What is Hartley Oscillator : Circuit, Working and Its Applications

The Hartley oscillator is an electronic oscillator circuit in which the oscillation frequency is determined by the tuned circuit consisting of capacitors and inductors, that is, an LC oscillator. The Hartley oscillator was invented by Hartley while he was working in the Research Laboratory of the Western Electric Company. The circuit was invented in 1915 by American engineer Ralph Hartley. The personal feature of the Hartley oscillator is that the tuned circuit consists of a single capacitor in parallel with two inductors are in series or a single tapped inductor, and the feedback signal needed for oscillation is taken from the center connection of the two inductors.

What are Hartley Oscillators?

Hartley oscillator is inductively coupled, variable frequency oscillators where the oscillator may be a series or shunt fed. Hartley oscillators is the advantage of having one tuning capacitor and one center-tapped inductor. This processor simplifies the construction of a Hartley oscillator circuit.

Hartley Oscillator Circuit and Working

The circuit diagram of a Hartley oscillator is shown in the below figure. An NPN transistor connected in a common emitter configuration works as the active device in amplifier stage. R1 and R2 are biasing resistors and RFC is the radio frequency choke, which provides the isolation between AC and DC operation.

At high frequencies, the reactance value of this choke is very high, hence it can be treated as an open circuit. The reactance is zero for DC condition, hence causes no problem for DC capacitors. The CE is the emitter bypass capacitor and RE is also be a biasing resistor. The CC1 and CC2 are the coupling capacitors.

When the DC supply (Vcc) is given to the circuit, the collector current starts raising and begins with the charging of the capacitor C. Once capacitor C is fully charged, it starts discharging through L1 and L2 and again starts charging.

This back-and-fourth voltage waveform is a sine wave which is a small and leads with its negative alteration. It will eventually die out unless it is amplified.

Now the transistor comes into the picture. The sine wave generated by the tank circuit is coupled to the base of the transistor through the capacitor CC1.

Since the transistor is configured as common-emitter, it takes the input from tank circuit and inverts it to a standard sine wave with a leading positive alteration.

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.

The frequency of oscillations in this circuit is

fo = 1/ (2π √ (Leq C))

Where Leq is the total inductance of coils in the tank circuit is given as

Leq = L1 + L2 + 2M

For a practical circuit, if L1 = L2 = L and the mutual inductance are neglected, then the frequency of oscillations can be simplified as

fo = 1/ (2π √ (2 L C))

Hartley Oscillator Circuit Using Op-Amp

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. The sine wave is generated by the feedback circuit and it’s coupled with the op-amp section. 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

fo = 1/ (2π √ (Leq C))

Where: Leq = L1 + L2 + 2M
Or
Leq = L1 + L2

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.

Av = L1 / L2

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

Av = (L1 + M) / (L2 + M)

Advantages

• Instead of two separate coils L1 and L2, a single coil of bare wire can be used and the coil grounded at any desired point along with it.
• By using a variable capacitor or by making core movable (varying the inductance), the frequency of oscillations can be varied.
• Very few components are needed, including either two fixed inductors or a tapped coil.
• The amplitude of the output remains constant over the working frequency range.

Disadvantages

• It cannot be used as a low-frequency oscillator since the value of inductors becomes large and the size of the inductors becomes large.
• The harmonic content in the output of this oscillator is very high and hence it is not suitable for the applications which require a pure sine wave.

Applications

• The Hartley oscillator is to produce a sine wave with the desired frequency
• Hartley oscillators are mainly used as radio receivers. Also note that due to its wide range of frequencies, it is the most popular oscillator
• The Hartley oscillator is Suitable for oscillations in RF (Radio-Frequency) range, up to 30MHZ

Thus, this is all about the hartley oscillator circuit theory working and applications. We hope that you have got a better understanding of this concept. Furthermore, any doubts regarding this concept or electrical and electronics projects, please give your valuable suggestions by commenting in the comment section below. Here is a question for you, what is the main function of Hartley Oscillator?

Photo Credits:

• Hartley Oscillator Circuit calctown