er to the required levels. Small transmitters including hand-held transceivers, mobile phones and the like use amplifiers that are semiconductor based. Even many high power transmitters still use semiconductors, but those which deliver many kilowatts use thermionic technology (valves or tubes) as they are still the most convenient way of generating these high levels of RF power. An RF amplifier is like any other form of amplifier having the same basic requirements and constraints. The only difference is that the frequencies at which the amplifier operates may be higher. Essentially a signal enters at the input and an amplified or larger version appears at the output. Where the signal being amplified contains elements that are dependent upon the level of the signal, i.e. amplitude modulated components, then the amplifier must be as linear as possible. Here the output level must be a linear function of the input, i.e. multiplied by a fixed number regardless of the input level. For systems running CDMA such as mobile phone systems linearity is again a requirement. In some transmitter applications this may not be the case. Good linearity is accomplished at the expense of poor power efficiency. In some cases efficiency is very important. For portable applications it is necessary to ensure that as much of the DC power entering the amplifier is converted into radio frequency power that can be transmitted. In this way the batteries can be made to last as long as possible. At the other end of the spectrum broadcast stations with transmitters that consume many hundreds of kilowatts find that the cost of the electricity is a major consideration. By increasing the efficiency of the amplifiers, significant cost savings can be made. The efficiency of the amplifier is simply the proportion of the DC input power supplied to the amplifier that is converted into radio frequency power to be transmitted. This can be expressed: Transmitters 183 Power output X 100% Power input There are a number of factors that affect the efficiency o