English: Single vacuum tube Armstrong regenerative receiver circuit, invented in 1913 by American engineer Edwin Armstrong when he was a 22 year old college student. It was widely used until the 1930s when it was replaced by TRF and superheterodyne receivers. In the regenerative receiver, the gain of the tube is increased by feeding back some of the energy from the output plate circuit into the input grid circuit. In the Armstrong circuit this is done by the "tickler coil" (L3).