GRID CURRENT MODULATION 113 



These equations are equally applicable to grid and to plate circuits 

 provided the potentials are small and the operating region is expressible 

 by eq. 5. This is not always the case in practice, and modifications in 

 the above comparatively simple analysis are required, which will be 

 treated below. A number of characteristic features of operation are 

 exhibited by the above analysis however and these we proceed to 

 discuss. 



If the preceding treatment is applied to the grid circuit we see that 

 in order to make the sideband potential across the grid a maximum 

 with fixed fundamental currents, the external grid impedance at the 

 sideband frequency must be made large compared to the effective 

 internal resistance of the tube. Further, if the generator potentials 

 and impedances are fixed it follows that the generator resistance should 

 be made to match the internal resistance of the tube at the fundamental 

 frequency — with a transformer, if necessary — in order to make the 

 fundamental currents as large as possible. This conclusion regarding 

 the ratio of grid impedances follows immediately without mathe- 

 matical analysis if we suppose the source of the higher order products 

 to lie in the variable impedance element so that it may be considered 

 equivalent to the presence of generators of the higher order frequencies. 

 The generator voltage is evidently maximum on open circuit, which 

 agrees with the above statement. 



In considering the form of external impedance to use for best 

 results, an inspection of eq. 10 shows that in the quantity Z-/(Ro + Z-), 

 which expresses the ratio of effective grid voltage to generated grid 

 voltage, the sideband impedance Z- should have a large reactive 

 component. This is illustrated by Fig. 1 which gives the ratio for 

 various relative external impedances having phase angles of and 90° 

 and shows that, with the external impedance fixed in magnitude, the 

 ratio has its greatest value for a pure reactance. 



Relative Phase of Grid and Plate Sidebands 



If the tube acted as a perfect amplifier of the potentials impressed on 



the grid, there would be no further distortion and the sideband current 



in the plate circuit would be obtained by multiplying eq. 10 by 



/x/(Z + Ro), where Z and Ro are the external and internal plate circuit 



resistances respectively, and /x is the amplification factor which is 



assumed constant here.^ Unfortunately this ideal situation does not 



exist of itself, and modulation of the amplified fundamentals takes place 



in the plate circuit, producing an additional sideband component to 



'' The distortion due to variable n as treated by Peterson and Evans in the Bell 

 System Technical Journal for July, 1927, represents but a small part of the total in 

 efficient modulators, although it is of importance in high quality amplifiers. 



