Plate Resistance

Author: E.E. Kimberly

When the plate current flows in a tube, a portion of the plate circuit voltage is required to force the electrons from cathode to plate. That voltage acts as an ordinary ri drop in the circuit; however, inasmuch as the plate characteristic is not linear, the voltage drop is not proportional to the plate current. The resistance which causes the voltage drop is called the plate resistance. The plate resistance, being a dynamic function of plate current, must be specified for some particular value of grid voltage to have any meaning. For any quiescent point on the tube characteristic, the plate resistance is equal to a small change of plate voltage divided by the resulting change in plate current. Thus,

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The plate resistance may be found most easily by drawing a straight line tangent to the plate characteristic at the quiescent point chosen and then calculating the resistance from the slope of the tangent line. Most manufacturers of tubes give the plate resistance at several quiescent points.