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what happens to reflected energy ?
On Jun 7, 12:19*pm, K7ITM wrote:
On Jun 6, 7:29*pm, walt wrote: ... And Tom, why would the source be reactive when the pi-network is tuned to resonance? And because the source resistance of the (tube) power amp is non-dissipative, its reflection coefficient is 1.0 by definition, and so it cannot absorb any reflected energy, and therefore re-reflects it. Walt, W2DU If you allow that the thing driving the pi network has an effective source impedance different from the load that the pi network presents to it, then clearly the output impedance seen at the other end (the "50 ohm" end) of the pi network won't be 50 ohms. *Try it with some numbers; for example, assume a pi network that transforms your 50 ohm load to a 4k ohm load to the amplifier output, and assume an amplifier output stage that looks like a 20k ohm source. *Design the pi network for a loaded Q of 10. *I believe you'll find that the source impedance seen by the 50 ohm load is about 11+j18 ohms. As Wim has pointed out, requiring an amplifier to be loaded and driven in a very particular way unnecessarily dismisses some very important classes of amplifier. *What do you do, for example, with a linear amplifier? *What do you do with an amplifier that drives a voltage very hard (and for which a simple pi network is inappropriate for matching to a load)? *Perhaps an even more basic question is: *why exactly do we tune a pi network to present a particular load to an RF amplifier stage? *Why should we operate a 6146 with, say, a 3000 ohm plate load? *Why not 1000, or 6000? And what if I set up a tube and pi network for operation such that the apparent output source impedance is 50 ohms (while driving a 50 ohm load), and then I add feedback to the amplifier in such a way that the operating conditions are not changed, but the impedance looking back into the plate is changed? How did we get to the source resistance of "the (tube) power amp" being non-dissipative? *I know there are some of us who don't buy into that... Cheers, Tom Hello Tom, Well, the reason I chose to discuss only the tube amp with a pi- network filter and impedance transformer is that that arrangement is the only one I've measured, and that I'm not sufficiently acquainted with other arrangements to discuss them. As to your question, why not use plate loads of 3000, 6000 or 1000? Because the transceivers I measured using two 6146s in parallel are Kenwood TS-830S and Heathkit HW-100. I don't remember the exact plate load with the HW-100, close to 1400 ohms, while the TS-830S was 1400 ohms. I had no control over those plate loads because the plate and grid voltages were preset, and 1400 ohms is the RL I measured at the input of the pi-network when the grid drive and the network were adjusted to deliver precisely 100w to the load 50-ohm load. Now concerning the non-dissipative source resistance of the tube-type power amp. There are two separate resistances in the amp, the cathode- to-plate resistance, Rpd, that accounts for all the dissipation in the tube; and the output-source resistance that is non-dissipative. It is a common myth that an RF power amp cannot have an efficiency greater than 50% when conjugate matched to the load, because half the RF power is dissipated in the source resistance. This is not true, because when the amp is operating properly, resistance Rpd is less than the output source resistance, thus allowing more power delivered to the load than that dissipated in the plate-to-cathode resistance. The output source resistance is derived from the voltage-current ratio E/I that appears at the output terminals of the pi-network. A ratio, as such, cannot dissipate energy, but the load it feeds does. Using an example from Terman's Radio Engineer's Handbook I explain this phenomenon in great detail in Chapter 19 in Reflections, and further in Chapter 19A, an addition to Chapter 19. Both are available on my web page at www.w2du.com. Chapter 19 appears in 'Read Chapters from Reflections 2', and 19A appears in 'Preview Chapter from Reflections 3'. The entire Chapter 19 appears in Reflections 3, which is now available from CQ. I invite you to review these Chapters. Walt, W2DU |
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