Basic Features of a Hartley Oscillator

The primary LC oscillator circuit does not have the option of controlling the amplitude of the oscillations. A weak electromagnetic coupling between first inductive coil L and second inductive coil L2 results in insufficient feedback and the oscillations would eventually die away to zero. Equally, a strong feedback causes the oscillations to increase in amplitude until they are limited by the circuit conditions producing distortion.

If the feedback is more than what is required, the amplitude of the oscillations can be controlled by biasing the amplifier in such a way that if the oscillations increase in amplitude, the bias is increased and the gain of the amplifier is reduced. If the amplitude of the oscillations decreases, the bias decreases and the gain of the amplifier increase, thus, increasing the feedback. This is the technique employed to keep oscillations constant and it is called automatic base bias. Efficient oscillator can be implemented by using a class-B or even class-C bias as the collector current flows during only part of the cycle and the quiescent collector current is very small. This self-tuning base oscillator circuit forms the basic configuration for the Hartley oscillator circuit.

In the Hartley oscillator, the tuned LC circuit is connected between the collector and the base of the transistor amplifier and as far as the oscillatory voltage is concerned, the emitter is connected to a tapping on the tuned circuit coil. A Hartley oscillator can be implemented from any configuration that uses either a single-tapped coil or a pair of series-connected coils in parallel with a single capacitor.

A Basic Hartley oscillator circuit

Let’s consider a basic Hartley oscillator circuit shown below:

During oscillation, the voltage at the point X (collector), relative to the point Y (emitter), is 180° out of phase with the voltage at the point Z (base) relative to point to the point Y. At the frequency of oscillation, the impedance of the collector load is resistive and an increase in base voltage causes a decrease in the collector voltage. Since there is a 180° phase change in the voltage between the base and collector and this along with the original 180° phase shift in the feedback loop provides the correct phase relationship of positive feedback for oscillations to be maintained. The position of tapping point of the inductor sets the amount of feedback. If it is moved nearer to the collector, the amount of feedback is increased, but the output taken between collector and ground is reduced and vice versa. Resistors R₁ and R₂ are used for stabilising the dc bias for the transistor while the capacitors act as dc blocking capacitors.

In this circuit, the dc collector current flows through a part of the coil and for this reason, the circuit is said to be series-fed with the frequency of oscillation of the Hartley oscillator as given as:

Where L = L₁ + L₂

Note that L is the total inductance if two separate coils are employed.

The frequency of oscillations can be adjusted by varying the tuning capacitor C or by varying the tap position by the inductive coil, giving an output over a wide range of frequencies making it very easy to tune.

It is also possible to connect the tuned tank circuit across the amplifier as a shunt fed oscillator as illustrated below:

In this type of Hartley oscillator circuit, both ac and dc components of the collector current have separate paths around the circuit. Because the dc component is blocked by the capacitor C₂, no dc component flows through the inductive coil L.  The radio frequency coil (RFC) L₂ is an RF choke which offers a high reactance at the frequency of oscillations so that most the RF current is applied to the LC tuning tank circuit via the capacitor, C₂ as the dc component passes through L₂ to the power supply. A resistor could be used in place of the RFC coil but the frequency will be less.

Related: Basic Features of Crystal Oscillators