Duality as a Guide in Transistor Circuit Design 



By R. L. WALLACE, JR. and G. RAISBECK 



{Manuscript Received Sept. 26, 1950) 



Because of a relationship which exists between the properties of a vacuum tube 

 triode and those of a transistor, it is possible to start with a known vacuum tube 

 circuit and to transform it into a completely different circuit suitable for use 

 with transistors. The nature of this transformation is discussed and a number 

 of examples are given. 



Introduction 



SINCE the invention of the transistor there has been a natural tendency 

 to compare its properties with those of a vacuum tube triode. This 

 comparison indicates that the two devices are different in many important 

 respects. For example, the grounded cathode vacuum tube is essentially a 

 voltage amplifying device with a high input impedance and a relatively 

 low output impedance, while the groundeil base transistor is essentially a 

 current ampHfying device with a low input impedance and a relatively high 

 output impedance. Furthermore, high gain vacuum tubes tend to be unstable 

 with open circuit terminations, while high gain transistors tend, on the 

 other hand, to be unstable with short circuit terminations. 



The properties of the two devices are, in fact, so radically different that 

 the development of the transistor has posed an entirely new set of circuit 

 design problems. If the vacuum tubes in a known circuit are simply replaced 

 by transistors (and appropriate changes are made in the supply voltages), 

 it is usually found that the transistor is not used to best advantage and the 

 circuit performance is not satisfactory. For this reason, circuit designers 

 heretofore have exercised considerable ingenuity in devising new circuits 

 which take into account the pecuHarities of the transistor and use them to 

 best advantage. It turns out that some of these circuits bear Uttle resem- 

 blance to vacuum tube circuits designed to perform the same function. 



Although there is a great difference between the electrical properties of 

 transistors and vacuum tubes, there is a very simple approximate relation- 

 ship between them. It is the purpose of this paper to show how it is possible, 

 taking this relationship into account, to start with a known vacuum tube 

 circuit and transform it into a completely different circuit suitable for use 

 with transistors. Circuits derived in this way tend to take advantage of the 

 pecuUarities of the transistor, and in a number of cases have shown excep- 

 tionally good performance. 



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