454 BELL SYSTEM TECHNICAL JOURNAL 



istic is controlled, in accordance with the procedure outlined previously, 

 by the elements in the ix circuit. The sharp slope just beyond the edge 

 of the useful band is obtained from a transformer anti-resonance. The 

 relatively flat portion of the characteristic near its intersection with the 

 asymptote is due partly to an anti-resonance of the /3 circuit with its 

 distributed capacitance and partly to an increase in the gain of the 

 third tube because of the filter-like action of the elements of Zk in 

 cutting out the local feedback on the tube in this region. 



The large margins in the design made it possible to secure a substan- 

 tial increase in feedback without instability. For example, with a loss 

 margin as great as 15 db the feedback can be increased by adjusting the 

 screen and plate voltages to increase the tube gains. A higher feed- 

 back can also be obtained by adjusting the resistance in the first inter- 

 stage. As this interstage was designed, an increase in the resistance 

 results in an increased amplifier gain and a correspondingly increased 

 feedback which follows a new theoretical characteristic with a some- 

 what reduced phase margin. The adjustment, in effect, produces a 

 change in the value of the constant k in equation (6). With this ad- 

 justment the feedback can be increased to about 40 db before the 

 amplifier sings. 



