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The Hartley Oscillator

In 1915, scientist and researcher Ralph V. L. Hartley invented and patented his design for a radio frequency oscillator. The circuit shown to the right is the modern incarnation of his design, which we still know as the Hartley oscillator.

The original Hartley oscillator used a vacuum tube — the only kind of amplifying device known at that time. Since the development of semiconductors, this design has been adapted for bipolar transistors and for FETs as shown here.

The Hartley oscillator is distinguished by the fact that it gets its feedback from a tap on the coil in the tuned circuit, as shown in the figure. The placement of the tap on the coil determines the amount of energy fed back to sustain oscillations. The output signal can be taken directly from the entire tuned circuit, from a tap on the coil, or from a secondary winding as shown here. In each case, the output signal will be a good quality sine wave. The output can also be taken from the drain (or collector) of the transistor, allowing the semiconductor to amplify the signal. However, in that case the waveform is distorted, containing significant harmonic energy.

When power is first applied to this circuit, some current will flow through Q. Since the source is grounded through a portion of coil L, source current will flow through that part of L, causing this inductor to induce a voltage across itself. This induced voltage will spread across all of L, in accordance with the self-inductance of this coil.

Because of capacitor C, however, the voltage across the entire length of L cannot change instantly. Instead, the energy inserted into L by the transistor starts to transfer to C. This begins the resonant "flywheel" effect in the LC pair, at the resonant frequency of the LC pair. This frequency is ω = 2πf = 1/.

The Hartley oscillator has several advantages. It can be designed to operate over a wide range of frequencies. The high frequency limit is determined primarily by the cutoff frequency of the transistor. At lower frequencies the coil and capacitor values get larger, and these components get more expensive. It is possible to design a Hartley oscillator to operate at audio frequencies, but it isn't cost effective.

Another advantage is that we can easily make the operating frequency variable, simply by using a variable capacitor. When this is done, the output signal remains at essentially the same amplitude regardless of the setting of the capacitor.

The coil need not be a single, tapped coil, although that is often the most practical. It is perfectly workable to use two different coils, which need not even be magnetically coupled together. In that case, frequency is determined by the total series inductance including any mutual inductance.

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