5W RF Amp

Taking a look at RF amplifiers. And the development of a driver and a PA output stage, with output somewhere around 5W.

DRIVER STAGE
 I tried a simple circuit using a pair of 2N3904 which quite a few people seem to use, but the results were poor with not much gain - 10dB or so at best.

My feeling is that this is due to a low fT (transition frequency) of the 2N3904, or these low cost versions of them.

I have now switched to a proper HF device, the 2N5109 (with an fT of 500MHz). And results are spectacular. This is the small circuit I am trying out.

Vcc = 12V, Ic = 52mA

Using a 100R potentiometer on the input from my Si5351 SIGGEN, and adjusting for 250mVrms input, the output is then 2.4Vrms, or 94mW. The highest output without clipping is around 3.9Vrms, or 290mW with an input of 420mVrms. Assuming (not unreasonably) an input and output impedance of 50R the power gain is

Pgain = 10 * log (3.9^2 / 0.42^2) ~ 20dB

Voltages were measured by my SmartScope running on my iMac. (Blue output, Green input)

OUTPUT STAGE
Also tried the (in)famous IRF510 as an output device, poor results - gain 10dB at best. So I plan to use the RD16FHH device. The final circuit is

This has an overall gain of around 40dB (20 + 20dB), and needs a signal of 35mVrms, after the 100R variable, for an output of 1W. I cannot measure the maximum output as the heat sink on the RD16HHF1 is too small to allow me to crank it up, but on a quick flip test it easily outputs 5W into a LPF and 50R load.

An LPF must be added to the output and a 40m one has been used in testing.