An apparatus includes: a first inverter configured to receive a first clock signal and output a second clock signal, wherein an input pin, an output pin, a power pin, and a ground pin of the first inverter connect to the first clock signal, the second clock signal, a first source node, and a second source node, respectively; a second inverter configured to receive the second clock signal and output a third clock signal, wherein an input pin, an output pin, a power pin, and a ground pin of the second inverter connect to the second clock signal, the third clock signal, the first source node, and the second source node, respectively; a first resistor connected to a first DC (direct-current) voltage to the first source node; and a second resistor connected to a second DC voltage to the second source node.

A low power crystal oscillator circuit has a high power part and a low power part. Crystal oscillation is initialized using the high power part. An automatic amplitude control circuit includes a current subtractor that decreases current in the high power part as an amplitude of the crystal oscillation increases. A current limiting circuit may limit current in the low power part in order to further reduce power consumption by the low power crystal oscillator circuit. Additionally, an automatic amplitude detection circuit may turn off the high power part after the amplitude of the crystal oscillation reaches a predetermined level in order to further reduce power consumption of the low power crystal oscillator circuit, and may turn back on the high power part after the amplitude of the crystal oscillation reaches a second predetermined level in order to maintain the crystal oscillation.