10W Class AB HF Amplifier - Initial Testing

I mentioned in this post that while the 2:1 transformers were successful, they still needed to be tested at higher power levels. This post covers that testing and some initial observations on transistors to be used in this amplifier stage.

10W(?) HF Test Amplifier using a CPU Heat Sink

Above is a picture of the amplifier using the two transformers I tested back to back in this post. At 4W continuous the transformer used in the output had no issues at 80m. I didn't get a chance to try other frequencies for an extended period because I was too busy letting out the smoke!

The initial transistor tested was a 2SC1096. From the datasheet we read:

·For audio frequency power amplifier and low speed switching applications
·Suitable for output stages of 3 to 5 watts car radio sets and car stereo

I expected this to work well at 80m and 40m given the Ft of 65MHz and it did so. Gain decreased with increasing frequency but across 160-15m I was able to achieve 4W continuous by adjusting the drive. 

2SC1096 Initial waveform at 1W output on 80m

 

The initial waveform at 1W output on 80m (above) suggested an increase in standing current would beneficial. I pushed the output up to around 40Vpp. 

2SC1096 4W Out

At this point I was not entirely happy with that downward sloping waveform after the positive peak of each cycle. But I pushed the bias too far and popped the transistor. However, my initial goal of testing the bias circuit and 50:12.5Ω transformers was achieved.

I replaced the transistor with a 2SC1226, which is broadly similar to the 2SC1096 i.e. it's not a RF transistor. Again I could get 4W easily.

2SC1226 at 4W Out

Of note is the initial downward sloping part of the waveform is around 0.06uS long, against the 0.08uS of the 2SC1096. Which sounds better though I suspect this is still not ideal.

The 50:12.5Ω transformer was replaced with a Ruthroff Type 9:1 on Small Binocular Ferrite. This gave me the potential to output more than 4W. I could still get 4W or so out but increasing the drive popped the transistor.

So progress has been good. I might have popped a couple of junk box non-RF transistors but I established the development board, biasing and transformers are all working.

Next time I will replace the transistor with an RF transistor so I have something to compare a number of readily available general purpose transistors against. 

73's

Richard