> My observations on most RF power amplifiers is that gain is higher at > lower drive levels so proportionally there is more output. e.g. full > drive =10w, half drive = 7w, quarter drive = 5w.
You are describing classic compression in the amplifier [chain]. Where you have cascaded amplifiers, measure the Po/Pi slope *for each device* and adjust the relative gains/drive levels so that each device has a similar curve - or at least all begin to compress at the same level - and operate below the onset of compression. > Yet these amplifiers sound fine in SSB so must have good linearity. You can't say that. Non-linearity will show up first in IMD products *outside the SSB bandwidth* - your neighbors will hear the distortion before your QSO partner does or you hear it in your monitor. Now, there is no issue using these amplifiers in CW/RTTY/JT-mode as all of those modes are "one tone at a time" and do not generate significant IMD when feeding even a class C amplifier (transitions will become a bit sharper/more "clicky"). However in general, one should keep any *amplifier chain* used for "linear" modes (SSB, PSK31/63/125, etc.) below the 1 dB compression point. In other words, if 10W drive gives 750W output you should need any more than 25W for 1500W output. The 1 dB compression point is generally associated with IMD in the -33 to -35 dB range provided other issues (e.g. crossover distortion, bias stability, etc.) are well controlled. Ideally one would keep any gain compression well below 1 dB (20%) but that is often difficult with solid stage devices at high power levels - particularly since solid state devices tend to be rated for saturated mode (class C, pulse) outputs without regard for compression. 73, ... Joe, W4TV On 2015-01-08 12:03 PM, Edward R Cole wrote:
David and Lexa, My observations on most RF power amplifiers is that gain is higher at lower drive levels so proportionally there is more output. e.g. full drive =10w, half drive = 7w, quarter drive = 5w. So one finds that an amplifier hits a minimum output with lowered drive that is not linear with change in drive level. Yet these amplifiers sound fine in SSB so must have good linearity. My 50w 2m transverter actually will produce nearly 60w with 1.5mw drive and 35w with 0.73 mw, but minimum output with 0.1mw is about 15w. I insert a 3-dB attenuator to get down to 7w output to drive my 150w amp to 55w which is the drive required for 1400w with my 2m-8877. I prefer this arrangement vs running the transverter near max at 55w when running digital modes (less heat stress on the transverter amp which also translates as less thermal frequency drift). My 25w 1296 transverter runs 20w at full output and about 8w with minimum drive. I've seen this consistently with RF power amplifiers (my HF amps operate similarly) 73, Ed - KL7UW http://www.kl7uw.com "Kits made by KL7UW" Dubus Mag business: [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
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