rs of this type typically have an efficiency of around 60 per cent. To improve the efficiency still further, the amplifier may be operated in class C. In this mode of operation the amplifier is biased beyond the cutoff point of the device. In this way it only allows short current pulses of less than half a cycle to flow, corresponding to the peaks in the input Transmitters Output current at this Voltage point Point at which Input transistor turns off Current Current pulses Conduction for less than half the waveform Figure 6.4 Class C operation waveform. In view of their mode of operation class C amplifiers are very non-linear and the output is rich in harmonics. This means that a filter is required to remove them before being passed to an antenna. The filter is tuned to the frequency of operation and only allows through the required frequency. In this way the required sine wave is extracted. In view of the fact that the amplifying device is biased beyond its cutoff point higher levels of drive are required for this type of amplifier when compared to a class A or class B amplifier. Class C amplifiers perform very well as frequency multipliers. Their output is particularly rich in odd harmonics, extending to many times the fundamental frequency. However, it is normal to select only the third or at the highest the fifth harmonic. This is achieved using the filter in the amplifier output circuit. If higher order harmonics are selected it is not normally possible to achieve sufficient rejection of the unwanted harmonics, and this results in unwanted signals being transmitted. 186 Newnes Guide to Radio and Communications Technology Output 0.001 Input Figure 6.5 A typical class C amplifier stage One very important aspect of a transmitter power amplifier is the amount of power that it can deliver to the antenna system. Radio frequency power is measured in exactly the same way as low frequency AC power. It is simply the voltage times the current, assuming they are in phase with one another. Assuming that the antenna load 