68 



BELL SYSTEM TECHNICAL JOURNAL 



The network R-aRa serves two important purposes. It is designed 

 to present toward the amplifier the same impedance as the amplifier 

 is to work into in service, and this in turn requires that the input and 

 output impedances of the amplifier be practically equal (or if not, 

 then small with respect to i?i) for otherwise the network R^Ri when 

 joined to R1R2 will not draw the same fraction of current as the 

 amplifier, thereby upsetting the comparison upon which the measure- 

 ments are based. Furthermore, the attenuation in R^Ri is to be 

 sufficiently large that variations in the impedance of the receiver and 

 its shunt as seen from R3R1 will not appreciably affect the impedance 

 into which the amplifier works as the receiver shunt setting is changed. 

 A simple calculation will show how great the attenuation must be in 

 any given case to satisfy these conditions. 



Proper values for the steps of the receiver shunt may be calculated 

 as follows, reference being made to Fig. 35 in which the currents and 

 potentials indicated are in accordance with the assumptions made 



R3 Rj 



regarding the attenuation in the various portions of the circuit. 

 Calling a the amplification to be measured it may readily be shown 

 that 



(a-l) 2 = ri 

 a r 2 



(9) 



Or if R is the impedance of the network RsRi as seen from the receiver, 

 and the shunt Sh is proportioned so that it also presents the impedance 

 R to the receiver, whence 



r 2 R 



R = r, 



then Equation (9) gives 



a = 



r 2 +R' 



R+r 2 



r 2 ' 



Taking account of the necessary approximations it is readily pos- 

 sible to measure current amplification to within 5%, for a range of 



