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BELL SYSTEM TECHNICAL JOURNAL 



Then, calling e g the input voltage, the voltage across the first tube is 



e g l«T 



S \z' 



the voltage across the primary of the second input transformer is 



e tL I** 



since 2 2 is numerically equal to r p \ and across the secondary is 



Z \ 2 \ 2 2 



Hence the voltage amplification of this portion of the amplifier is 



1*1 1*1 



\! 2 \ 2 2 ' 



and a similar argument applies to the following stages. 



The measurement of amplification can be accomplished by the 

 obvious procedure of determining the magnitudes or the relative 

 magnitudes of the input and output current. This can be done for 

 either a single stage or for several stages at once. 



Fig. 34 



A very satisfactory circuit for measuring amplification is illus- 

 trated in Fig. 34 and through use of a vacuum tube voltmeter (see 

 Sec. 56) as a comparison means, it is capable of an accuracy of about 

 2%. The conditions which the resistances Ri and R 2 , etc., should 

 satisfy are very simple. The network connected between the oscilla- 

 tor and amplifier input should present an impedance, looking into 

 it from the right, equal to the input impedance of the amplifier; like- 



