THE OPERATION OF VACUUM TUBES 29 



This paper will deal particularly with the type of amplifier used for 

 the amplification of the modulated carrier. It will be assumed that 

 a linear relation between input voltage and output current is the de- 

 sired characteristic of such amplifiers and that the more nearly this 

 relation is attained, the less will be the distortion produced. 



An approximate graphical method for the calculation of the dynamic 

 output characteristic from the static characteristics of a tube is out- 

 lined which is capable of considerable accuracy. The exciting voltage 

 is taken to be a sinusoidal voltage of varying amplitude, and only the 

 fundamental component of the output current is considered. The 

 very important question of the distortion introduced by the non- 

 linearity of the characteristic and by the resonance effect of the output 

 circuit, as well as the question of the suppression of harmonics in the 

 antenna circuit, is considered to be beyond the scope of this paper. 



Theoretical Development 



Class B ^ amplifiers have been defined as those which operate with 

 a negative grid bias such that plate current is practically zero with 

 no excitation grid voltage, and in which the power output is propor- 

 tional to the square of the excitation voltage. 



Class C amplifiers have been defined as those which operate with a 

 negative grid bias more than sufficient to reduce the plate current to 

 zero with no excitation grid voltage, and in which the output varies 

 as the square of the plate voltage between limits. 



There is actually very little distinction between the two types as 

 the fundamental principles of operation are the same in that the plate 

 current flows in pulses and becomes zero during part of the cycle, the 

 Class C type being merely the case where the duration of the pulses is 

 shorter. For the purposes of this paper, a Class B amplifier shall be 

 regarded as one in which the grid bias is either just sufficient or is not 

 sufficient to reduce the plate current to zero with no excitation grid 

 voltage, and a Class C amplifier as one in which the grid bias is more 

 than sufficient to reduce the plate current to zero with no excitation 

 grid voltage. 



Let us consider the plate circuit of a Class B or C amplifier to be 

 represented schematically in Fig. 1. We will omit the grid circuit and 

 assume that the excitation of the grid merely varies the internal tube 

 resistance, Rp, which it does in effect. If at the start, the grid is so 

 biased that no plate current flows, the condenser Co will charge up to 

 a potential £& as indicated in Fig. 1. Let it be assumed that Co is of 

 sufficiently large capacity that it presents negligible impedance at the 



« 1931 Standardization Report, Year Book of the I. R. E., 1931, p. 71. 



