Solid-state RF power amplifiers - Classes, system and technologies

This article and the following [E 1 611] deal with solid-state RF power amplification. The fields of application concern all systems that have to deliver a significant output power for the product under consideration. They can be roughly divided into two categories, according to linearity constraints:

• non-linear applications, such as constant-envelope telecommunications, pulsed devices (radar, medical) and industrial applications;

• linear applications that are increasingly essential to modern telecommunications services.

The frequency range covered here extends from a few tens of kilohertz up to today's technological limits, i.e. over a hundred gigahertz. In most applications, the power amplifier sub-assembly excites an antenna designed to transmit electromagnetic waves. In industrial applications at lower frequencies, the system's output is usually a solenoid inducing a magnetic field.

For our purposes, the notion of power is quite relative: in the industrial sector, for example, we're talking about tens or hundreds of kilowatts, whereas in some areas of telecommunications, output power can be as low as a watt (cell phones, for example).

In all cases, however, power is involved in the sense that the amplification sub-assembly consumes most of the energy required to operate the complete system, and must therefore be the focus of particular attention during design, so as to optimize the consumption budget and degrade signal definition parameters as little as possible. These two aspects are common to all power amplification applications, regardless of the absolute powers involved.

If these criteria are a constant, we'll see that the proposed solutions can differ, essentially for technological reasons, depending on the absolute power and frequency ranges of the systems.

These particularities have two fundamental consequences for the design of power amplifiers: on the one hand, optimizing conversion efficiency from the power supply to the radio-frequency signal, and on the other, taking non-linear distortions into account. A third aspect, underlying power implementation, concerns the understanding of thermal phenomena and the treatment of power dissipation.

Compared with the amplification of small signals, where dissipation remains low and distortion negligible, power amplification...