A radio frequency (RF) transmitter uses two digital-to-RF conversion modules to convert digital baseband signals into RF signals. In cartesian mode, baseband signals are conveyed to the conversion modules for RF conversion. In polar mode, baseband signals are converted into amplitude and phase data parts. Phase data part is converted into I (inphase) and Q (quadrature) data parts to be converted into RF signals which are modulated in a power amplifier by amplitude data part through the power supply of the power amplifier. Each digital-to-RF module uses parallel unit cells to perform digital-to-analog (D/A) conversion and upconversion functions by an IF signal. Each unit cell, adapted to receive a control voltage indicative of a data signal value, is a mixer cell type converter having a differential data switch section connected in series to a differential LO (local oscillator)-switch pair. LO-switch is further connected in series to a current source.