This paper presents a low-power rail-to-rail amplifier with a paralleled folded-cascold structure input stage and a feed forward class AB output stage.the switch in the bias circuit is switching off to hoost the driving capability when the input single is low,or else,the amplifier works on the switching on state to save power.simulations using SMIC 0.35um process cooled and cadence specre at 33V supply voltage,the amplifier performs almost 70dB dc gain,the CMRR reaches 140dB and the lowest static power consumption is just 0.6mW.this amplifier is used in the PGA of hige precision sigma-delta ADC as a buffer to meet the input sigle range of 2.5V when the gain of the PGA is 1 or 2.