This paper shows that CMOS Class-E PAs are capable of high Power-Added Efficiency (PAE), even when delivering large output powers at Radio Frequency (RF). In particular, a cascode device is used to obtain high efficiency while assuring reliable operation. A differential solution has been adopted to maximize 2^nd harmonic suppression and minimize potential on-chip interference. Prototypes realized in 0.13μm CMOS technology using thick oxide devices show the following performances: 31dBm maximum output power at 1.7GHz with 67% drain efficiency and 58% PAE, -51dBc and -39.5 Bc suppression for 2^nd and 3^rd harmonics, respectively.