Dear All,

Stefan raises an interesting issue.

On 2011 Jan 13, at 21:36, Stefan Schäfer wrote:
> My LF 1 kW class E PA i using a single IRFP360 at > 96 % efficiency

I'm sure you are getting high efficiencies. But measuring it at these numbers 
close to 100% actually makes quite interesting demands on the accuracy of the 
instrumentation.

You presumably measure DC input power from the PSU as Vin * Iin, and RF output 
power as Vout * Iout.
The efficiency is then (Vout * Iout) / (Vin * Iin). The percentage error of the 
result (the efficiency) is the sum of the percentage errors of the four 
measurements.
You might instead measure the output power into a dummy load as Vout^2/R, but 
you still have a percentage error of twice twice the error of the voltage 
measurement.

If you measure the output on a scope, personally I wouldn't trust a scope 
measurement to better than a couple of percent. The Vout and Iout measurements 
(or two Vout measurements) will amount to a few percent error each, so your 
efficiency of 96% could really be 96% +/-4% or even 96% +/-6%.
(If you measure Vin and Iin on a multimeter, they could easily have errors of 
0.1% or better, and not contribute appreciably to the overall error.)

John G3PAI brought up this interesting problem when he first built a class E 
amplifier and measured efficiencies of over 100%. It was immediately obvious to 
him that the measurement accuracy was a limitation.

73,
Chris G4OKW

-----------------------
Dr Chris Trayner
School of Electronic & Electrical Engineering,
The University of Leeds,
Leeds LS2 9JT, United Kingdom
Tel: +44 113 34 32053
Fax: +44 113 34 32032