

I then examine the emitter voltage (Ve) with my CRO. It must be greater than zero at all times when delivering the required 1W for a Class A amplifier. The exact value needed to avoid distortion varies with the transistor used. The minimum Ve is a function of standing current so if you are bottoming out then a small adjustment to the base resistor to ground is needed.

Parallel output transistor at this power level at HF are easy to heatsink using resistors as a thermal bridge and cheap. I recommend this approach over expensive RF devices in this application.

73's

Richard