This paper presents a 77GHz power amplifier (PA) in 40nm CMOS technology for automotive radars. A symmetrical current-mode power combiner is adopted in the PA design to improve efficiency, extend bandwidth and maximize output power. The peak small signal gain of the PA is 16.4dB at 74GHz with a 3-dB bandwidth from 68 to 85GHz. The proposed PA achieves a saturated power (Psat) of 17dBm, Psat-1dB bandwidth of 30GHz from 70-100GHz, and 20% peak power-added efficiency (PAE).