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Efficiency optimization of envelope amplifier based on slow envelope technique

Efficiency optimization of envelope amplifier based on slow envelope technique . P. Cheng.

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Efficiency optimization of envelope amplifier based on slow envelope technique

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  1. Efficiency optimization of envelope amplifier based on slow envelope technique P. Cheng High frequency dc-dc switching converters are used as envelope amplifiers in RF transmitters. The dc-dc converter should operate at very high frequency to track an envelope in the MHz range to supply the power amplifier. One of the circuits suitable for this application is a hybrid topology composed of a switched converter and a linear regulator in series that work together to adjust the output voltage to track the envelope with accuracy. This topology can take advantage of the reduced slew-rate technique where switching dc-dc converter provides the RF envelope with limited slew rate in order to avoid high switching frequency and high power losses, while the linear regulator performs fine adjustment in order to obtain the exact replica of the RF envelope. Project sponsored by Hybrid series converterwithslow-envelopetechnique Hybrid series converter Background Theschematic of hybrid series converter EER technique RF poweramplifierdominatesoperateor’senergybudget Specifications: VIN=12V, Rload=10 ohm, Envelope bandwidth = 2MHz, Average output power = 3.5W, peak output power = 14.4W Envelopamplifier = Theswitchingbuckconverter+ linear regulator Slow envelope technique FFT of slow envelopes VSW • Theslowenvelopecorrespondstolowerslewratethanthe RF signalenvelope. • Theamplitude of theslow-envelope has tobeinstantaneouslyhigherthanthe RF signalenvelope. • Thelowerslewratecorrespondstolowerswitchingfrequency of converter in ordertogeneratetheslowenvelope. Time Relationship between slew rate and bandwidth of the generated envelopes Simulation and experimental results CONCLUSIONS The efficiency of the hybrid series envelope amplifier can be improved by applying the technique of the reducing the slew-rate of the envelope signal. Depending on the reduction applied to the slew rate of the original envelope signal, the required switching frequency for the switched converter is reduced but the power losses of the linear are increased. A trade-off has been found and an optimum slew-rate has been identified.

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