Distortion

It is also common to modify the gain of an amplifier operating class C so as to produce amplitude modulation. If done in a linear manner, this modulated amplifier is capable of low distortion. The output signal can be viewed as a product of the input rf signal and the modulating signal.

The development of fm broadcasting improved fidelity by using a greater bandwidth which was available in the vhf range, and where atmospheric noise was absent. fm also has an inherent ability to reject noise, which is mostly amplitude modulated. Valve technology suffers high-frequency limitations due to cathode-anode transit time. However, tetrodes are successfully used into the vhf range and triodes into the low GHz range. Modern fm broadcast transmitters use both valve and solid state devices, with valves tending to be more used at the highest power levels. fm transmitters operate class C with very low distortion.

Today's digital radio that carries coded data over various phase modulations (such as gmsk, qpsk, etc.) and also the increasing demand for spectrum have forced a dramatic change in the way radio is used, e.g. the cellular radio concept. Today's cellular radio and digital broadcast standards are extremely demanding in terms of the spectral envelope and out of band emissions that are acceptable (in the case of gsm for example, −70 dB or better just a few hundred kilohertz from center frequency). Digital transmitters must therefore operate in the linear modes, with much attention given to achieving low distortion.