An amplifier applicable to an intra-band non-contiguous carrier aggregation (NCCA) band includes a first amplifier circuit and a second amplifier circuit. The NCCA band includes at least a primary component carrier (PCC) channel and a secondary component carrier (SCC) channel not adjacent to each other. The first amplifier circuit receives a first input signal, and generates a first output signal for undergoing down-conversion of one of the PCC channel and the SCC channel. The second amplifier circuit receives at least one second input signal, and generates a second output signal for undergoing down-conversion of another of the PCC channel and the SCC channel. The at least one second input signal received by the second amplifier circuit is provided by the first amplifier circuit according to the first input signal.