Hartley oscillator
All LC oscillators, whether Meissner oscillator, Colpitts oscillator or Hartley oscillator, work with a frequency-determining oscillating circuit from which a signal part is fed back and maintains the oscillating state of the oscillating circuit.
In the Hartley oscillator, the inductance of the resonant circuit is tapped and the tapped frequency signal is fed back to the base or emitter of the transistor, depending on the circuit design. If the feedback signal is applied to the base, then it must undergo a phase rotation of 180° in the feedback path, because it is again rotated by 180° in the transistor. If, on the other hand, the signal is applied to the emitter, then there is no phase rotation. The feedback signal portion results from the winding ratio between the number of windings to the tap and the total number of windings of the coil.