Block diagram of a regenerative radio receiver, a type of radio receiver widely used before World War 2.  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 distinctive feature of the regenerative receiver is that the gain of the amplifier is increased by regeneration (positive feedback); the signal from the output of the amplifier is fed back into the bandpass filter by a feedback loop  This also decreases the bandwidth of the receiver.
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