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In theory, if an ideal capacitor is connected in parallel with an ideal inductor, they form a resonant circuit that, once it begins oscillating, will oscillate forever as the energy is transferred back and forth between the capacitor and the inductor.
In practice, however, the two components are not ideal. Real inductors and capacitors are equivalent to an ideal component in parallel (or in series) with a resistance; a real resonant circuit is equivalent to an ideal capacitor, inductor, and resistor connected in parallel.
If a negative resistance equal in magnitude to this positive resistance can be connected in parallel with the above circuit, then the two resistances will cancel and the circuit will oscillate forever as described earlier.
Although no passive negative resistors exist, some active devices exhibit a negative resistance characteristic. Examples of such devices are some tetrode thermionic valves (or tubes) and the tunnel diode. Using these devices to provide the negative resistance causes the tuned circuit to oscillate as long as power is provided to the active device. The power supplied compensates for resistive losses and power extracted from the oscillator. The oscillation is self-starting as the resonant circuit is excited by the inherent noise of the active device.
The negative resistance exhibited by tetrodes is due to secondary emission from the anode when operated at a lower voltage than the screen grid. Secondary emission is usually an unwanted phenomenon, and more modern valves are treated to minimize it. Older valves make good dynatron oscillators.
This type of oscillator is commonly known as a 'dynatron' oscillator, though, technically, only the version using the tetrode valve is a true dynatron oscillator. Later models of tetrodes were treated to reduce the (usually undesirable) secondary emission characteristics of the plate, making them less suitable for the dynatron oscillator. An oscillator that uses the negative-resistance characteristic of screen grid secondary emission in a pentode is called a "transitron oscillator".
M. G. Scroggie described the dynatron oscillator in detail in earlier editions of his Radio Laboratory Handbook (later Radio and Electronic Laboratory Handbook). He recommended the old type 77 pentode, connected as a tetrode, as a good dynatron tube. The main advantage of this type of oscillator was extreme simplicity: a very simple dynatron circuit could be built without resonant elements, and when a combination of inductor and capacitor was connected would oscillate at its natural frequency; a very wide frequency range could be covered changing only the tuned circuit.
- Terman, F.E. "Electronic and Radio Engineering," 4th ed. McGraw-Hill, NY 1955, pp. 197,503
- Irving M. Gottlieb Practical oscillator handbook Elsevier, 1997 (reprint) ISBN 0-7506-3102-3, page 79