On 14 mayo, 02:57, Richard Clark wrote:
On Fri, 13 May 2011 16:28:52 -0700 (PDT), K7ITM wrote:
Though it's a red herring typical of audio-speak, most modern high
fidelity audio amplifiers have a very low output impedance, a small
fraction of an ohm, so they can claim a high damping factor. *Off
topic: *the reason it's a red herring is that the impedance of the
speaker connected to the amplifier must be included to figure the
damping, and that impedance (even just the DC resistance of voice
coils) changes by considerably more than the amplifier's output
impedance (resistance) just because of heating on audio peaks.

Especially when reactive components (inductors and capacitors) couple
power between a source and a load, you can get stresses--voltages and/
or currents--well beyond what's safe when you try to operate the
source into a load it's not intended to handle. *That's true even when
the net power delivered to the load is considerably LESS than the
rated output power of the source. *Wim's example of the class-E
amplifier is true enough, but it's not necessary to ask the source to
deliver more net load power than it's rated to deliver, to establish
conditions that cause trouble. *Thus, even sources that have an output
impedance at or very close to the rated load impedance will have
circuits to protect against loads that could destroy things inside the
amplifier.

I'm going to slip a mickey into this by a careful, editorial change of
focus BACK to the subject line.

Though it's a red herring typical of Ham-speak, most modern retail
100W RF transmitters for amateur service *have a very low output
impedance, a fraction of an ohm [ editorial: until, of course, it goes
to the Z transformer that precedes the bandpass filter].

Richard, the text below represents real world better:

"Though it's a red herring typical of Ham-speak, most modern retail
100W RF transmitters for amateur service require to see a very low
impedance at their drains/collectors, a fraction of an ohm
[ editorial: until, of course, it goes to the Z transformer that
precedes the bandpass filter]."

A push-pull 100W PA (non-switching, just a linear amplifier) fed from
13.8 Vdc will produce a voltage swing of about 20Vp at the primary of
the wide band impedance transformer. I know it can be somewhat more,
but with increased IMD. To extract 100W, you need to create a 2 Ohms
load between the collectors (or drains).

This 2 Ohms has no relation with the actual output impedance of the
active devices (as this depends on the impedance seen from the base/
gate, semiconductor process, amount of saturation, etc).

With kind regards,


Wim
PA3DJS
In case of PM, remove abc first.