A FM receiver comprises an IF filter, a demodulator and a decoder. The IF filter generates an RSSI and a FM modulated signal in response to a FM signal. The demodulator comprises a duty-to-voltage amplifier for amplifying a peak of a MPX signal. The duty-to-voltage amplifier comprises a current source, a switch and a current splitter. The current source generates a current. The switch controls a flow of the first current in response to a PWM signal. The current splitter splits the flow of the current into a sub-flow in response to the RSSI. The peak of the MPX signal is proportional to the sub-flow. The decoder receives the MPX signal to generate an audio signal to play sound.