with the capacitor-coil resonance, all frequencies other than the tuned frequency will tend to be absorbed (the tank will appear as nearly 0Ω near DC due to the inductor's low reactance at low frequencies, and low again at very high frequencies due to the capacitor); they will also shift the phase of the feedback from the 0° needed for oscillation at all but the tuned frequency.
Variations on the simple circuit often include ways to automatically reduce the amplifier gain to maintain a constant output voltage at a level below overload; the simple circuit above will limit the output voltage due to the gate conducting on positive peaks, effectively damping oscillations but not before significant distortion (spuriousharmonics) may result. Changing the tapped coil to two separate coils, as in the original patent schematic, still results in a working oscillator but now that the two coils are not magnetically coupled the inductance, and so frequency, calculation has to be modified (see below), and the explanation of the voltage increase mechanism is more complicated than the autotransformer scenario.
A quite different implementation using a tapped coil in an LC tank feedback arrangement, still called a Hartley oscillator (or sometimes "the" Hartley Oscillator circuit[2]) is to employ a common-grid (or common-gate or common-base) amplifier stage, which is still non-inverting but provides voltage gain instead of current gain; the coil tapping is still connected to the cathode (or source or emitter), but this is now the (low impedance) input to the amplifier; the split tank circuit is now dropping the impedance from the relatively high output impedance of the plate (or drain or collector).
Comparison of Hartley and Colpitts oscillator
The Hartley oscillator is the dual of the Colpitts oscillator which uses a voltage divider made of two capacitors rather than two inductors. Although there is no requirement for there to be mutual coupling between the two coil segments, the circuit is usually implemented using a tapped coil, with the feedback taken from the tap, as shown here. The optimal tapping point (or ratio of coil inductances) depends on the amplifying device used, which may be a bipolar junction transistor, FET, triode, or amplifier of almost any type (non-inverting in this case, although variations of the circuit with an earthed centre-point and feedback from an inverting amplifier or the collector/drain of a transistor are also common), but a junction FET (shown) or triode is often employed as a good degree of amplitude stability (and thus distortion reduction) can be achieved with a simple grid leak resistor-capacitor combination in series with the gate or grid (see the Scott circuit below) thanks to diode conduction on signal peaks building up enough negative bias to limit amplification.
Easy to create an accurate fixed-frequency crystal oscillator variation by replacing the capacitor with a (parallel-resonant) quartz crystal or replacing the top half of the tank circuit with a crystal and grid-leak resistor (as in the Tri-tet oscillator).
Record, F. A.; Stiles, J. L., An Analytical Demonstration of Hartley Oscillator Action, Proceedings of the IRE, June 1943, 31 (6), ISSN 0096-8390, doi:10.1109/jrproc.1943.230656
Rohde, Ulrich L.; Poddar, Ajay K.; Böck, Georg, The Design of Modern Microwave Oscillators for Wireless Applications: Theory and Optimization, New York, NY: John Wiley & Sons, May 2005, ISBN 0-471-72342-8
Vendelin, George; Pavio, Anthony M.; Rohde, Ulrich L., Microwave Circuit Design Using Linear and Nonlinear Techniques, New York, NY: John Wiley & Sons, May 2005, ISBN 0-471-41479-4
