Comparison of Transistor Connections

Author: Leonard Krugman

The analyses of the three basic connections and their operating characteristics apply equally to both point-contact and junction transistors. However, due to the difference in comparative values of the internal transistor parameters, r_e, r_b, r_c, and r_m, the performance of the two basic transistor types is considerably different. In practice, the point-contact transistor is unstable, and has negative input and output resistances. On the other hand, the junction type is generally cheaper to produce, has better reliability, better reproducibility, higher available gain, and a lower noise figure than the point-contact type. It is safe to predict the gradual displacement of the point-contact transistor by the junction transistor in all but a few specialized applications, particularly since the frequency range of the junction type is steadily being increased by new manufacturing techniques. In view of this, the remainder of the book will deal primarily with the junction transistor, and unless specified, typical junction characteristics will be assumed.

At this point in the book all the basic design formulas have been derived for the three transistor connections. Thus, a comparison between the general characteristics of the three fundamental connections is now in order.

The grounded base connection is similar to the grounded grid circuit in electron tubes. This connection is characterized by low input resistance, high output resistance, and no phase inversion. Although its current gain is less than one, it provides respectable voltage and power gains. It is well suited for d-c coupling arrangements and for preamplifiers that require a low input and high output impedance match.

The grounded emitter circuit is the transistor equivalent of the grounded cathode connection in the vacuum tube circuit. This transistor connection is the most flexible and most efficient of the three basic Connections. The grounded emitter connection reverses the phase of the input signal. Its matched input resistance is somewhat higher than that of the grounded base connection; its matched output resistance is considerably lower. The grounded emitter usually provides maximum voltage and power gain for a given transistor type.

The third connection, the grounded collector, is the transistor equivalent of the grounded-plate vacuum tube. It is characterized by a voltage gain that is always slightly less than unity. Its current gain is in the same order as that of the grounded emitter. It has a relatively low output resistance, a high input resistance, and does not produce phase reversal. It offers low power gain, but is capable of supplying reverse power gain. The grounded collector circuit is used primarily as a matching or buffer stage.