In one aspect this invention provides a DC-DC converter that has a switch mode part for coupling between a DC source and a load, the switch mode part providing x amount of output power; and that further has a linear mode part coupled in parallel with the switch mode part between the DC source and the load, the linear mode part providing y amount of output power, where x is preferably greater than y, and the ratio of x to y may be optimized for particular application constraints. In a further aspect there is a radio frequency (RF) transmitter (TX) for coupling to an antenna, where the TX has a polar architecture having an amplitude modulation (AM) path coupled to a power supply of a power amplifier (PA) and a phase modulation (PM) path coupled to an input of the PA, where the power supply includes the switch mode part for coupling between a battery and the PA and the linear mode part coupled in parallel with the switch mode part between the battery and the PA.