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| View Original: | UY224 tetrode plate current negative resistance.svg (1374x1118) | |||
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| Keywords: UY224 tetrode plate current negative resistance.svg plate anode and screen grid current vs plate voltage in a UY224 1920s era tetrode vacuum tube showing region of negative resistance grey used in dynatron oscillator circuit I<sub>P</sub> Plate anode current I<sub>G2</sub> Screen grid current I<sub>C</sub> Cathode current is equal to the sum of I<sub>P</sub> and I<sub>G2</sub> Control grid potential V<sub>G1</sub> ˆ’1 5 V Screen grid potential V<sub>G2</sub> 75 V When electrons from the cathode hit the plate they knock secondary electrons out of the metal a process called secondary emission If the screen grid is made positive with respect to the plate the secondary electrons will be attracted to it representing a current away from the plate For a certain plate voltage range grey an increase in plate voltage will cause a greater increase in secondary electrons than primary electrons so the plate current will decrease Thus the differential plate resistance is negative r<sub>P</sub> ΔV<sub>P</sub> / ΔI<sub>P</sub> < 0 This negatve resistance was used in the dynatron oscillator circuit to cancel the resistance in a tuned circuit creating oscillations The dynatron oscillator was used in some instruments in the 1930s and 40s In modern tetrodes the surface of the plate is treated to greatly reduce secondary emission so they don't have negative resistance Note Due to the fact that the screen grid screens out the electric field of the plate the cathode current I<sub>C</sub> is almost constant with plate voltage So the plate and screen currents are almost mirror images of each other Within the range V<sub>P</sub> 20 - 60 V each electron striking the plate actually knocks out more than one secondary electron so the plate current actually reverses - more electrons leave the plate than enter it 1952 Tracing of a graph retrieved 18 October 2013 from http //www tubebooks org/Books/intro_army_theory pdf Basic Theory and Application of Electron Tubes US Dept of the Army Technical Manual TM 11-662 February 1952 US Dept of Defense p 90 fig 71 on Pete Miller's http //www tubebooks org/index html tubebooks org website Original work K E Thiebaud Derived work Chetvorno PD-USGov Uploaded with UploadWizard Vacuum tube diagrams Device current-voltage characteristics | ||||
