An oscillator circuit comprising first and second resonator terminals for connecting to respective terminals of a resonator. The oscillator circuit also comprises a first inverting amplifier connected between the first and second resonator terminals in a first mode of operation; and a back to back pair of second inverting amplifiers connected between the first and second resonator terminals in a second mode of operation. There is also provided a controller configured to compare an operational parameter of the oscillator circuit to a switchover threshold, and switch the oscillator circuit from the first mode of operation to the second mode of operation when the operational parameter exceeds the switchover threshold.