 receivers are only perhaps 2 to 3dB from this limit in their very costly, there has been no great need for increased transmitter preamplifiers, and also have further unavoidable limitations due to power, and that system gain improvements, as we shall see later, preselector filtering losses. In recent years, the Noise Figure of have been made by relatively inexpensive improvements in receiver typical microwave communications receivers has followed the sensitivity. curve of Figure 50.29. This also shows the curve extended in the The predicted increase in transmitter power output in the future future towards the approachable, but unachievable limitations may occur due to the fact that there is very little further improve- presented by KTB noise and filter losses. ment in receiver sensitivity to be had. The exact technology that will Due to possible future improvements in both transmitter power be used remains to be determined, but GaAs FET amplifier technol- ogy looks most promising. Unfortunately, it seems to be a law of nature that along with the benefits possible from increased transmit- ter power there are also severe penalties in cost and reliability. The thrust of new technology is generally to reduce those penalties. Presently, the cost for transmitter power is strongly dependent on microwave frequency, i.e. power costs much more at higher fre- quencies. In many systems, present microwave transmitter output power is already more than adequate, and in some cases is sufficient to degrade system performance by severely overloading the micro- wave receivers. Often the excess transmitter power is thrown away by using power attenuators. In these cases, the potential benefits to be gained by using low power are not realised. Not only is low power significantly less costly, but also the associated decreased equipment temperature, decreased parts count, and decreased d.c. load can yield improved equipment reliability. For example: a typical solid-state power amplifier (either 2GHz or 6GHz) consists of several stages 