EMRFD Message Archive 12491

Message	Date	From	Subject
12491	2016-03-08 20:53:01	rodwall1234	Transmitter efficiency
Hi all, Is someone able to tell me if the following is used. If the output impedance of a fixed frequency transmitter is 50ohm. and the Antenna and transmission line are also 50ohm. Only 50% of the transmitter power gets to the Antenna load. If the transmitter is a fixed frequency say 100M watt. Then a lot of power is wasted. My question is. Do they design the transmitter to be as efficient as possible with a low output impedance. And then use a high Q matching circuit to match it to the load? Note that the frequency is fixed. Regards, Roderick Wall, vk3yc.
12492	2016-03-08 21:01:09	Andy	Re: Transmitter efficiency
Transmitters are nonlinear.  The power transfer principles you think you know, apply only to linear circuits. In the old days we used to study the efficiency of a class-A amp, class-AB, class-B, and class C.  Now there are other classes as well.  Class C and most digital switching amps achieve efficiencies much greater than 50%.  In principle, digital switching amps could approach (but not achieve) 100% efficiency -- nothing lost in the amp itself. It's a different mindframe than the one about linear circuits and matching impedances.  Forget about that, and you'll be better off. Regards, Andy
12493	2016-03-08 21:56:43	Russell Shaw	Re: Transmitter efficiency
12494	2016-03-09 08:12:55	Bill Carver	Re: Transmitter efficiency
Hi Roderick. The usual high power amplifier goal is to produce the maximum power in the load. Typically, having roughly a Vcc to ground voltage swing and a current swing that matches the device capability at the same time. The load impedance at the drain/collector is typically a much, much, much lower impedance than the drain/collector dynamic impedance. The matching network "optimizes" the load presented to the active device, but that is NOT an impedance match. Bill W7AAZ