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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. |
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