New communication services and the use of complex waveforms have created a demand for highly linearized power amplifiers. Deviations from linearity are manifest by spectral distortion components in the output of these amplifiers—that is, undesired energy, not contained in the original signal, inside or outside the frequency band of interest. Linearization techniques seek to reduce these distortion components, restoring signal integrity and allowing an amplifier to operate at its best spectral and power efficiency for the specific application.
To date, a number of linearization approaches have been attempted. Most of these techniques, however, are useful only in circumstances where the input signals occupy low-percentage bandwidths (typically 10% or less) or do not change frequency rapidly over time.
For example, existing digital pre-distortion (DPD) techniques, which use non-linear transfer characteristics of an amplifier in order to generate inverse distortion products that can be supplied along with input signals to the amplifier, require time to converge and must have access to the baseband or digital intermediate frequency (IF) signal components (I and Q or IFI and IFQ). Moreover, digital signal processors, ASICS or FPGAs used in DPD-based designs tend to be limited in bandwidth (due to their clock frequencies) and processing capacity.
Current analog pre-distortion techniques are likewise limited in bandwidth and require adjustment when the input signal center frequency is changed. Adaptive linearization can eliminate the limitations to the change in the center frequency of these designs, but tend to be slow to respond to changes in system characteristics and can be very complex to implement.
Feed-forward linearization techniques are also complex to implement (often making them unsuitable for use with existing amplifiers), are limited in bandwidth and often do not provide significant gains in efficiency because of the relatively high power consumption of the error or auxiliary amplifier. Other conventional linearization solutions, such as power back-off schemes, envelope elimination and restoration designs, linear amplification using nonlinear components, and Cartesian feedback designs all have their own limitations as well. In addition, the increasing complexity of semiconductor-based power amplifiers has memory effect or inertia and therefore limits the use of converging linearization techniques in frequency hopping applications due to the reaction time to correct.