Tam/WB2TT wrote:

"Rich Grise" wrote in message
news
On Fri, 25 Feb 2005 17:59:56 -0500, Tam/WB2TT wrote:


"gwhite" wrote in message
...
Richard Clark wrote:

On Wed, 23 Feb 2005 19:08:20 GMT, gwhite wrote:

RF transmitters are not ....

Sorry OM,

This was all nonsense.

Nice articulation. I don't know who OM is, but RF transmitter power
amps
are
not "impedance matched." Neither are audio power amps for that matter.

My stereo amp has a spec on output impedance. As I recall, it was around
0.16 Ohms. Intended load is 4 - 16 Ohms.


That works because the transmission line is less than 0.01 wavelength.
So impedance matching becomes moot. If the speaker line were 1/4
wavelength
long, there would be almost no signal transferred at all.

Cheers!
Rich


There is nothing wrong with driving a transmission line/antenna from a zero
impedance source. It does NOT change the SWR. The point is that an audio
amplifier with a damping factor of 50 is NOT conjugate matched.

Somebody mentioned Motorola Application note 721. This is what it says:

************************************************** **************************************
" ..the load, in first approximation, is not related to the device, except
for VCE(sat). 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. "
************************************************** *****************************************

When device people talk about "matching", they mean matching the load to
what the transistor wants to see, which is not the conjugate of the output
impedance. The way this is done is to build an amplifier, and vary the load
until maximum output power is reached. The transistor is then removed, and
the impedance looking into the coupling network is measured. The conjugate
of this is sometimes listed as "output impedance" on data sheets. Newer data
sheets will have an asterisk * next to that, and a note explaining what it
means. If you look at Philips literature, you will see exactly the same
explanation.

Nice. Exactly: "what it wants to see" is perfect. Of course, "varying the
load" requires load pull test equipment and that can be expensive. When load
pull equipment is not available, we're stuck with other methodology. In that
case, my first order cut is the AC load line, a harmonic short at the device,
and enough flexibility in the layout to pull it in by cut and try. I haven't
used ADS or Microwave Office's Harmonic Balance simulators. I suppose with good
behavioral models and a good simulator, a good deal of cut and try could be
circumvented.

Obviously people don't have 100 W (or more!) network analyzers looking into the
output and pretending the device is similar to a linear small signal device.