Huth-Kühn circuit
The Huth-Kühn circuit is an oscillator circuit . Although it has no discernible feedback, it generates vibrations very reliably. It was created by Ludwig Kühn, who worked for Dr. EF Huth worked in Berlin, applied for a patent in 1917.
functionality
The resonant circuit on the grid of the triode consists of a variable capacitor 13 and a coil 12. The resonant circuit on the grid is coupled to the resonant circuit on the anode of coils 7, 10 and variable capacitor 11 via the internal grid-anode capacitance of the tube. Both oscillating circuits have slightly different resonance frequencies. The slight detuning ensures that due to the strong dependence of the phase shift on the frequency, the necessary phase out of phase between grid voltage and anode voltage is established. The anode battery is connected to 5. Capacitor 6 bridges the anode battery for the high frequency.
It must be emphasized that the function of this circuit has nothing to do with a possible inductive coupling of the coils and, as a rule, cannot be suppressed by a special arrangement of the coils.
The inductance on the grid can be replaced by a quartz oscillator , thereby stabilizing the generated frequency.
Suppression of the tendency to oscillate
There are several ways to reduce the oscillation tendency of a selective amplifier with oscillating circuits at the input and output, despite the Huth-Kühn principle:
- Connect a resistance of 1 Ω to 1 kΩ between the grid (base, gate) and the amplifier input. This resistance is a resistance in series resonance circuit of the internal capacitance of the amplifier component and the external inductor and reduces the resonant circuit quality Q .
- Use an amplifier component with a smaller internal capacity. The lower capacitance between the collector (drain, anode) and base (gate, grid) is no longer sufficient for oscillation. Use a pentode instead of a triode for tube circuits . That was a major reason for the introduction of the screen grid in the early days of tube technology.
- A small part of the output voltage is fed back to the grid with the desired (opposite) phase in order to have a neutralizing effect. This is usually done using a capacitive voltage divider and is common with power amplifiers for shortwave transmitters.
- The emitter circuit (source circuit, cathode base circuit ) is not used, but the base circuit (gate circuit, grid base circuit ), which is characterized by an extremely low feedback capacitance. In this case the base (gate, grid) is connected to ground and acts like a shield. This circuit is often the only way to build power amplifiers in the VHF area.
Applications
This oscillator circuit is preferably used as a power oscillator for a few kilowatts in industrial generators for induction furnaces or dielectric heating . The frequencies are in the ISM band around 27.12 MHz.
Individual evidence
- ↑ Huth GmbH, Ludwig Kühn: DE Patent 310152 , switching method for generating vibrations with vacuum tubes. registered on Sep 28 1917
- ^ Heinrich Schröder: Electrical communications engineering, volume II. Chapter EI6, p. 567ff.
- ↑ A442 screen tube from Philips from 1927