English: Block diagram of a tuned radio frequency (TRF) receiver, a type of radio receiver circuit invented in 1916 by Ernst Alexanderson and widely used in the vacuum tube receivers of the 1920s. It consisted of one or more tuned RF amplifiers, each consisting of a tuned circuit which functioned as a bandpass filter followed by an amplifier; a detector (demodulator) to extract the audio waveform from the radio carrier wave; followed by an audio amplifier.