This paper analyses the pure- and mixed-mode operation of class-B outphasing amplifiers and provides the analytical expressions for the drive profiles, when assuming ideal devices. In pure-mode operation, output power is controlled by phase modulation only. In mixed-mode operation, amplitude control replaces phase control at the deep back-off power levels to save driving power and hence improve efficiency. Envelope simulations using a 10.6 dB PAR W-CDMA signal verify the analysis of an ideal class-B outphasing amplifier model with only 10 dB of transistor's power gain. In mixed-mode operation, they predict up to 65% of average total-efficiency which is 7%-points higher than an equivalent asymmetrical (1:3) Doherty amplifier.