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Hartley oscillator From Wikipedia, the free encyclopedia Jump to: navigation, search

The Hartley oscillator is an electronic oscillator circuit in which the oscillation frequency is determined by a tuned circuit consisting of capacitors and inductors, that is, an LC oscillator. The circuit was invented in 1915 by American engineer Ralph Hartley. The distinguishing feature of the Hartley oscillator is that the tuned circuit consists of a single capacitor in parallel with two inductors in series (or a single tapped inductor), and the feedback signal needed for oscillation is taken from the center connection of the two inductors.

Contents

1 Operation 2 History 3 See also 4 References 5 External links

Operation Common-collector Hartley circuit

The Hartley oscillator is distinguished by a tank circuit consisting of two series-connected coils in parallel with a capacitor, with the feedback signal needed for oscillation taken from the center connection between the coils; the coils act as a voltage divider. 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. Op-amp version of Hartley oscillator

The frequency of oscillation is approximately the resonant frequency of the tank circuit. If the capacitance of the tank capacitor is C and the total inductance of the tapped coil is L then

f = {1 \over 2 \pi \sqrt {LC}} \,

If two uncoupled coils of inductance L1 and L2 are used then

L = L_1 + L_2 \,

However if the two coils are magnetically coupled the total inductance will be greater because of mutual inductance k[1]

L = L_1 + L_2 + k*\sqrt{L_1*L_2} \,

The actual oscillation frequency will be slightly lower than given above, because of parasitic capacitance in the coil and loading by the transistor.

Advantages of the Hartley oscillator include:

The frequency may be adjusted using a single variable capacitor, one side of which can be earthed The output amplitude remains constant over the frequency range Either a tapped coil or two fixed inductors are needed, and very few other components 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). Italic text