Paul Burridge wrote:

On Sun, 04 Jan 2004 15:44:04 +0000, Leon Heller
wrote:



Paul Burridge wrote:

On Sat, 3 Jan 2004 16:50:52 -0800, "Tim Wescott"
wrote:



Normally you'd expect the final amplifier to be fairly "stiff" - the output
power should be inversely proportional to the resistor you put on it.


??? Shirley, some mistake.

Both AF and HF power amplifiers generally have a very low output impedance.

Motorola's AN721 app note on impedance matching networks for RF
transistors explains this.

This doesn't apply to QRP output stages, though.


Hi, Leon,

Do you know Brian (G4BCO) by any chance?
My - admittedly little - understanding of the situation is that the
power transferred into a resistor (regardless of power level) will be
maximised when Rs=Rl,. whereupon half the power is dissipated in the
final device and half in the load. Tim's posts seems to refute this
basic truism. Clearly the statement that the output power should be
inversely proportional to the load resistance *must* be false.

No, that isn't the case. From AN721:

"The load value is primarily dictated by the required output power and
the peak voltage; it is not matched to the output impedance of
the device."

Leon
--
Leon Heller, G1HSM
Email:
My low-cost Philips LPC210x ARM development system:
http://www.geocities.com/leon_heller/lpc2104.html