In article , gwhite wrote:
Ken Smith wrote:


The strongest argument for dropping the impedance matching concept is PA
efficiency, and therefore maximum signal swing. Obtaining maximum swing is a
load line issue.

What do you mean by "maximum signal swing" in this context. I can get a
bigger swing by leaving the output completely unloaded and hence causing
the actual efficiency to be zero.

LOL. Sure, the purpose of a power amp is to actually extract power. This is a
good start.

No, the purpose of the power amp is to deliver power, not extract it.


Perhaps a simplistic (and of course idealized) class A example would help. And
I want to remind that this is a simplification of the first order design cut.

Don't bother with the over simplified Class A case. RF power
amplification is rarely done class and and it is a digression from the
actual topic.

[...]

Our circuit loaded with 10 ohms delivers twice as much power as with the lesser
5 ohms or greater 20 ohms. That is, extracted output power is peaking at some
finite non-zero value. This is also easily seen to be most efficient point for
this simplistic example.

At some point as you decrease the resistance, the output will drop to zero
as the amplifier fails or it will start to decrease in some more
controlled manner as the protection circuits take control. If we assume
the latter case, it is easy to see that the power reaches a maximum value
and then decreases as the resistance is lowered. The point at which the
power is at the maximum is the point at which the load is matched. If you
make a small change in the load and observe the voltage and current when
that small change is made, you will see that that is indeed the output
impedance of the amplifier. I think this is the part you are not
grasping.


--
--
forging knowledge