r.ici'UAf rcnii (\scii.i..iroKs 



511 



ilii- )i\k\ circuit resistiinci- causts a change <>f opposite si^n in the 

 Noltagc drop across the plate circuit resistance, the two voltage drops 

 heing referred to the potential of the filament. The freciuency contrt>l 

 unit is in the form of a series circuit containing inductance, capacity, 

 .md resistance. Two resistance elements are used for conplini; tn the 

 input and to the output of the amplifier. 



I.et us tirst consider the properties of the vacuum tui)e amplilier. 

 In Fig. 3 the voltage develoix-d across the junction A is plotted as a 

 function of the voltage across the junction B. I.et us assume, for 

 the present at least, that this curve holds for all frequencies. Obviously 



VOLTAGE AT B 

 l'"ig. S .Amplifier output cli.ir.u tiri>lic- 



the voltage across the junction A depends upon the impedance looking 

 into the frequency control unit, but if the resistance Ri is small in 

 comparison with the resistance R, the voltage will be practically 

 independent of the frequency control unit. This curve represents 

 a familiar characteristic of the vacuum tube amplifier. It shows 

 that as the voltage upon the grid of the first tube is increased, a point 

 is reached where the amplitude of the output is no longer propor- 

 tional to the amplitude of the input. If this is carried far enough a 

 point is ultimately reached where a continued increase in the voltage 

 on the grid fails to produce any further increase in the voltage across 

 the output. For our present purpose the data contained in this 

 curve will be more useful if plotted in a less familiar form. 



