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MIT Research Tweaks Smartphone Amplifier Voltage To Gain Battery Life

Posted by timothy on from the ok-there's-the-car-analogy dept.

hypnosec writes "Two MIT electrical engineering professors, Joel Dawson and David Perreault, have claimed that they have cracked the age old efficiency problem related to the power amplifier in smartphones by designing a new amplifier that consumes just half the power as compared to their current counterparts. Current transistor-based power amplifiers consume power in two modes – standby and output signal mode. The only way to reduce power consumption and increase battery life is to use the least possible power when in standby mode. The problem here is that if the power is kept very low when in standby mode, because of sudden jumps from low-power standby mode to high-power output mode, signals get distorted. This is why current technologies waste a lot of electricity as standby power levels are kept at a relatively higher level to avoid distortion. The new technology, dubbed asymmetric multilevel outphasing, is basically a blazingly fast electronic gearbox that would select the best possible voltage to send across to the transistors that would minimize power consumption."

6 of 47 comments (clear)

  1. I hope they've done the needful... by bogaboga · · Score: 3, Interesting

    ...and filed a patent for the implementation or call it execution....otherwise some clever company, that is better known for suing others in the industry,; that's had one of its famous patents invalidated recently, will file...and sue.

  2. Duplicate story from Oct 31 by MattskEE · · Score: 5, Informative

    This is a duplicate story posted on Slashdot on Oct 31: http://hardware.slashdot.org/story/12/11/01/0021213/breakthrough-promises-smartphones-that-use-half-the-power

    Luckily this time the summary includes a link to an actual technical paper. The summary and the news article make it sound like this is an Envelope Tracking amplifier, but if you read the paper this is actually something different, it is more complicated and more interesting.

    They are starting from an outphasing amplifier which divides a variable envelope signal into two constant amplitude but variable phase signals which can be amplified more efficiently since the amplifier doesn't need to output both large and small signals. But combining the signals is inefficient because the combiner must absorb some of the power when the two halves of the signal are very out of phase with each other. What the MIT researchers are doing is extending the outphasing technique to allow multiple discrete amplitudes on each amplifier to minimize the combiner inefficiency. It's more efficient than plain outphasing, I'm not sure how it compares to envelope tracking since the authors did not compare it to this in their paper.

    1. Re:Duplicate story from Oct 31 by Andy+Dodd · · Score: 4, Informative

      Zero crossings and low output amplitudes with outphasing techniques are a bitch. If you use a hybrid combiner, nearly all of the energy goes into a dummy load on the difference port. If you use a "lossless" combiner such as a matching network, the VSWR seen by the amp elements skyrockets and - best case, efficiency is crap. Worst case, you fry the output stages of the amp.

      Around a decade ago I worked on an outphasing system - I fried a LOT of hardware by accidentally feeding the system a low or zero amplitude, which effectively caused two amps to fight each other 180 degrees out of phase. http://www.google.com/patents/US6930547

      What they seem to be doing here is reducing the voltage provided to the amp elements, reducing the power output, which allows them to reduce total output power efficiently while keeping the two amp elements relatively in-phase, and also allowing them to "fight" each other without frying themselves like they would if running at full power.

      E.g. it's probably something like outphasing for 70-100% envelope levels, and power supply modulation below that. Likely not a "hard" threshold - they probably transition from one approach to the other somewhere in the 30-70% operating region.

      http://slashdot.org/comments.pl?sid=3222705&cid=41842229 for some more details.

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  3. hybrid A/theta modulator by cats-paw · · Score: 5, Interesting

      One way to do high efficiency is to do separate phase and amplitude modulation, the so called A-theta modulator. you get your amplitude control by adjusting the PA voltage _continuosly_ and do your phase modulation with an IQ modulator.

    For wide bandwidth modulation formats this is a bit of a pain since you need a very wideband, high current, power supply. so they are doing an A/theta modulator but trying to simplify the bias control on the PA to avoid that.

    What I'm not clear on is why they are doing this when they have a predistortion loop anyway. a pure predistortion loop should be able to achieve very similar results without any need for the PA bias adjust. you can also do it with 1 PA instead of two.

    maybe it simplifies the PD loop substantially, I can't tell yet from the paper, but it trades this off against needing two PAs and PA bias control. And that PA bias control is a bit of a headache two, because you can't just stick a switch in there. The selectable bias levels have to come from a switching power supply which switches between different output voltages. doing this is also non-trivial.

    I don't know, looks like somebody's thesis to me. Doesn't look like it's particular practical.

    Also, first rule of looking at schemes like this. How much of that power they saved is being used in the more complicated digital circuitry. That's the reason you don't see PD loops in cell phones. It's a wash, you spend so much power analyzing the signal to do PD that you burn up the savings . Now if you have a 10W transmitter, PD makes lots of sense.

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    1. Re:hybrid A/theta modulator by MattskEE · · Score: 4, Insightful

      For wide bandwidth modulation formats this is a bit of a pain since you need a very wideband, high current, power supply. so they are doing an A/theta modulator but trying to simplify the bias control on the PA to avoid that.

      No, they're doing something more complicated than A/theta modulation (aka envelope tracking aka envelope elimination and restorataion aka ...). They are doing an outphasing amplifier which splits the original signal into two constant amplitude variable phase signals which will reproduce the original signal when they are recombined, but they are adding some envelope tracking elements to further improve efficiency since wideband combiners will absorb the differential section of the input signals as loss.

      What I'm not clear on is why they are doing this when they have a predistortion loop anyway. a pure predistortion loop should be able to achieve very similar results without any need for the PA bias adjust. you can also do it with 1 PA instead of two.

      A predistorter doesn't have much of anything to do with PA efficiency (the point of this MIT research), PD's are for linearity. High efficiency PA topologies typically take a hit to linearity. Fancy techniques like outphasing and envelope tracking are better than some but they still reduce linearity. Poor linearity means increased distortion. Distortion increases error rates in digital transmissions, and it also leads to signal leaking into adjacent signal bands, which isn't allowed in the tight cellular spectral environment. So they put a PD in there to linearize things a bit further. That's probably why they are using discrete bias levels instead of continuously variable - they can optimize their PD loop to work best in these four well-characterized states which gets most of the efficiency gain while making the linearization easier.

      I don't know, looks like somebody's thesis to me. Doesn't look like it's particular practical. Also, first rule of looking at schemes like this. How much of that power they saved is being used in the more complicated digital circuitry. That's the reason you don't see PD loops in cell phones. It's a wash, you spend so much power analyzing the signal to do PD that you burn up the savings . Now if you have a 10W transmitter, PD makes lots of sense.

      These are not fundamental limitations. It's making more and more sense as digital processing and DSP chips get faster, cheaper, and lower power. The point of research is to try to push the state of the art and it may not be "practical" when the research paper is hot off the press. If it's good enough then it will be practical some time down the road, and if it isn't good enough then it will be left aside as a lesson learned. Apparently the researchers think its good enough to found a startup company, so they're either foolish or they understand it better than you. Time will tell.

  4. Re:Age old? by __aaltlg1547 · · Score: 3, Informative

    It's clumsily worded. It's an age-old problem in power amplifiers that are used in situations where power varies, such as radio receivers and transmitters that need to work at multiple power levels. But I don't understand why they identify smartphones as a place where this has a great potential to improve battery life. It's really more applicable to non-smart cellphones where the audio amp and transmitter PA are bigger proportions of the total battery draw.

    Cellphone power is often dominated by processor and display power.