280 BELL SYSTEM TECHNICAL JOURNAL 



Perhaps it is in order to inquire as to what makes an ampUfier free 

 from frequency distortion over a wide range. The answer might well 

 be: attention to impedance relations. A compact, efficient amplifier 

 requires several pieces of reactive apparatus such as transformers, 

 retardation coils, and capacitors. One must remember that an in- 

 ductance of one henry is equivalent to an impedance of 250 ohms at 

 40 c.p.s. but that it is nearly 100,000 ohms at 15,000 c.p.s. ; that the 

 grid circuit of the vacuum tube is not actually an open circuit even 

 though the grid is maintained negative with respect to the cathode, 

 but has a reactance which becomes important at high frequencies or 

 with large ratio input transformers. Many years of development in 

 this field have advanced the art to the point where transformers 

 transmitting extremely wide bands now can be designed. The com- 

 mercial production of such designs requires rigid inspection including 

 shop transmission measurements under the actual conditions of use. 

 The transformer must be designed for the particular type of vacuum 

 tube with which it is to be used. First, however, the tube must be 

 designed to permit its use under the proposed conditions and then it 

 must be manufactured to close limits, every tube of a type like every 

 other tube of that type. 



This is, then, the general requirement for a wide frequency range 

 amplifier: (1) attention to impedance relations; (2) meticulous design 

 of each component for the particular job it has to do, and rigid inspec- 

 tion to insure that it does that job. 



One might suppose that when the tube designer and the coil designer 

 each had done his part the job was done. Such is not the case. The 

 various pieces of apparatus have to be gathered together into a unit 

 (often a current supply set for supplying anode, cathode, and grid 

 potential is assembled with the amplifier) and out of this electrical and 

 physical association is apt to arise "feed-back" and "noise." 



When there is coupling between two parts of the amplification circuit 

 which are at different potential or different phase there is feed-back. 

 Feed-back sometimes is employed designedly to modify an amplifier 

 characteristic, but, feed-back which may arise as a result of a particular 

 arrangement of apparatus or wiring ordinarily will cause more or less 

 severe frequency distortion. It may be induced due to stray electro- 

 magnetic or electrostatic fields, which must be eliminated by rear- 

 rangement of apparatus or by shielding; or it may be caused by 

 common circuit impedance, requiring circuit modifications. In 

 general, a low gain amplifier or one with limited frequency range 

 presents no feed-back problems, but a study of a high-gain wide- 

 range equipment usually is necessary in order to determine the best 



