124 BELL SYSTEM TECHNICAL JOURNAL 



lively low frequencies used in carrier telephony, and by the compara- 

 tively low alternating and battery potentials which raise the relation- 

 ship of plate and grid voltages to grid current, to importance. 



We have now to examine the electrical properties of available 

 circuit elements in the light of our previous analysis, so that their 

 assembly will yield the most favorable results. 



Vacuum Tubes 



The effect of the shape of the grid-current — grid-voltage curve on 

 the modulating properties of the grid circuit is not as pronounced at 

 large amplitudes as might be expected from experience with plate 

 current modulators at comparatively low amplitudes. As is well 

 known this characteristic of ordinary tubes is much more variable 

 between tubes of the same type than the plate-current — plate-voltage 

 curve. But it has been found that a change of tubes having static 

 grid characteristics varying within wide limits does not vary the 

 modulating gain of a grid current modulator more than one T.U. The 

 reason for this may be seen most easily in the case of an external grid 

 impedance consisting of a pure resistance. If the tube grid resistance 

 were comparatively small for all positive voltages the positive half 

 of the wave would be completely suppressed, and the analysis of 

 Appendix 1 would accurately represent the wave. Even when the 

 tube grid resistance varies considerably it does not alter the wave 

 shape appreciably so long as it remains small compared to the external 

 resistance. This condition may be satisfied with particular ease for 

 large input voltages, and may also be satisfied in a qualitative sense, 

 when reactances are used in place of resistance. The principal effect 

 of a change in grid resistance is then to change the input impedance, 

 which affects the net gain only through the mismatch of impedance at 

 input frequencies. 



As a consequence of the tube circuits and range of operating po- 

 tentials used in the grid current modulator, the details of the grid 

 current characteristics become of relatively small importance and 

 attention is focussed on the functioning of the plate circuit. The 

 plate circuit is used purely for amplification purposes as mentioned 

 above, so that the criteria of usefulness of a tube as a grid current 

 modulator come down ordinarily under the stated operating conditions 

 to the criteria of usefulness of a tube as an amplifier. 



Filter and Transformer Networks 

 Input Filters and Modulating Gain 

 Since the gain obtainable in a grid current modulator depends 

 primarily on the ratio of external to total grid circuit impedance, it is 



