Hartley circuit

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

Schematic diagram

functionality

In the Hartley circuit , two inductors are connected in series and connected in parallel to form a capacitor. This creates a parallel oscillating circuit that determines the oscillating frequency. Since three connection points are present, which belongs Hartley oscillator to the three-point circuits .

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

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

A detailed analysis for tubes was published by FA Record and JL Stiles as early as 1943, in which the currents in the coils are considered instead of voltages.

Frequency of the generated vibration

The generated frequency f _res is determined according to Thomson's resonance formula by the resulting inductance L _{0 of} the coils L ₁ and L ₂ connected in series and the capacitance of the capacitor C :

${\displaystyle f_{\mathrm {res} }={\frac {1}{2\pi {\sqrt {C\cdot L_{0}}}}}}$

The parasitic capacitances of the remaining components, the series resistance of the inductances and the parasitic inductances of the connections reduce this calculated frequency.

The total inductance of two by a factor coupled individual inductors and is: ${\displaystyle L_{0}}$ ${\displaystyle k}$ ${\displaystyle L_{1}}$ ${\displaystyle L_{2}}$

${\displaystyle L_{0}=L_{1}+L_{2}+k\cdot {\sqrt {L_{1}\cdot L_{2}}}}$

history

Patent drawing

The Hartley circuit was developed by Ralph Hartley and filed for a patent on June 1, 1915 in the United States under number 1,356,763. In the patent, the electron tube works in a cathode base circuit, this corresponds to the FET source circuit. The inductance of the resonant circuit is divided into two coils, one in the grid circle (3 in patent drawing) and the other in the anode circuit (4). The resonant circuit capacitor (2) connects the grid with the anode. There is a phase shift of 180 ° between the two capacitor plates , as well as between the grid and the anode.

use

The Hartley circuit is very suitable for a tuning oscillator in the superhet receiver, since only a coil with a tap and a variable tuning capacitor are required, which can be connected to ground on one side. For higher frequencies, the Clapp circuit is often more advantageous, in which two capacitors connected in series are used.

If value is placed on a sinusoidal signal with as little distortion as possible, it must be decoupled with high resistance on the resonant circuit; however, the oscillation frequency is then significantly influenced by the input capacitance of the subsequent stage. Alternatively, as shown in the patent, it can be coupled out inductively.

In the 1960s, Hartley oscillators were used in the Philicorda organs as the main frequency-determining oscillators, the oscillations of which were then converted into the required octaves by synchronized sawtooth generators.

Examples

Emitter bipolar transistor

Hartley oscillator with bipolar transistor in common emitter circuit

The resonant circuit of the Hartley oscillator with bipolar transistor in emitter circuit and consists of the two inductances L ₁ and L ₂ and the variable capacitor C ₁ . The voltages at the two connections of the capacitor have a phase shift of 180 °. The amplifier consists of transistor Q ₁ and also rotates the phase between input (base) and output (collector) by 180 °, so that the phase shift of 360 ° required for an oscillation results. L ₁ and L ₂ form an inductive voltage divider. The inductance of L ₁ and thus the high-frequency AC voltage across L ₁ is five times greater than that of L ₂ . The inductive voltage divider effects an impedance matching between the upper high-resistance resonant circuit connection with L ₁ and the lower low-resistance connection with L ₂ . The resistors R ₁ to R ₃ determine the operating point of Q ₁ . At the desired emitter current of 5 mA, a voltage of approximately 0.7 V should be present at the resistor R ₁ . The resistor R ₄ limits the base current and thus prevents the transistor from saturating . The amplitude is limited by R ₁ . Due to the component tolerances, it is often necessary to make R ₁ adjustable in order to achieve both goals, a reliable start of the oscillation and low harmonics .

The capacitors C ₂ and C ₃ are permeable to high-frequency alternating current, but not to direct current. The RC element R ₅ , C ₄ sifts the operating voltage. The load resistance R _L at the low-resistance end of the resonant circuit is not part of the oscillator circuit, but rather simulates the load on the oscillator through the following stages. In the simulation, the parallel resistance R _P reduces the quality factor of the resonant circuit to Q = 100 according to the formula:

${\displaystyle R_{P}={\frac {Q}{2\pi f\,C}}=2\pi f\,L\,Q}$

The resistor R _P is a replacement element for the resonant circuit losses; in a real circuit, R _{P is contained} in the components L ₁ , L ₂ and C ₁ . The values ​​of load resistance and quality factor are important for the dimensioning or the circuit simulation .

Gate-connected junction field effect transistor

Hartley oscillator with JFET in gate circuit

The resonant circuit of the Hartley oscillator with junction field effect transistor (JFET) in gate circuit according to consists of the two inductances L ₁ and L ₂ and the variable capacitor C ₁ . The transistor in the gate circuit has a phase shift of 0 °. The two resonant circuit inductors form an autotransformer or an inductive voltage divider. The total inductance of L ₁ and L ₂ and thus the high-frequency AC voltage at the lower connection of L ₁ is ten times greater than that at the lower connection of L ₂ . The inductive voltage divider effects an impedance matching between the low-resistance amplifier input (source) and the high-resistance amplifier output (drain). The quality factor of the parallel resonant _circuit at resonance frequency is simulated as a parallel resistance R _p1 as described above. The HF choke L ₃ prevents the high-frequency current from flowing off via R ₁ . The resistor R ₁ determines the working point of J ₁ . The amplitude is limited by R ₁ . Because of the components tolerances, it is often necessary R ₁ run set both of these ends, safe start of oscillation and low harmonics to achieve.

The capacitors C ₂ and C ₃ are permeable to high-frequency alternating current, but not to direct current. The RC element R ₂ , C ₄ sifts the operating voltage. The load resistance R _L at the low-resistance end of the resonant circuit is not part of the oscillator circuit, but rather simulates the load on the oscillator through the following stages. With R _P3 , the quality factor of the HF choke L _{3 is set} to Q = 65. The values ​​of load resistance and quality factor are important for the dimensioning or the circuit simulation .

See also

• Meissner circuit
• Colpitts circuit
• Clapp circuit

Individual evidence

1. ^ FA Record, JL Stiles, "An Analytical Demonstration of Hartley Oscillator Action". Proceedings of the IRE, Volume: 31, Issue: 6, June 1943, ISSN  0096-8390
2. ^ RV L Hartley, Oscillation Generator, US patent 1356763 Link
3. ^ Wes Hayward: Radio Frequency Design . ARRL, 1994, ISBN 0-87259-492-0 , chapter 7.3 Further LC oscillator topics, p. 280 .
4. ↑ Eckart Moltrecht: Amateur Radio Course Technology Class A . vth, 2007, ISBN 978-3-88180-389-2 , Lesson 7: Oscillator and RF amplifier, p. 90 .
5. ↑ Eckart Moltrecht: Chapter 7: Oscillator and high frequency amplifier. Retrieved July 12, 2016 . 
6. ^ Paul Falstad: Circuit Simulator Applet. Retrieved July 8, 2016 . 
7. ^ Wes Hayward: Radio Frequency Design . ARRL, 1994, ISBN 0-87259-492-0 , chapter 7.3 Further LC oscillator topics, p. 280 .
8. ^ Paul Falstad: Circuit Simulator Applet. Retrieved July 8, 2016 .