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Allan Herriman wrote:
On Sat, 26 Feb 2005 04:53:03 +0000 (UTC), "Reg Edwards" wrote: The phrase "output impedance" in connection with amplifiers is ambiguous and likely to result in arguments. The correct description is "internal impedance" or "internal resistance" and should always be used. S22 is fairly well defined. Just to bring back to the original discussion and reiterate: S22 is a small signal (linear) parameter, by definition. It does not apply to the large signal environment. This idea is somewhat related to the idea that power amps should be tuned for "maximum transfer of power," which is a small signal (s-param) issue, and requires conjugate matching. The idea is incorrect because it ignores the practical large signal non-linearity and *any* consideration of DC to RF efficiency (which is prime for PA design). Linear parameters provide *no* recognition of things like DC to signal power efficiency and therefore practical issues like supply rails. First order matching of an RF PA to a load involves transforming the load to the optimum point on the AC load line (for example, more or less equal positive and negative swing limits for class A). That's what "matching" is for an RF PA. It makes no statement about actual "output impedance" of the source. What is said is that "such and such RF PA will deliver X power into some specifed impedance within some VSWR circle." That's all. The concept of output impedance begins to break down for large signal devices. |
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