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ENHANCED FILTERING METHOD FOR ENVELOPE ELIMINATION AND RESTORATION AMPLIFICATION

IP.com Disclosure Number: IPCOM000009037D
Original Publication Date: 1999-Jan-01
Included in the Prior Art Database: 2002-Aug-02
Document File: 2 page(s) / 112K

Publishing Venue

Motorola

Related People

J. Staudinger: AUTHOR [+2]

Abstract

Many of the second generation cellular and wireless systems have adopted spectrally efficient digital modulation methods and as a consequence, the modulated rf signal envelope in many cases exhibits time varying amplitude characteristics. Due to the amplitude varying envelope, a linear (low distortion) high efficiency power amplifier is required in the transmitter unit in contrast to non- linear saturated ones common to analog FM sys- tems. To achieve the desired level of linearity, typi- cal single ended amplifiers (e.g, class E, F, . ..) must be operated in the gain backoff region where the achievable efficiency is much lower than desired. In general, an inherent trade-off exists between amplifier efficiency and spectral linearity. An alter- native amplifier topology which has the potential to achieve reasonable spectral linearity and simultane- ous high efficiency is the Envelope Elimination and Restoration (EER) class of amplifiers.

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M-LA Technical Developments

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ENHANCED FILTERING METHOD FOR ENVELOPE ELIMINATION AND RESTORATION AMPLIFICATION

by J. Staudinger and R. Sherman

BACKGROUND

  Many of the second generation cellular and wireless systems have adopted spectrally efficient digital modulation methods and as a consequence, the modulated rf signal envelope in many cases exhibits time varying amplitude characteristics. Due to the amplitude varying envelope, a linear (low distortion) high efficiency power amplifier is required in the transmitter unit in contrast to non- linear saturated ones common to analog FM sys- tems. To achieve the desired level of linearity, typi- cal single ended amplifiers (e.g, class E, F, . ..) must be operated in the gain backoff region where the achievable efficiency is much lower than desired. In general, an inherent trade-off exists between amplifier efficiency and spectral linearity. An alter- native amplifier topology which has the potential to achieve reasonable spectral linearity and simultane- ous high efficiency is the Envelope Elimination and Restoration (EER) class of amplifiers.

supply voltage operation. The modulated drain sig- nal is developed in the upper signal path where an envelope signal is first obtained via a detector. The envelope signal is efficiently amplified via a class S modulator, passed through a low pass filter and applied to the amplifiers drain terminal. In addition, Figure 1 illustrates a pole/zero compensation net- work and pre-distortion element in the upper signal path to further condition the envelope signal as required. Ideally, the amplifier functions as a time- domain voltage multiplier such that the signal at the output of the amplifier is the product of the enve- lope and the input rf signal (V,,t &t) = k V&t) *

Vin RF(t))

  Several practical aspects which must be addressed relate to the low pass filter - including fil- ter order, time delay introduced by the filter and limited filtering of the sampling signal - all of which have a direct impact on the performance of the EER system, particularly spectral linearity.

PROBLEM SOLUTION

  One form of an EER amplifier system using an open topology (i.e, w/o linear feedback) is shown in Figure 1. Briefly, the system functions where a sig- nal applied to the input of the amplifier is split into two separate ones, individually conditioned, and later recombined, with all signal conditioning and recombination performed very efficiently. Referring to...