246 



BELL SYSTEM TECHNICAL JOURNAL 



small, or where the voltage cannot be detected except by several 

 stages of amplification such as are obtained in the current analyzer. 

 To adapt it for this purpose it is only necessary to precede it by a 

 simple circuit such as shown in Fig. 14. This consists of a single 



VOLTAGE AMPLIFIER 



/THERMOCOUPLE 



ST'D. 

 ^ REFERENCE OSC. 



VOLTAGE TO BE 

 ANALYZED 



TO 

 CURRENT ANALYZER 



Fig. 14 — Voltage amplifier 



vacuum tube having a large resistance across the grid and filament. 

 This resistance should be greater than the impedance across which the 

 voltage is to be measured. The output side works into a step-down 

 transformer through a resistance of several times the output impedance 

 of the tube. This tends to straighten out the characteristic and to 

 lower the tube modulation level. The object of the step-down trans- 

 former is of course to secure greater efficiency in working into the low 

 input impedance of the heterodyne current analyzer. 



The measuring procedure would be to apply the voltage to be 

 evaluated across the high impedance input and adjust the analyzer in 

 the usual manner; then substitute the output from the standard 

 oscillator of the saiue frequency across the amplifier and adjust the 

 amplitude to give the same meter deflection. In order to determine 

 the voltage applied, the oscillator may be connected across a known 

 resistance in parallel with the input, and the current into this resistance 

 measured. The IR drop will then be a measure of the voltage applied. 



The range of voltage which can be measured of course depends upon 

 the biasing potential on the amplifier grid as it is not desirable that grid 

 current flow through the high resistance and increase the tube modula- 



