Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

INPUT IMPEDANCE OF PENTODES 3139i = 9o + K9m (3)where g is the residual conductance with zero plate current and k 4 isa constant for a particular tube and a particular frequency. Inspectionof the graphical data shows that for these tubes at the frequenciesinvestigated the residual conductance, g , in (3) is so small that it canbe neglected. This makes possible the expression of the variation ofinput resistance with transconductance by:R t=(4)k±gmIf plotted in logarithmic coordinated (4) would give a straight linewith a negative slope of 1.Compensation of Input Resistance VariationThe variation in input resistance of a vacuum tube may be substantiallyreduced by means of negative feedback through an unbypassedcathode resistor. This consideration is of particular value inthe operation of tubes with varying bias voltage.Also the use of anunby-passed cathode resistor makes possible an increase in the effectiveinput resistance for either fixed or variable bias operation. Figure 5gives measurements of input resistance with unby-passed cathoderesistors for those tubes which had their suppressors brought out ina separate base connection. The cathode resistor was interposed inplace of the connection kk in Figure 2.The effect of an unby-passed cathode resistor on the input impedanceof a tube is discussed in report LB-450. Neglecting higher orderfrequency terms and making a number ofapproximations the inputconductance of a tube with unby-passed cathode resistor may beexpressed as:g{ = gi— g tRk9m —