General Purpose Transistors in RF Amplifiers - Driver Test Results

Having reworded the amplifier chain to use 2:1 transformers instead of 22uH collector chokes I have commenced testing. My approach was:

1. Build the amplifier with transistors that were known to work and measure
2. Transfer the output and buffer transistors to a know board and start substituting SOT-89 packaged driver transistors
   
3. With a chosen driver transistors transfer everything to another new board and after confirming a SOT23 buffer would work start substituting final transistors.

Being a known good design the amplifier worked with the original transistors. I adjusted the output for 10v pp at 1MHz. The roll-off at 20MHz was 2.9dB. Looking at the inter-stage levels I concluded that most of the roll-off was split equally between the buffer and the driver. The output stage was almost flat (-0.2dB). 

After testing 10 different SOT89 driver transistors I found the average of the roll-off at 20MHz for the entire amplifier was now -3.7dB, a deterioration of 0.8dB. I explain that by the difference in Ft: the original driver, a 2N4427, has a transition frequency of at least 500MHz, compared with the transistors tested having a typical transition frequency of 100MHz. 

While I only tested one device from each packet, the best transistor was the BCX56 which was just 0.4dB worse than the 2N4427. I'm comfortable using that transistor because it also appears to have the highest Ft at the collector current being used (~180MHz at 42mA from the datasheet chart).

The worst transistor had a claimed 210MHz typical Ft at 500mA. But clearly the Hottech 2SC4672 does not perform very well at a 42mA collector current. That's probably why there is no chart! 

And be wary of just buying any BCX56. I checked a few datasheets and in this application I would only use a BCX56 where there was a chart of Ft versus current, or a stated Ft in the vicinity of 50mA. Of course, it you buy from Aliexpress then you have entered a lottery.

Conclusion:

In a class A amplifier around 30mW output you can use a 10cent transistor at HF. This BCX56 is now my standard and should work very well in my 80m Weaver transceiver being developed. This removes the need for a 2N4427 for low-mid HF applications.

Now to test some higher power alternatives.

73's

Richard