Meissner circuit

Meißner oscillator, patent specification

A Meissner oscillator , or Armstrong oscillator , is a feedback amplifier with a frequency-determining resonant circuit , which belongs to the group of sine oscillators . The circuit is named after its inventor Alexander Meißner , who patented it in 1913.

In the Meißner oscillator, the resonant circuit is at the output of the amplifier component. In Edwin Howard Armstrong's Audion circuit , the resonant circuit is at the input of the amplifier component.

construction

The oscillator can be implemented with different active components as an amplifier such as a bipolar transistor , field effect transistor or also by means of an electron tube . The actual resonant circuit is formed from a coil L ₂ and a capacitor C ₂ . In addition, a second winding L ₁ , which is magnetically coupled to L ₂ like a transformer, feeds back the voltage on the resonant circuit with the appropriate phase . In the first circuit, the bipolar transistor Q serves as an amplifier, in the second circuit the junction field effect transistor (JFET) J ₁ .

Meissner oscillator with bipolar transistor

Circuit diagram of a Meissner oscillator with a bipolar transistor

For the circuit to generate an undamped oscillation, the loop gain must be 1 and the feedback must be in phase (0 ° or another multiple of 360 °). Since the Meißner circuit in the circuit example is an emitter circuit , the signal is inverted by the transistor . This is reversed by the transformer, since L ₁ and L ₂ have opposite winding directions, indicated in the circuit by the two black dots that indicate the beginning of the winding.

The first circuit shown is an emitter circuit with negative current feedback, in which the gain is equal to the ratio of the collector resistance (alternating current resistance of the resonant circuit) and the emitter resistance R ₃ . Since conventional LC parallel resonant circuits have very small resistances outside the resonance frequency, the loop gain is only greater than one for the resonance frequency. The transformation ratio of the transformer is chosen so that the loop gain for the resonance resistance of the LC circuit is certainly greater than one, and the voltage at the input does not overdrive the transistor.

Meissner oscillator with JFET

Meissner oscillator with JFET

In the JFET circuit according to FIG. ₁ , the resonant circuit consists of C ₁ and winding L _{1 of} the transformer L ₁ -L ₂ . C ₁ is often a variable capacitor . The JFET J ₁ in gate circuit has a phase shift of 0 °. The amplifier input is the source and the amplifier output is the drain. The transformer does not produce any phase shift. After switching on, the JFET works in class A mode . The voltage at the drain of the JFET is roughly the operating voltage . A small change in the voltage at the drain of the JFET due to the thermal noise is coupled to the source via L ₁ -L ₂ and C ₂ . This small change in the entrance is amplified. The amplitude of the alternating voltage on the resonant circuit increases until the voltage on the drain oscillates approximately between the voltage on the source and twice the operating voltage. The JFET is now working in class C mode . During a small current flow angle at the time of the minimum drain voltage, the JFET operates in the linear region (ohmic region). How long the JFET stays in the linear range is determined by R ₁ . So that the sinusoidal output signal contains few harmonics , the amplifier should only compensate for the losses in the resonant circuit and the outflow to R _L. Due to the component tolerances, it is often necessary to make R ₁ adjustable in order to achieve both goals, reliable oscillation and low harmonics.

The transformer L ₁ -L ₂ has a transformation ratio of the impedances of 10: 1. As a result, the load resistance R _{L has} only a slight damping effect on the resonant circuit L ₁ -C ₁ . The load resistance R _L no longer belongs to the oscillator, but simulates the load on the oscillator through the following stages. The values ​​of load resistance and quality factor are important for the dimensioning or the circuit simulation . The HF choke L ₃ prevents the high-frequency current from flowing away from L ₂ via C ₂ and R ₁ . The resistance R _S symbolizes the quality factor of the unloaded resonant circuit. The result would be a quality factor of Q = 100 (characteristic resistance is 190 Ohm), but this is not achieved in the circuit. At 30 MHz, the transformer can be implemented with a toroidal core made of iron powder. The RC element R ₂ -C ₃ decouples the oscillator from other assemblies. The capacitor C ₃ short-circuits the supply voltage in terms of high frequencies (blocking capacitor) and thereby prevents, among other things, the propagation of the high frequency on the power supply line.

The output frequency is calculated according to Thomson's oscillation equation :

${\displaystyle f_{O}={\frac {1}{2\pi \cdot {\sqrt {L_{1}\cdot C_{1}}}}}}$

Dimensioning

If Meißner oscillators are dimensioned unfavorably in terms of L / C ratio, operating point or translation, they may oscillate, but the oscillation deviates noticeably from the sinusoidal shape. Typically, the total gain of the oscillator is slightly greater than 1 when it is switched on and is automatically reduced to exactly 1 during operation due to limitation, operating point shift or amplitude-dependent gain (transient process). For the amplitude limitation, the FET uses the property that the voltage gain depends on the gate voltage. With LC resonant circuits, the ratio from to cannot be selected arbitrarily if a high quality is to be achieved. The so-called characteristic impedance or characteristic resistance of the resonant circuit results from the resonance frequency and inductance or capacitance via the complex resistance. They should only be so high that the circuit does not dampen the resonant circuit too much. ${\displaystyle L}$ ${\displaystyle C}$

Calculation example

The calculation refers to the picture with the bipolar transistor.

