The invention relates to an amplifier for amplifying an input signal to generate an output current, comprising an impedance matching network for receiving the input signal to perform impedance matching; a first resistor for receiving the input signal to generate the matched signal; a first transistor having a channel thermal noise to establish a first noise voltage; a second resistor for receiving the channel thermal noise to establish a second noise voltage; and a transconductance circuit, comprising a first transconductance circuit which has a first transconductance coefficient and receivesthe first noise voltage to generate a first noise current; a second transconductance circuit which has a second transconductance coefficient and receives the second noise voltage to generate a secondnoise current such that the first and the second noise currents are counteracted with each other to reduce the noises in the output current. In contrast to the prior art, the invention can efficiently reduce the noises in the output current.