A line driver includes the following. A current replication unit replicates a reference current according to an input signal. A current mapping unit adjusts a number of P channel transistors connected in parallel and inside the current mapping unit according to control information, so as to amplify the reference current according to a magnification value. A first resistor is electrically connected between the current replication unit and a direct-current voltage. A second resistor is electrically connected to the current replication unit and the current mapping unit and generates an output signal. A signal detection unit performs integration on a part of a difference between the input signal and the output signal, thereby generating an integration signal. A magnification control unit gradually adjusts magnification information related to a reference signal, and updates the control information by the magnification information when the reference signal is equal to the integration signal.