A typical small-signal transistor has a direct current gain B of approximately B = 100 and a base-emitter voltage U _BE = 0.65 V.

Furthermore, let I _C = 2 mA (collector current at the operating point) and U _B = 15 V (supply voltage of the circuit)

The voltage drop at R ₃ should be 1 V, so:

${\displaystyle R_{3}={\frac {1\,\mathrm {V} }{I_{C}}}={\frac {1\,\mathrm {V} }{2\,\mathrm {mA} }}=500\,\Omega }$

So the voltage divider of R ₁ and R ₂ has to supply this 1 V plus the base emitter voltage of approximately 650 mV. The voltage divider is loaded by the base current I _B = I _C / B = 2 mA / 100 = 20 µA; if one takes ten times the cross current of 0.2 mA, then one can neglect the base current and get:

${\displaystyle R_{2}={\frac {1\,\mathrm {V} +0{,}65\,\mathrm {V} }{0{,}2\mathrm {mA} }}=8{,}2\,\mathrm {k\Omega } }$

${\displaystyle R_{1}={\frac {15\,\mathrm {V} -(1\,\mathrm {V} +0{,}65\,\mathrm {V} )}{0{,}2\,\mathrm {mA} }}=67\,\mathrm {k} \Omega }$

The coil L ₁ has an inductance 22 mH and the capacitor C ₂ is 33 nF. This results in a resonance frequency of:

${\displaystyle f_{O}={\frac {1}{2\pi \cdot {\sqrt {22\,\mathrm {mH} \cdot 33\,\mathrm {nF} }}}}=5{,}9\,\mathrm {kHz} }$

In order not to overdrive the transistor Q and to generate a good sinusoidal signal, the fed back voltage must not be significantly greater than 1.5 V _pp (peak-to-peak voltage). The voltage on the resonant circuit is around 28 V _pp . This results in a reduction of 1:18 and a gain of v = 18 for the circuit , for which the collector resistance must be at least 9 kΩ (including the output resistance of the transistor of around 100 kΩ).

Assuming the quality factor g  = 50, then the resistance of the LC circuit is at the resonance frequency

${\displaystyle R_{LC}=g\cdot {\sqrt {\frac {L}{C}}}=50\cdot {\sqrt {\frac {22\,\mathrm {mH} }{33\,\mathrm {nF} }}}=41\,\mathrm {k} \Omega }$

That seems sufficient and corresponds to an ohmic coil resistance of 16 Ω.

The coupling capacitors C ₁ and C ₃ only allow the alternating voltage to pass and do not change the operating point of the transistor. C ₁ works on the input resistance of the emitter circuit (approx. R ₂ ). C ₃ and the input resistance of the next stage load or detune the resonant circuit. A decoupling at R ₃ which is obvious in this respect , however, rarely delivers a good sinusoidal signal.

Application examples

The Meißner circuit is rarely used because the transformer is a considerable effort; the Hartley and Colpitts circuit , and in some contexts also the Clapp circuit, are mostly preferred, especially when only one transistor is to be used. Further oscillator circuits are possible with several transistors .
The use of the Meißner circuit as the FuG 23 tracking transmitter for the Fieseler Fi 103 ("V1") cruise missile used by the Air Force in World War II is remarkable .

A widespread application of the Meissner circuit were straight-ahead radio receivers called feedback audion such as. B. the people's receivers . Here the positive feedback is not used to generate vibrations, but to amplify the input signal, i.e. to improve radio reception.

literature

• H. Barkhausen : Textbook of electron tubes and their technical applications. Volume 3: feedback . Hirzel, Leipzig 1951.
• Andrei Grebennikov: RF and Microwave Transistor Oscillator Design. Wiley, Chichester, et al. a. 2007, ISBN 978-0-470-02535-2 .
• Günter Kurz, Wolfgang Mathis : Oscillators. Circuit technology, analysis, properties . Hüthig, Heidelberg 1994, ISBN 3-7785-2251-5 .
• U. Tietze, Ch. Schenk: Semiconductor circuit technology. 12th edition. Springer, Berlin a. a. 2002, ISBN 3-540-42849-6 .
• O. Zinke, H. Brunswig: High frequency technology . 2: Electronics and signal processing. 5th revised edition. Springer, Berlin a. a. 1999, ISBN 3-540-64728-7 , ( Springer textbook ).

Individual evidence

1. ↑ Patent DE291604 : Device for generating electrical vibrations. Registered on April 13, 1913 , inventor: Alexander Meissner ( Online @ DepatisNet ). ‌
2. ↑ Patent US1113149 : Wireless Receiving System. Registered on October 29, 1913 , inventor: EH Armstrong. ‌
3. ^ Wes Hayward: Radio Frequency Design . ARRL, 1994, ISBN 0-87259-492-0 , chapter 7.3 Further LC oscillator topics, p. 283 .
4. ^ Paul Falstad: Circuit Simulator Applet. Retrieved July 8, 2016 .