Transistor power amplifiers

The factors governing the power-handling capabilities of p-n-p junction transistors in sinusoidal amplifiers are discussed. It is shown that, although the maximum allowed collector dissipation, Pc max, is an important transistor design criterion, it does not necessarily indicate the limit to the maximum output power possible; limitations set by other factors, notably the variation of current gain with emitter current, become particularly significant in Class B amplifiers because of their very high efficiencies. The relative importance of Class A and Class B amplifiers is considered, and the three possible Class B push-pull arrangements are analysed in detail. It is shown that load-line techniques, such as are used in designing thermionic-valve power amplifiers, are not really suitable for transistors, and a different approach is made. It is concluded that the common-collector Class B push-pull amplifier has a number of attractive features which favour its use. The design of a driver stage for this amplifier is described. Complementary arrangements of n-p-n and p-n-p transistors are examined, but it is found that their advantages are not very great. Finally, some non-sinusoidal power-amplifier applications are mentioned, and it is seen that transistors can be very important in this context.