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.
Cheng, P., Garcia, O., Vasic, M., Alou, P., Oliver, J., Montoro, G., Cobos, J. Envelope amplifier based on a hybrid series converter with the slow-envelope technique. A: IEEE Energy Conversion Congress and Exposition. "Energy Conversion Congress and Exposition (ECCE), 2012 IEEE". Raleigh: 2012, p. 1-5.