[BC] Solid State FM PAs

[John Lyles]

One thing to watch out for, modern RF power transistors, unlike tubes, have about a short lifetime, I don't mean they fail, just that they don't get marketed and manufactured anymore. Replaced with a better generation, that is incompatible performance and spec-wise. There are some standards that have survived longer, like the venerable MRF150, 151G, and some of the bipolars from Motorola and ST, but in general, watch out for amplifiers using odd devices that are near end of market life. Its hard to tell unless you ask, and look up the datasheet for the date, but a lot of 50 and 100 watt parts are discontinued. Companies buy out others, like M/ACom buying the MRF line, and Tyco now owning M/ACom, and Microsemi buying former Thomson Montgomerville,PA line and shutting that place down. Some power amplifiers used selected and matched parts in push pull layouts. These are even harder to come by if the original part is near end of production life. 

This becomes more of a problem for Wireless amplifiers, 900 MHz and up, as the transistor technology has made many rapid improvements past 10 years. Even broadcast gear, though, is susceptable to this problem. Whatever you select, keep in mind that some bargain-priced power amplifiers may use very old designs (look for mica clad capacitors and trimmer capacitors in the layout), bipolar transistors that are no longer popular, or MOSFETs that are near end of production life, and cheap. 

I was just at a tube manufacturer, and they also pointed this out to me, and said that tubes tend to stay in production longer than silicon-based power devices. 

Happy 2008,
John