Hartley Oscitor

The Hartley Oscillator uses two inductive coils in a series of parallel capacitors to create the resonance tank circuit that produces sinusoidal oscillations.

hartley oscillator

One of the main drawbacks of the basic LC Oscillator circuit, which we examined in the previous lesson, is that there are no means to control the amplitude of oscillations, and also that it is difficult to adjust the oscillator to the required frequency. If the cumulative electromagnetic link between L1 and L2 is too small, there is insufficient feedback and the oscillations eventually drop to zero.

Similarly, if feedback was too strong, oscillations would continue to increase in amplitude until they are limited by circuit conditions that produce signal distortion. Therefore, it becomes very difficult to "adjust" the oscilator.

If the amplitude of oscillations decreases, the polarity voltage decreases and the gain of the amplifier increases, thereby increasing feedback. In this way, the amplitude of oscillations is kept constant using a process known as Automatic Base Polarity.

It has the advantage that the collector current flows only during part of the oscillation cycle, so the motionless collector current is very small. Then this "self-adjusting" basic oscillator circuit forms one of the most common LC parallel resonance feedback oscillator configurations called Hartley Oscillators.

hartley oscillator
Hartley Oscitor Tank Circuit

In the Hartley Oscillator, the tuned LC circuit is connected between the collector and the base of a transistor amplifier. In the case of oscillation voltage, the emitter is connected to a stage point on the adjusted circuit coil.

The feedback part of the adjusted LC tank circuit is taken from the center point of the inductor coil or even from two separate coils in series parallel to a variable capacitor C, as shown.

The Hartley circuit is often called a split induced inducing oscilator, since the coil L is connected to the center. In reality, the inductacus L acts like two separate coils very close with the current flowing along the coil section XY, the following coil section induces a signal to AI.

The Hartley Oscillator circuit can be made from any configuration that uses either a single-stage coil (similar to an autotransformer) or a pair of coils connected to a parallel series with a single capacitor, as shown below.

hartley oscillator
Basic Hartley Oscitor Design

When the circuit oscillates, the voltage at point X (collector), according to the Y point (transmitter), the voltage at point Z (base) is 180 o-phases compared to the Y point. In the oscillation frequency, the impedance of the collector load is resistant, and an increase in base voltage leads to a decrease in collector voltage.

Therefore, there is a phase change of 180 o inthevoltage between the Base and collector, which, together with the original phase shift of 180o inthe feedback loop, ensures the correct phase relationship of positive feedback for the protection of oscillations.

The amount of feedback depends on the location of the "guide point" of the inductor. If this is brought closer to the collector, the amount of feedback increases, but the output between the Collector and the soil decreases, and vice versa. Resistors, R1 and R2,provide dc polarization, which normally stabilizes for the transistor, while capacitors act as DC blocking capacitors.

In this Hartley Oscillator circuit, the DC collector current passes through part of the coil, and therefore the circuit is said to be "Serial-fed" and is given as the oscillation frequency of the Hartley Oscillator.

hartley oscillator

Note: LTis a cumulatively combined inductive if two separate coils are used, including mutual inducings M.

The frequency of oscillations can be adjusted by changing the "adjustment" capacitor C or changing the position of the core inside the coil (inductive adjustment), which gives an output in a wide frequency range that makes adjustment very easy. In addition, the Hartley Oscilator produces an output amplitude that is constant in the entire frequency range.

In addition to the Series-fed Hartley Oscillator above, it is also possible to connect the tuned tank circuit along the amplifier as a shunt-fed oscillator, as shown below.

hartley oscillator
Hartley Oscillator circuit with shunt feed

In the shunt-fed Hartley oscillator circuit, both ac and DC components of the collector current have separate paths around the circuit. Since the DC component is blocked by the capacitor, dc does not flow from the C2 inductive coil, less power is wasted in the L and tuned circuit.

Radio Frequency Coil (RFC), L2, is an RF coil with high reactance at the oscillation frequency, so that most of the RF current is applied to the LC adjustment tank circuit via the capacitor, C2, as the DC component passes through L2. A resistance can be used instead of rfc coil L2, but the efficiency is less.

Hartley Oscitor Question Example 1

A Hartley Oscillator circuit with two separate inductors of 0.5 mH each is designed to resonate in parallel with a variable capacitor that can be adjusted between 100pF and 500pF. Calculate oscillation upper and lower frequencies, as well as bandwidth of Hartley oscillators.

We can use this formula to calculate the frequencies of oscillations:

hartley oscillator

The circuit consists of two series-connected inductive coils, so the total inducing is given as follows:

hartley oscillator

Top Frequency

hartley oscillator

Sub-Frequency

hartley oscillator

Bandwidth

503 – 225 = 278kHz

Hartley Oscitor Using OPAMP

In addition to using bipolar connection transistor (BJT) as the active stage of the Hartley oscillator, we can also use an area-effective transistor(FET)or a transactional amplifier (op-amp). The operation of the Hartley Oscillator using op-amp is exactly the same as the transistor version with the operating frequency calculated in the same way.

hartley oscillator

The advantage of creating a Hartley Oscillator using a transactional amplifier as its active stage is that the op-amp gain can be adjusted very easily using R1 and R2 feedback resistors. As with the transistor oscillator above, the gain of the circuit should be much or slightly larger than the L1/L2 ratio. If the two inductive coils are wrapped in a common core and there is mutual inductive M, the ratio is (L1+M)/(L2+M).

Summarize

To summarize, the Hartley Oscillator consists of a parallel LC resonator tank circuit, whose feedback is obtained through the inductive divider. Like most oscillator circuits, the Hartley oscillator is found in various forms, and the most common form is the transistor circuit above.

This Hartley Oscillator configuration has a tank circuit set depending on the resonance coil to feed part of the output signal back to the transistor emitter. Since the output of the transistor emitter is always in the "same phase" as the output in the collector, this feedback signal is positive. The oscillation frequency, which is the sine wave voltage, is determined by the resonance frequency of the tank circuit.

In the next lesson about oscillators, we will look at another type of LC oscillator circuit, which is the opposite of the Hartley oscillator called the Colpitts Oscillator. The Colpitts oscitor uses two capacitors in series to create a center-stage capacitance in parallel with a single inductance within the resonance tank circuit.