A 6.78 MHz wireless power transfer (WPT) transmitter with two calibration schemes, adaptive dead-time control and transistor-width switching, is proposed for power supply of implantable medical devices (IMDs). The adaptive dead-time control scheme with on-time delay compensation for power transistors ensures zero voltage switching (ZVS) operation for the Class-D power amplifier (PA). The transistor-width switching scheme for power stage optimizations improves the transmitter efficiency over an extended loading range. To verify the effectiveness of the two calibration schemes, the transmitter is implemented in a 0.18 μm CMOS process. Simulation results show 15.9% and 3.6% improvement in the transmitter efficiency at the light load and the heavy load respectively. Under the output power range of 0.2 to 2 W, the transmitter efficiency maintains above 92%, and the peak efficiency of 93.2% is achieved when the output power is 0.7 W.