Introduction

In the past, design techniques for setting bias conditions on RF power amplifiers required the use of either an unreliable mechanical potentiometer or a "hammer-and-nut" solution, typically using an 8-bit DAC. The former is extremely unreliable, and often makes nonsense of MTBF calculations. The latter approach is relatively expensive, since the designer will make little or no use of the DAC performance capabilities that they are paying top dollar for.

An innovative new technique has emerged. Using a novel element called a DCP (digitally-controlled potentiometer,) designers can radically improve the cost and reliability of RF power amplifiers. This is particularly valuable in demanding applications such as cellular base stations using high-linearity and high-efficiency LDMOS technology.

The DCP is ideally suited for this application and exceeds the reliability, programmability and cost specifications of this requirement completely.

The new generation DCP contains no charge pumps and, therefore, no external noise is introduced on the wiper, even when programming. It is possible therefore to dynamically reprogram the DCP and still maintain CW (continuous wave) power on the amplifier with the wiper resistance executing a smooth transition between the old and new states. This is an important issue for re-biasing amplifiers that share different modulation standards, which consequently may require different operating points for optimum performance.

DCPs have established millions of problem-free field hours in commercial PAs installed in communication networks all over the world.

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