Duality as a Guide in Transitor Circuit Design

S I N C E 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 grounded base transistor is essentially a current amplifying 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 peculiarities of the transistor and use them to best advantage. It turns out that some of these circuits bear little resemblance 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 relationship between them.