Frequency modulation (FM) can be used to transmit analog voltage level signal with good noise immunity. The transmission itself might employs different methods, and this circuit uses free air optical medium. This optical receiver circuit is built using a L14G2 detector, two stages of gain, and a FM demodulator. To obtain better sensitivity, we can use more stages of stabilized gain with AGC.
[Circuit’s schematic diagram source: seekic.com]