This article discusses the radio system requirements and design trade-offs of a low-power 2.3/2.4 GHz IEEE 802.15.6-narrowband transmitter for body area network applications. The IEEE 802.15.6 narrowband PHY adopts nonconstant envelope DPSK modulations. Transmitter architecture selection is a major challenge because it needs to provide both amplitude and phase modulation, with good efficiency and strict requirements on modulation accuracy and linearity for non-constant envelope modulations. A PLL-based polar transmitter with a Sigma-Delta digitally controlled power amplifier (SD-DPA) is employed for its excellent energy efficiency. Finally, this article discusses the circuit design considerations and implementation results. With balance between analog and digital implementation, and careful planning of the radio system, the polar transmitter presented in this article can meet all of the specifications defined in IEEE 802.15.6 with abundant margin, while having extremely low power consumption of 5.4 mW and excellent energy efficiency of 5.4 nJ/bit.