Hartley oscillator

An electrical three-point oscillator circuit, two inductors and a capacitor form a parallel resonant circuit in which it is referred to as a Hartley or Hartley oscillator circuit. It is named after Ralph Hartley, who in 1920 received a patent for it.


In Hartley circuit two inductors are connected in series and connected in parallel with a capacitor. So that a parallel resonant circuit is formed which determines the oscillation frequency. Since three connection points are available, the Hartley circuit is one of the three-point circuits.

In the principle diagram of a junction field- effect transistor is used in the drain circuit, wherein the gate of the ( high resistance ) is input, which is connected to one end of the parallel resonant circuit. The output signal at the source is connected to the connection between the two coils. The amplifier acts as a voltage -to-current amplifier; an increase of the input voltage leads to an in-phase current rise in the lower coil, so that the resonant circuit energy is supplied.

A coil is used instead of tap with two non- coupled coils, the tapped coil is a transformer, so that the product of the voltage gain of the amplifier and up- transform factor must be greater than one in order to satisfy the oscillation condition.

A detailed analysis for tubes was in 1943 by FA Record and J. L. Stiles published in which the currents in the coils to be considered instead of stress.

Frequency of the vibration generated

The generated frequency fres is determined by the Thomson's resonant formula with the resultant inductance L0 of the series-connected coils L1 and L2 and the capacitance of the capacitor C:

Reduce the parasitic capacitances of the remaining components and the parasitic inductances of the compounds of this calculated frequency.

The total inductance of two coupled with the factor of individual inductances and is:


The Hartley circuit was developed by Ralph Hartley and filed under the number 1,356,763 on June 1, 1915 as patent. In the patent, the electron tube operates in cathode -base circuit, which corresponds to the FET common-source configuration. The inductance of the resonant circuit is divided into two coils, one in the grid circuit and the other in the anode circuit. The tank capacitor connects with grid anode. Exists between the two capacitor plates, a phase rotation of 180 °, as well as between the grid and anode.


For a tuning oscillator in superheterodyne receiver, the Hartley circuit is very appropriate, since only one coil with tap and a variable tuning capacitor is needed, which may be unilateral are grounded. For higher frequencies, the Clapp circuit is often advantageous, in which two capacitors connected in series are used.

If value is set to a low distortion sine wave as possible, must be coupled with high impedance at the resonant circuit; However, then the oscillation frequency is significantly affected by the input capacitance of the next stage. Alternatively, as shown in the patent, can be coupled inductively.


Bipolar transistor in common collector circuit

The resonant circuit consists of the two inductors L1 and L2 and capacitor C1. C2 includes the resonant circuit. The voltages at the two terminals of the capacitor are opposite in phase ( phase difference 180 °). The amplifier Q1 reverses the phase between input and output likewise 180 °, so that the results in the need for an oscillation phase shift of 360 °. Of the voltage divider R1, R2 is a base voltage. The amplitude control is carried out with R3. C3, the output of the oscillator is extracted.

The junction field effect transistor in common-source

The resonant circuit consists of the two inductors L1, primary winding of the transformer TR1 and the capacitor C1. The resonant circuit is closed by the battery. The voltages at both terminals of the capacitor C1 are in phase opposition ( phase shift 180 °). The amplifier Q1 reverses the phase between input and output likewise 180 °, so that there is required for an oscillation phase shift of 360 °.