A low noise amplifier includes a preamplifier, first differential amplifiers, second differential amplifiers, a signal adder, and a load circuit. The preamplifier receives an input signal, and amplifies the input signal to generate a first signal. The input signal and the first signal have the same phase. The first differential amplifiers receive the first signal and a first reference signal and generate a first output differential signal pair. The second differential amplifiers receive the input signal and a second reference signal and generate a second output differential signal pair. The signal adder adds up the first output differential signal pair and the second output differential signal pair. The load circuit is coupled to the signal adder, and generates a third output differential signal pair according to the addition result.