A simple envelope detector circuit, used to demodulate amplitude modulated (AM) radio signals in AM radio receivers.  It consists of a semiconductor diode (D) with a bypass capacitor.  The diode allows current to pass in only one direction, rectifying the AC radio signal to a pulsing DC current, whose peak value is proportional to the modulation.  The capacitor C charges up to the peak value of the pulses, following the envelope of the modulation. It serves as a low pass filter, smoothing the pulsing current, removing the pulses of radio frequency carrier, leaving the audio frequency modulation.
Own work
Chetvorno