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Envelope tracking (ET) describes an approach to Radio Frequency (RF) amplifier design in which the power supply voltage applied to the power amplifier is continuously adjusted to ensure that the amplifier is operating at peak efficiency for power required at each instant of transmission. A conventional RF amplifier designed with a fixed supply voltage operates most efficiently only when operating in compression. Amplifiers operating with a constant supply voltage become less efficient as the crest factor of the signal increases, because the amplifier spends more time operating below peak power and, therefore, spends more time operating below its maximum efficiency. ==Background== The need for greater efficiency arises particularly as modulation schemes become more complicated and their peak to average power ratio increases. Older modulation schemes based on phase or frequency modulation with no amplitude information carried on the signal can use amplifiers that are driven into compression and offer high levels of efficiency. As of 2014 mobile communications basestations consumed ~1% of global electricity.〔 However many new communications systems from WiMAX to LTE do use amplitude information. The amplifier cannot be run into compression, because the amplitude information becomes distorted. These amplifiers can only achieve their peak efficiency on the peaks of the amplitude. The remainder of the time power is being dissipated unnecessarily.〔(Radio-Electronics.com Envelope Tracking Tutorial )〕 Thus signals with a high peak to average power ratio mean that low efficiency levels are achieved. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Envelope tracking」の詳細全文を読む スポンサード リンク
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