How does hartley oscillator work

This gives rise to an electric field across the capacitor plates in the direction shown. When the key K is closed, the capacitor discharges itself through inductor and there is a flow of electrons as indicated by arrow head.

The current flow sets up a magnetic field around the inductor coil. Due to inductive effect the current builds up slowly up to a maximum value which is attained when the capacitor is fully discharged.

At that instant the electrostatic energy is zero but due to maximum current flow the magnetic field energy around the inductor coil is at maximum. As the capacitor is fully discharged the magnetic field starts decreasing.

As the magnetic flux linked with circuit changes, an e. This current recharges the capacitor in opposite direction with its upper plate negative and lower plate positive. Finally the magnetic field fully collapses. At this stage the magnetic field energy is zero and electrostatic energy is again at maximum. As soon as the magnetic field is zero, the capacitor which is fully recharged begins to discharge, due to which the current flows in opposite direction and a magnetic field is again set up around the inductor coil.

The magnetic field energy becomes a maximum when electric field energy is zero. Hartley Oscillator is a device that generates oscillatory output sinusoidal. It consists of an amplifier linked to an oscillatory circuit, also called LC circuit or tank circuit. The function of tank circuit is to tune a certain frequency. LC oscillators are designed to operate in the radio-frequency range.

Its inductance will be in micro Henries. However they can also be designed to produce oscillations in the low audio-frequency range. This ensures that the fraction of the output signal fed back from the tuned circuit collector load to the emitter via the capacitor C2 provides the necessary positive feedback. C2 also forms a long time constant with the emitter resistor R3 to provide an average DC voltage level proportional to the amplitude of the feedback signal at the emitter of Tr1. This is used to automatically control the gain of the amplifier to give the necessary closed loop gain of 1.

The emitter resistor R3 is not decoupled because the emitter terminal is used as the amplifier input. The base being connected to ground via C1, which will have a very low reactance at the oscillator frequency. Its operation is supposed to be rather like a water tank or cistern that can supply a continuous flow of water from an intermittently flowing external supply.

The tank circuit in the oscillator contains high values of circulating current topped up regularly by smaller amounts of current from the amplifier. Because most of the current flowing in the oscillator is flowing just around the resonant tank circuit rather than though the amplifier section of the oscillator, LC oscillators generally produce a sine wave with very little amplifier sourced distortion.

Another feature of the tank circuit is to provide the correct amount of positive feedback to keep the oscillator running. This is done by dividing the inductive branch of the circuit into two sections, each having a different value, the inductor therefore works in a similar manner to an autotransformer, the ratio of the two windings providing the appropriate amount of signal to be fed back to the input of the amplifier.

Because in Fig. Therefore waveform X across L1, and waveform Y across the whole circuit are in phase. As a common base amplifier is being used, the collector and emitter signals are also in phase, and the tank circuit is therefore providing positive feedback.

In other Hartley designs, using common emitter amplifiers for example, similar tank circuits are used but with different connections, so that the feedback signal is always in phase with the input signal, therefore providing the necessary positive feedback. It is common in LC sine wave oscillators to use automatic class C bias.

In class C the bias voltage, that is the base voltage of the transistor is more negative than the emitter voltage, making V be negative so that the average centre voltage of the input wave is located on the negative portion of the V be axis of the characteristic curve shown in Fig. Because only the tips of the waveform are amplified in class C, the amplifier cannot produce an undistorted output wave. This does not matter however, in an LC oscillator.

All the amplifier is required to do is to provide pulses of current to the LC resonant circuit at its resonating frequency. The sine wave output of the circuit is actually produced by the resonating action of the LC circuit. The amplitude of the signal produced will depend on the amount of current flowing in the LC tuned circuit. As the amplifier provides this current it follows that, by automatically controlling the amount of collector current I c flowing each time a pulse is produced, the amplitude of the output sine wave can be controlled at a constant level.

Thus the transistor provides amplification along with inversion to amplify and correct the signal generated by the tank circuit. The mutual inductance between L1 and L2 provides the feedback of energy from collector-emitter circuit to the base-emitter circuit. Where Leq is the total inductance of coils in the tank circuit is given as. The Hartley oscillator can be implemented by using an operational amplifier and its typical arrangement is shown in the below figure.

This type of circuit facilitates the gain adjustment by using feedback resistance and input resistance. In transistorized Hartley oscillator, the gain depending up on the tank circuit elements like L1 and L2 whereas in Op-amp oscillator gain is less depends on the tank circuit elements and hence provides greater frequency stability. The operation of this circuit is similar to the transistor version of the Hartley oscillator. Then this wave is stabilized and inverted by the amplifier.

The frequency of an oscillator is varied by using a variable capacitor in the tank circuit, keeping the feedback ratio and the amplitude of the output is constant for over a frequency range. The frequency of oscillations for this type of oscillator is the same as the above-discussed oscillator and is given as. To generate the oscillation from this circuit, the amplifier gain must and should be selected greater than or at least equal to the ratio of two inductances.

If the mutual inductance exists between L1 and L2 because the common core of these two coils, then the gain becomes.