LOAD CARRYING CAPACITY OF AMPLIFIERS 575 



for i\ more complete analysis of the problem. Three kinds of measure- 

 ments were made. 



1. The gain of the amplifier was measured under load conditions 

 which varied from a point well below its carrying capacity to a 

 point where it was obviously overloaded. The results from tests 

 of this kind show that the gain is uniform at low outputs, begins 

 to fall off when the output reaches a certain level and falls off 

 more and more rapidly as the load is further increased. The 

 point at which the gain begins to fall off has sometimes been 

 taken as a criterion of the load carrying capacity of the 

 amplifier. 



2. With a single frequency (1,000 c.p.s.) input to the amplifier the 

 output was analyzed at a number of points along the load-gain 

 curve and the percentage of harmonic to fundamental in the 

 output plotted against the same scale of energy output as for 

 the load-gain curve. 



3. The input voltage was made to produce a horizontal deflection 

 in a cathode ray oscillograph while the output voltage or current 

 was made to produce a vertical deflection. In effect this is a 

 convenient means of drawing the input-output characteristic of 

 the amplifier so that its general curvature and the loads at which 

 any sudden changes of curvature occur may be easily observed. 

 For an amplifier that produced no distortion or phase shift, the 

 resultant figure would be a straight line whatever the wave 

 shape of the input. If there were phase shift but no distortion 

 the result would be an ellipse or circle depending on the phase 

 and amplitude relationships of the input and output provided 

 the input were a pure frequency. In general for the practical 

 case the result is a distorted ellipse showing that the wave 

 undergoes both distortion and change of phase in passing through 

 the amplifier. With increasing load the distortion becomes more 

 and more pronounced. 



The gain measurements were made by methods in principle the same 

 as those embodied in standard gain measuring sets of the Bell System, 

 and while it is not the intention of this paper to give a detailed descrip- 

 tion it may be desirable to give a brief statement of the principles in- 

 volved. For the purposes of this paper the gain of an amplifier is de- 

 fined as the logarithm of the ratio of the power delivered into its load 

 impedance to the power that would be delivered if the amplifier were 

 removed and replaced by the best possible passive network. 



