368 lUiLL SYSTEM TECHNICAL JOURNAL 



The method used for circuit analysis may be grouped under the following 

 headings: 



1. Linear problems, like low-level amplifiers or the question of onset of 

 oscillations. Such problems visualize the transistor as making only 

 small excursions from an assumed operating point and are best treated 

 by the method of small-signal analysis. The unit is assigned an equiv- 

 alent circuit or, in mathematical terms, is dealt with by means of linear 

 equations. 



2. Slightly non-linear problems, like Class A power amplifiers. Here the 

 excursions about the operating point are large enough to bring in 

 higher-order effects like harmonic generation or intermodulation, but 

 still small enough so that these efifects can be treated by adding to 

 the equivalent circuit certain distortion generators. Mathematically, 

 some terms need to be added to the linear equations but these terms are 

 of the nature of corrections, not big changes. 



3. Highly non-linear problems, such as Class B or C amplifiers, oscillators, 

 switches, harmonic generators. Here the excursions about the charac- 

 teristic are so large as to reduce the linear approximation to the status 

 of a qualitative guide or perhaps to invalidate it entirely; mathema- 

 tically, the small signal series either require many terms for accuracy or 

 else do not converge at all. These large-signal problems usually have to be 

 treated by methods special for each problem. Frequently one uses graph- 

 ical constructions from the static characteristics, or analytical methods 

 starting from reasonable approximations to the static characteristics. 



4. Finally, in certain highly non-linear problems the non-linear features 

 are in a sense subsidiary ; one is really interested in the behavior of a 

 superposed small signal subject to a linear analysis. The non-linear 

 part of the problem may appear in the form of circuit parameters or 

 frequency shifts which may be left for empirical determination. Such 

 problems are exemplified by mixers, modulators, or switches. 



The subsequent discussion will emphasize mainly the linear problems 

 where the methods of circuit analysis are most effective, but will touch on 

 some of the other fields occasionally. 



The Type A Transistor 



Perhaps at this i)oint is the ])lace to pay our respects to the physics of 

 the transistor. A view of the Tyi)e A transistor^ currently being made in 

 small quantities, is shown in Fig. 1. It is about \ inch long and ^ inch 

 in diameter. Two small phosphor bronze ''cat-whiskers" make point contacts 

 close together to a block of germanium. A large area ohmic contact to the 



3 "Type A Transistor," R. M. R)dcr, Bell Laboratories Record, March 1949, pp. 89-93. 



