GRID CURRENT MODULATION 119 



A similar expression exists for A£, when /„ is substituted for J ,,. The 

 reciprocal of a\ will be recognized as the internal resistance of the 

 variable element for small potential variations. 

 From these 



so that the two fundamental circuits share equally in the power 

 dissipation due to sideband flow. This means that when the two 

 modulating currents are not of the same amplitude, the smaller current 

 will have the larger resistance change due to sideband flow, and there- 

 fore will suffer a greater percentage amplitude change. This discussion 

 serves to emphasize the point that the tube impedances depend upon 

 the impedance-frequency characteristics of the circuit to which the 

 tube is connected, so that this point must be kept in mind in the design 

 and measurement of modulating circuits. 



Grid Current Modulator, Large Alternating Grid Potentials 

 The comparatively simple analysis we have just employed is not 

 capable of very wide application because of the assumed form of the 

 grid current equation. In the practical forms of grid current modu- 

 lators, from which comparatively large amounts of modulated power 

 are required, the grid potentials are increased and the grid is maintained 

 negative during an appreciable part of the cycle. The above method 

 then becomes too involved to be extended to this case, since a large 

 number of terms would be required for an accurate representation of 

 the tube characteristic. When we have a large external grid resistance, 

 however, as appeared to be desirable from eq. 10, a fairly exact 

 solution for the modulation products can be obtained by another 

 method which is capable of direct application. 



If we determine the relation between impressed potential and output 

 current in this particular case we find that on passing from negative to 

 positive potentials, the plate current curve breaks sharply at about 

 zero grid potential, and becomes nearly parallel to the x-axis, as shown 

 in Fig. 4. We can therefore consider the positive lobe of the input 

 wave to be cut off at zero grid potential under these conditions and the 

 problem can be handled analytically.^ 



We are indebted to Mr. F. Mohr for computations on the sideband 

 amplitude as given by eq. 20, of the Appendix, in which the sideband is 

 expressed in terms of a multiple of P, as function of the ratio QjP. 

 The relationship between these quantities is given as a single-valued 

 function. For our own purposes, however, we have plotted the 



^ Appendix 1. 



