EMRFD Message Archive 13265
Message Date From Subject 13265 2016-11-05 16:02:49 Chris Howard w0ep how to determine proper bias for a MOSFET power amp I'm building a kit, a 4sQRP SS-TX40, 40 meter transmitter.
It has a MOSFET final: STP16NF06L
My question is about setting the idle current / bias.
Is there either a calculation or some empirical measurement
for setting the idle current on such a thing?
The build instructions say I should see 95 mA or some
such figure as the total power supply current with no
drive and bias set to 0. And then the instructions
say to use the bias-setting trimmer to run that up by 40 mA
and you're done.
But my board has 265 mA with no drive, bias set to 0.
I ran that up 40mA and my RF output is 1 watt instead
of the advertised 5 watts. I may be completely
off track and should be searching for the difference
between 95 and 265. I have attempted! but nothing
I have asked about this on the yahoo group dedicated
to this particular project. But I thought it might
be a good opportunity to learn something. Where did
the 40 mA come from?
I have a suspicion you are going
to ask me the class of the amplifier. Answer, I don't know.
Maybe that can be deduced from the 40 figure?
13280 2016-11-08 10:52:18 dixonglennb Re: how to determine proper bias for a MOSFET power amp Hi Chris,Did the other forum help you? If you are still trying to troubleshoot your circuit, we can try a few things here.73,Glenn AC7ZN 13281 2016-11-08 13:03:18 cwfingertalker Re: how to determine proper bias for a MOSFET power amp Chris,I would like to learn about this too. I built a 12 watt 40 meter CW transmitter with the RD16HHF transistor. I fooled around with the bias until I got 15 watts out. Not scientific I know but I guess tuning for maximum smoke worked for me. No idea what class amplifier I am running. I am eager to learn though.73Bill N7EU 13282 2016-11-08 13:28:07 Dana Myers Re: how to determine proper bias for a MOSFET power amp 13283 2016-11-08 14:27:24 Chris Howard w0ep Re: how to determine proper bias for a MOSFET power amp I did get an answer to my question.
My direct problem was a soldering issue on my board.
But I also learned a bit about the biasing. It seems that
it was arrived at through watching the output for quality
on a spectrum analyzer.
13286 2016-11-09 09:11:24 Dana Myers Re: how to determine proper bias for a MOSFET power amp