We report a DC/DC converter based on GaN HEMTs with a switching frequency above 200 MHz that can be used to generate envelope-modulated power supply voltages for use in envelope tracking power amplifiers. The converter consists of switching circuits using 0.7-um GaN HEMTs, inductor, and low pass filter, and can provide output voltages above 20V. Generation of envelope power supply voltages for 20 MHz LTE signals was demonstrated using 200 MHz switching rates with efficiency of 64% (including dissipation in final and drivers stages; final stage efficiency was 90%). To the best of our knowledge, this is the first demonstration of high speed dc/dc converters for envelope amplifiers using GaN HEMTs.