We present a digitally-controlled class-D power amplifier (PA) utilizing a new conduction angle calibration technique for harmonic suppression. The optimum conduction angle is derived to minimize chosen harmonics. The calibration circuit adjusts the conduction angle by changing the voltage transfer characteristic of the input buffer. Operating at 2.4GHz, this single-ended PA achieves a drain efficiency of 39% with an output power of 1.2dBm from a 1V supply. The even-order harmonics can be suppressed to below -49dBm. Integrated with an RF frequency modulator, the PA fully meets the spectral mask requirements for Bluetooth Smart applications.