Block diagram of a regenerative radio receiver, invented by Edwin Armstrong in 1912, is a  type of radio receiver widely used up until the 1930s..  It consists of a tuned circuit that serves as a bandpass filter to select the desired radio signal out of all the signals picked up by the antenna, and a combined amplifier-detector to increase the power of the signal and extract the audio modulation (sound) signal from from the radio carrier wave.  The gain of the amplifier is increased by regeneration (positive feedback, feeding some of the energy from the output of the amplifier back into the tuned circuit at the input.  Regeneration can not only increase the gain of the amplifier by a factor of 10,000, it also decreases the bandwidth of the filter by the same factor, increasing the selectivity of the receiver greatly. The resulting audio signal can be amplified in a separate audio amplifier before being applied to the speaker.
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