An amplifying circuit is provided. The amplifying circuit includes a bias circuit receiving an operating voltage from a power supply circuit and generating a first bias voltage, a resistance circuit connected between the bias circuit and a gate node and transferring the first bias voltage to the gate node, a start-up circuit generating a high-level start-up voltage and supplying the start-up voltage to the gate node before the operating voltage is supplied, based on a control signal, and an amplifier started-up by receiving the start-up voltage and then receiving the operating voltage and the first bias voltage to amplify a high frequency signal input through the gate node.