A crystal oscillator circuit that can be controlled for fast start-up and for efficient operation is disclosed. The control includes adjusting a voltage applied to a body terminal of a transistor in order to control the amplification of the crystal oscillator. The amplification can be increased, relative to a motional resistance of the crystal oscillator, at start-up to reduce a start-up time necessary for oscillation. The amplification can also be decreased in order to maintain oscillation after start-up more efficiently. In some implementations, the transistor for control is a fully depleted silicon on insulator (FDSOI) transistor that accommodates a wide range of body bias voltages.