Hartley Oscillator - Operation

Operation

A Hartley oscillator is essentially any configuration that uses positive feedback into two series-connected coils and a single capacitor forming a resonant LC tank circuit (see Colpitts oscillator for the equivalent oscillator using two capacitors and one coil). 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, 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.

It is made up of the following:

• Two inductors in series, which need not be mutual
• One tuning capacitor

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).

Disadvantages include:

• Harmonic-rich output if taken from the amplifier and not directly from the LC circuit (unless amplitude-stabilisation circuitry is employed).

Note that, if the inductance of the two partial coils and is given (e.g. in a simulator), the total effective inductance that determines the frequency of the oscillation is (coupling factor ):

(see )