This paper presents a 40 GHz varactor-less class-C VCO with long-term reliability. The class-C mode is achieved by a transformer magnetic feedback loop and a control circuit that lowers the bias gate voltage of the VCO's cross-pair transistors. The tail current source is removed from the VCO, ensuring a high voltage swing in addition to the high efficiency of class-C VCO. The design benefits from the body-biasing offered by the FD-SOI technology to fine-tune the frequency with the parasitic capacitance of the transistors. Thus, low-quality factor varactors at mm-wave frequencies are avoided. The capacitor bank is placed inside the transformer to reduce the parasitic inductance due to lead wires and the silicon footprint. The circuit reliability is improved by using a cascode structure in the active device to mitigate the hot-carriers generation. A capacitive feedback loop added to the active device boosts the transconductance. The VCO was implemented in 28 nm FD-SOI technology and covers the frequency between 35.58 GHz and 42.36 GHz (tuning range of 17.1%). The measured phase noise is −88.33 dBc/Hz at 1 MHz offset from a 35.58 GHz carrier with a power consumption of 7.3 mW under 1 V supply voltage. The resulting FoM and FoMT are 170.72 dBc/Hz and 175.38 dBc/Hz, respectively.Read less <