Signal amplifier by using a doherty amplifier

PURPOSE: A signal amplifying apparatus is provided to reduce the size of the apparatus by controlling the characteristic impedance of Doherty output network. CONSTITUTION: A signal amplifying apparatus comprises a splitter(100) for splitting an input signal into first and second paths; a first bias control network(300) and a second bias control network(310) for generating bias signals by varying operating modes in accordance with the power level of the input signal; a carrier amplifier(210) disposed on the first path, and which reduces idle current consumption at a low power level in accordance with the bias signal output from the first bias control network, and operates as an AB-level amplifier at a high power level; a peaking amplifier(220) disposed on the second path, and which is turned off at a low power level in accordance with the bias signal output from the second bias control network, and operates as an AB-level amplifier at a high power level; and a Doherty output network(400) for matching the signals amplified by the carrier amplifier and the peaking amplifier, and outputting the matched signals.