A crystal oscillator circuit comprising: a crystal oscillator; a tunable loading capacitance circuit, which is connected to the crystal oscillator such that the crystal oscillator and the tunable loading capacitance circuit define an impedance of the crystal oscillator circuit, wherein a real part of a negative resistance of the impedance of the crystal oscillator circuit is equal to or larger than a resistance of a resistor of the crystal oscillator in order to start-up and maintain oscillation; and an injection frequency generating circuit, separate from the tunable loading capacitance circuit, said injection frequency generating circuit being configured, during start-up of the crystal oscillator, to sense a signal of the crystal oscillator and amplify the sensed signal, the injection frequency generating circuit being further configured to inject the amplified signal to the crystal oscillator, wherein the injection frequency generating circuit comprises a two-stage operational amplifier circuit for sensing and amplifying the signal of the crystal oscillator wherein the injection frequency generating circuit is connected to pins (114, 116) on opposite sides of the crystal oscillator for sensing the signal and for injecting the amplified signal, wherein inputs of the two-stage operational amplifier circuit are connected to the pins (114, 116); wherein the crystal oscillator circuit is configured such that the crystal oscillator receives the amplified signal during an initial start-up period of the crystal oscillator and stops receiving the amplified signal at an end of the initial start-up period. a crystal oscillator; a tunable loading capacitance circuit, which is connected to the crystal oscillator such that the crystal oscillator and the tunable loading capacitance circuit define an impedance of the crystal oscillator circuit, wherein a real part of a negative resistance of the impedance of the crystal oscillator circuit is equal to or larger than a resistance of a resistor of the crystal oscillator in order to start-up and maintain oscillation; and an injection frequency generating circuit, separate from the tunable loading capacitance circuit, said injection frequency generating circuit being configured, during start-up of the crystal oscillator, to sense a signal of the crystal oscillator and amplify the sensed signal, the injection frequency generating circuit being further configured to inject the amplified signal to the crystal oscillator, wherein the injection frequency generating circuit comprises a two-stage operational amplifier circuit for sensing and amplifying the signal of the crystal oscillator wherein the injection frequency generating circuit is connected to pins (114, 116) on opposite sides of the crystal oscillator for sensing the signal and for injecting the amplified signal, wherein inputs of the two-stage operational amplifier circuit are connected to the pins (114, 116); wherein the crystal oscillator circuit is configured such that the crystal oscillator receives the amplified signal during an initial start-up period of the crystal oscillator and stops receiving the amplified signal at an end of the initial start-up period.