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.


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.


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