Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

:INPUT IMPEDANCE OF PENTODES 307general, circuit values and structural features were designed to simulateconditions found in typical circuit applications.For these reasons the graphically presented data is given in termsof smoothed out curves representing gross phenomena.Input ResistanceThe data is given in terms of input resistance since this is ofpractical interest to the designer. In the discussion, however, inputconductance is also used because, when complex, it is more readilyresolved into its component factors.Figure 1 gives the results of measurements on a number of tubesillustrating the relationship between the input resistance and thefrequency.These tubes were operated under the conditions given inTable 1. The circuit connections are shown in Figure 2, where C hasthe values 4000 and 500 micromicrofarads for thefrequency ranges10-50 and 50-200 megacycles respectively.The input conductance of a vacuum tube with fixed operatingvoltages may be considered as varying with frequency according to theexpressiong l= k l + k af + k^p (1)where the fe's are constants for a specific tube and for specific operatingvoltages. The terms k t and hj are primarily dielectric losses due toleakage and hysteresis respectively, and distinguished from k 3f 2 whichis electronic. The electronic term, which is of particular interest athigh frequencies, is generally resolved into two factors, i.e. the motionof electrons in the cathode-to-plate space and the reaction of the platecurrent through any residual coupling element between the plate andgrid circuits.Although these factors could be analysed by means ofelectron ballistics and circuit theory an empirical method isas being more practical for engineering purposes.indicatedIf k tand k zf were negligible (1) could be written in terms of inputresistance as:1R< =—(2)Plotted in logarithmic coordinates (2) would give a straight line witha negative slope of 2. This is shown in Figure 1 for the purpose ofcomparison with the measured values.The departure at low frequencies from the theoretical curve ia dueto the increasing importance of k t and kj.