We present a novel fully differential input/output distributed transformer topology used for the design of millimeter-wave power amplifiers. Input/output distributed transformers are used to feed the input signal to four differential couples and to combine their output power. This topology improves the stability and the efficiency of the power amplifier, minimizing the chip area. The PA prototype realized in a standard 65nm CMOS technology, supplied with 1.2V, achieves a 1dB single-ended output power compression point of 14dBm at 56GHz with a PAE of 8.3%, occupying only 0.2mm².