Negative Resistance Oscillators

Posted on Feb 6, 2014

The schematic of its mixer part shows such an oscillator. The oscillator LC circuit only has two connections to the valve. It has no tap or tickler coil. Oscillations will only the occur, if the tube provides a negative resistance which compensates the losses in the LC circuit. The negative resistance has to be smaller than the positive resistance of the LC circuit.

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Here the screen-grid and plate electrodes are both positive, and the suppressor is sufficiently negative to produce a virtual cathode between the suppressor and screen. A high resistance is placed between the suppressor and its bias potential, and the suppressor and screen electrodes are tied together with a by-pass condenser. With this arrangement the screen-grid circuit offers a negative resistance to alternating currents. This is because the screen and suppressor electrodes are at the same potential with respect to alternating currents, and, if the potentials of both electrodes vary together, the current to the screen will decrease with increasing potential, and vice versa. The reduction in screen current comes about because the screen current consists largely of electrons that have returned from the virtual cathode produced by the negative suppressor. As the suppressor-grid potential increases, more electrons are drawn from the virtual cathode to the plate, leaving fewer to return to the screen and hence reducing the screen current. The magnitude of the negative resistance obtained in this manner can be controlled by varying the control-grid potential. With ordinary tubes values as low as 3000 or 4000 ohms are obtainable. A number of other negative resistance arrangements using vacuum tubes have been proposed but none of these except the magnetron, [which is discussed above, ] has had much practical use. Negative...

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