A programmable gain amplifier includes an active load module, a first differential pair, a second differential pair and a power source module. The first and second differential pairs are electrically connected to the active load module. The power source module is electrically connected to the first current source end of the first differential pair and the second current source end of the second differential pair. The power source module supplies a first current to the first differential pair through the first current source end. The power source module supplies a second current to the second differential pair through the second current source end. The power source module adjusts the potential of the first current, the potential of the second current, or both.