With the introduction of the 3G standards, the implementation of the analog radio transceiver circuitry has become easier with much of the channelization and filtering being performed within the digital domain. This has not been the case for the power amplifier design — the introduction of higher-order modulation schemes (3G, EDGE), the linearity of the power amplifier has been a key area of interest. With GSM, power amplifier design was simplified by the fact that the modulation scheme employed had a constant RF envelope. This allowed GSM to achieve low distortion, high-efficiency power amplification. This is not the case for 3G applications where high linearity and low distortion are necessities — impacting heavily on efficiency. In such situations circuits known as linearizers are employed that assist the power amplifier to achieve higher power and efficiency for the same level of distortion.

The main goal of the power amplifier is to produce the highest RF power possible while maximizing efficiency — with any resultant distortion products within acceptable limits.

The problem with this approach is most amplifiers in mature networks operate at less than the maximum-rated RF power. For example, in a mature GSM network, the power amplifiers may be backed off by 3 dB from full power. This is likely for emerging standards such as EDGE and 3G where the deployment of infrastructure will be initially at full power and later reduced as more cells are added to support extra capacity. In addition, engineering margins need to be added to ensure that the equipment will be compliant at full-rated power over all conditions. Finally, the RF power required during quiet periods of the day (early hours of the morning) is likely to be much lower than that required at peak periods. Therefore, the average amplifier power delivered may be significantly less than the maximum rated.

Unfortunately, linearizers that work best at maximum-rated power, offer little benefit at lower RF powers. This results in power amplifier efficiencies that are lower than expected in operation with a significant overhead of redundant linearization circuitry.

What can be done to improve efficiency for lower RF powers below the maximum rated for the power amplifier?

The answer is to ensure that the linearizer provides good benefit over a range of powers. This can be achieved using adaptive bias techniques. These can vary the quiescent current and supply voltage across the power amplifiers in sympathy with the average or instantaneous RF power requirements.

By employing a hybrid combination of adaptive bias and linearization, it is possible to improve amplifier efficiencies over a range of RF powers for a varied type of modulation scheme. In addition, the linearizer provides an error signal indication that can be used to determine the optimum bias conditions for the power amplifier while maintaining an acceptable distortion.

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