the invention relates to a sense amplifier comprising the following element : a first current mirror unit coupled to a high voltage source , outputting a first current and a second current according to a first reference current , wherein the second current is twice the first current ; a second current mirror unit coupled to a high voltage source , outputting a third current according to a second reference current ; a first impedor coupled to the second current and a low voltage source ; a second impedor coupled to the third current and a low voltage source ; a third current mirror coupled to the first , second and third currents , and the first current is regarded as the reference current of the third current mirror unit , thus , the current which flows through the first impedor is the first current , and the current which flows through the second impedor is a fourth current .