Embodiments disclosed herein relate to oscillators including methods of operating the same, for example for use in radio frequency circuits. In an embodiment, an oscillator has cross-coupled transistors connected between a resonant circuit and a tail circuit. The resonant circuit and tail circuit have respective supply connections for powering the oscillator with an external power supply and the cross-coupled transistors have a bias circuit coupled to respective gates of the cross-coupled transistors and arranged to bias said transistors in an active region of operation. The tail circuit has a current source, a tail capacitor and a tail resistor coupled between a common node of the cross-coupled transistors and the supply connection of the tail circuit.