Amplitude control by amplitude vs phase shift - efficency? - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: July 12, 2020, 07:07:57 am
content: Say we have two linear sine wave power amplifiers feeding opposite ends of a load. We can control the amplitude of the sine wave signal fed to the load by either driving the amplifiers 180 deg out of phase and simply changing the output level of both amplifiers, or we can drive both amplifiers to the same maximum just before clipping and shift the phase of one with respect to the other. Max output when they are 180 deg out of phase, and zero output when they are exactly in phase.Question - being linear amplifiers, neither method is particularly efficient, but is one method better than the other or are they identical?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 12, 2020, 10:48:14 am
content: Phase shifting is rather difficult, so you'd tend not to do it in general. For wideband applications, say: you need a flat phase shift over the whole band.When you do have a variable phase shift, and maybe can take advantage of harmonic distortion in the output, i.e. to improve efficiency while filtering it from the output signal -- or don't care at all -- yes, that's doable.I'm not sure if that's done for RF, because the load varies -- you're going from 100% common mode, to 100% differential mode. If the load is differential only, then the finals are perfectly unloaded when in phase. Which isn't going to do your efficiency any favors, but might blow them up, too (if they're no good at handling SWR). If you have some way to recycle CM, yes, it would be usable, and efficient.(RF of course has the better solution of phase modulating the carrier, and amplitude modulating the finals -- you can do PAM explicitly.)For power switching, it's quite practical and abbreviated as PSPWM. Again, when in phase, you have full switching but no load current, so the inverters are hard switching just their capacitance, for all that power dissipation and no output. Not the greatest situation. And, not only does hard switching increase EMI, but the fact that you're explicitly driving 100% common mode into the load helps even less...Tim

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: July 12, 2020, 10:57:47 am
content: You see this with RF amplifiers for achieving higher power efficiency. An amplifier is much more efficient when it is driven quite hard. Taking two amplifiers like that and then playing with the phase difference allows you to make an AM constellations. If you can change the phase of both separately, you can not only make AM modulation, but any complex modulation. This is called out-phasing. (For lower-speed signals such as WiFi and cellular, you can actually use class D/E/F etc amplifiers to generate this CW tone, further increasing power efficiency)Few difficulties:The efficiency calculations of this are actually quite a challenge. Because you are working with power amplifiers, the behavior depends a lot on load, and so even though you are (trying) to have two amplifiers output a constant output power, the actual output power of both will change because it sees the other amplifier as a form of load modulation. (load modulation can actually improve overall efficiency further - this is the principle used in Doherthy amplifiers).This load modulation also ruins your constellation because the phase of the output signal depends on the load, and if you change the load, you change the output phase, even though you don't change the input phase.Another common issue is that these amplifiers have difficulty with small signals. They can get good EVM at high power (which is where more classic amplifiers struggle as they start saturating) but at low power, you get large errors. To get a small value, you subtract to very large values, and if your large values have a small amount of noise, that noise blows up with these small values (EG, say one amplifiers output level is 1.01, and the other is 0.99 - both just 1 percent error. But if you want to get a signal of almost 0 by canceling the two out, you are left 0.02, which might be a big error).

author: Circlotron
author_rank: Super Contributor
timestamp: July 12, 2020, 11:18:39 am
content: Sorry, I should have added, not RF. Just something like 50Hz from two big audio power amplifiers to make UPS type power. Not a real project, just a thought experiment.

author: duak
author_rank: Super Contributor
timestamp: July 12, 2020, 07:03:33 pm
content: That's an interesting thought. At first, I wondered if the load current in the load will be in phase with the voltage generated by the power amps. To each power amp, if the current is not in phase with the output voltage it will appear to drive a reactive load. If memory serves, current leads in a capacitive reactance and this causes greater dissipation in the driver devices. However, after thinking about it some more, no, the current is still in phase with the voltage and three phase systems, with their 12- deg phase shift between phases do not have power factor related problems with a purely resistive load. At one time I would have been easily able to calculate the dissipation in the power devices - something about calculus.A single frequency +/- 180 deg phase shifter is easy to build to try out - see attached doc.