315 BELL :iySTEM TECHNICAL JOURNAL 



This expression neglects the area under the characteristic on the left of 

 the Une AEq , Fig. 1. The formula is useful in selecting a tube for closer 

 consideration. 



When a tube is used as a preamplifier in a wave analyzer an error of meas- 

 urement may occur if two input frequencies intermodulate in the amplifier 

 to produce a current of the same frequency as the one being measured. For 

 instance, the fundamental R cos rl and the second harmonic W cos wt may 

 intermodulate to form the third harmonic. If Ir-w is the disturbing current 

 and Ip the wanted output, then 



T F — Ft, R W 



20 log ^ = 20 log ^' ^^° + 6 - 20 log ^ - 20 log ^ 



The Rectifier 



The portion of the plate current resulting from the rectification of a 

 signal P cos pt is 



4 Ae 

 If several frequencies were present, Pi cos />/, Pz cos Pzt and so on, 



I^ = l^{Pl + Pl-h--') 

 4 Ae 



Thus Ide is proportional to the square of the root-mean-square voltage input. 



This property of the parabolic tube of measuring the root-mean-square 



voltage is often useful in the measurement field. 



If Ae is the paraboUc range of the tube and Ag the corresponding change 



in g, the largest possible rectified current obeying the root-mean-square 



Ae 

 law will obtain for an amplitude P = — . Then 



/max = Ts^g^e. 



The Frequency Doubler 

 The second harmonic is given, as before, by 



4 Ae 

 The largest possible output current is 



/max = Tff^gAe. 



