Hartley oscillator

[1] In 1946 Hartley was awarded the Institute of Radio Engineers Medal of Honor "for his early work on oscillating circuits employing triode tubes" and for his work in information theory (which largely paralleled Harry Nyquist) about "the fundamental relationship between the total amount of information which may be transmitted over a transmission system of limited band-width and the time required.

The original 1915 version used a triode as the amplifying device in common cathode configuration, with three batteries, and separate adjustable coils.

The circuit illustrates the Hartley oscillator operation:[dubious – discuss] 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 (spurious harmonics) may result.

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.

The Hartley oscillator has several advantages: The output is harmonic-rich if taken from the amplifier and not directly from the LC circuit (unless amplitude-stabilisation circuitry is employed).