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Dynatron Parasitic Oscillations

February 24th Fact-of-the-Day

Resistive loads make resonant oscillations die out sooner, because some portion of the oscillating energy is consumed by resistance on each cycle. That is well known, but what would be the effect of a negative resistive load? A negative resistive load would put energy into an oscillating system on each cycle. Consider a child's swing. Pushing against the swinging action will make oscillations die out sooner, because the pushing resists the motion. However, pushing the swing synchronously with the natural cycles will do the opposite and will add negative resistance. Cycle amplitudes will grow, rather than diminish. A variety of operating conditions can cause vacuum tubes to exhibit negative resistance that feeds energy into parasitic oscillations. For example, when the grids of some power amplifier tubes swing positive, the grid-to-cathode circuit has negative resistance, because of secondary electron emission from the grid. Oscillations due to negative resistance are called dynatron oscillations. ©2005 Martek International All rights reserved.


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