English: Diagram of an Armstrong oscillator circuit using a FET as the active device. The Armstrong oscillator, invented by Edwin Armstrong in 1913, was one of the earliest oscillator circuits. It is an LC oscillator, in which the frequency is determined by a tuned circuit consisting of the inductor L1 and capacitor C. In the Armstrong circuit, a little of the energy from the output of of the transistor, the feedback necessary for oscillation, is fed back into the input (gate) circuit by a small coil L2, called the "tickler coil", inductively coupled to the tank circuit coil L1