1. A phase shift detector comprising:a comparator including:
a first-stage circuit with a first input end and a second input end,
a second-stage circuit electrically connected to the first-stage circuit and including a first-controlled and a second-controlled transistors and outputting a first and a second output voltages, and
a third-stage circuit electrically connected to the second-stage circuit and which outputs a reference output voltage; and
an offset calibration circuit electrically connected to the comparator and receiving the first output voltage, the second output voltage and the reference output voltage, wherein the offset calibration circuit outputs a first bulk voltage and a second bulk voltage to the first-controlled transistor and the second-controlled transistor respectively according to the first output voltage, the second output voltage and the reference output voltage for controlling a threshold voltage of the first-controlled transistor or the second-controlled transistor and wherein the first-controlled and second-controlled transistors are the only transistors in the comparator that are controlled by the offset calibration circuit.