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The Problem of the Power Amplifier July 5, 2008

Posted by OldGuy in Power amplifiers.
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We’re getting pretty good at efficient appliances plugged into the wall.  But wireless devices are tough.  Reception isn’t too bad, but sending (transmitting) is a real challenge.

Radio transmitters push signals through the air based on the amount of energy released by the power amplifier. But they only go so far.  And the power fades as distance increases.  (That’s why radio signals fade as you drive down the highway.)  If you need a strong signal over a long distance, you need to start out with more power than you will have at the receiver.

State of the art for some of the medium frequency power amplifiers is around 10%.  That means, for every 10 watts in, you only get 1 watt out.  5 watts of signal only goes a couple of miles.  If you need to receive 5 watts from 25 miles away, it takes 50 watts output power.   And that means, for one military data freqency (the “common” data link – 14.5 to 15.35 GHz), you need to push almost 700 watts in to get that 50 watts out!  Over 90% of the energy is wasted. because the state of the art of power amplifiers is so inefficent.

And what happens to the other 600+ watts?  It becomes wasted heat.  That’s a problem for electronics in small spaces.  You need extra cooling, which takes its own power, to get that heat away from the active electronics, or the circuits of the radio become really innefficient, and eventually quit working.

What we need is more research into newer methods of manufacturing radio power amplifiers.  Because we can’t afford to operate these critical devices at current efficiencies.