Abstract: A low noise amplifier with back-to-back connected diodes and a back-to-back connected diode with high impedance thereof are provided. The low noise amplifier includes a first operational amplifier (OP) and at least two first back-to-back connected diodes. The back-to-back connected diode with high impedance is formed from at least one MOS FET operated within a cut-off region. The first back-to-back connected diodes are connected electrically between the first input end and the first output end, and between the second input end and the second output end, of the first OP respectively. By the implementation of the present invention, the low noise amplifier is not only low noise, but also with low energy consumption, high stability, low circuitry complexity, and easily controlled manufacturing process.