Hartley Oscillator is a type of Harmonic Oscillator which was invented by Ralph Hartley in 1915. These are the Tuned Circuit Oscillators which are used to produce the waves in the range of radio frequency and hence are also referred to as RF Oscillators. Its frequency of oscillation is decided by its tank circuit which has a capacitor connected in parallel with the two serially connected inductors, as shown by Figure 1.
Here the RC is the collector resistor while the emitter resistor RE forms the stabilizing network. Further the resistors R1 and R2 form the voltage divider bias network for the transistor in common-emitter CE configuration. Next, the capacitors Ci and Co are the input and output decoupling capacitors while the emitter capacitor CE is the bypass capacitor used to bypass the amplified AC signals. All these components are identical to those present in the case of a common-emitter amplifier which is biased using a voltage divider network. However, Figure 1 also shows one more set of components viz., the inductors L1 and L2 and the capacitor C which form the tank circuit (shown in red enclosure).
On switching ON the power supply, the transistor starts to conduct, leading to an increase in the collector current, IC which charges the capacitor C. On acquiring the maximum charge feasible, C starts to discharge via the inductors L1 and L2. This charging and discharging cycles result in the damped oscillations in the tank circuit. The oscillation current in the tank circuit produces an AC voltage across the inductors L1 and L2 which are out of phase by 180o as their point of contact is grounded.
Further from the figure, it is evident that the output of the amplifier is applied across the inductor L1 while the feedback voltage drawn across L2 is applied to the base of the transistor. Thus one can conclude that the output of the amplifier is in-phase with the tank circuit’s voltage and supplies back the energy lost by it while the energy fed back to amplifier circuit will be out-of-phase by 180o. The feedback voltage which is already 180o out-of-phase with the transistor is provided by an additional 180o phase-shift due to the transistor action. Hence the signal which appears at the transistor's output will be amplified and will have a net phase-shift of 360o.
At this state, if one makes the gain of the circuit to be slightly greater than the feedback ratio given by