Andrew Wright, Director, Product Research
Oliver Nesper, DSP Design Engineer
1．Introduction
Non-linearity is a fundamental property of high power RF semi-conductor transistors.
Consequently, any amplifier design approach will be burdened by the management of non-linear spectral regrowth and the degradation of signal int. Figure 1 below illustrates the incumbent feed forward approach. Operation is intrinsically quite simple, with a second error amplifier and reference signal cancellation circuit being utilized to extract and amplify only the distortion components created by the main amplifier. The balanced output of the error amplifier is subtracted from the output of the main amplifier to leave a near perfect signal. In practice this approach works very well, but it is encumbered with the utilization of a second amplifier which often consumes exactly the same amount of power as the main amplifier. This significantly limits the efficiency of the assembly. Furthermore, to ensure that the circuit provides a significant reduction in distortion products the main and error loops have to be critically adjusted to ensure distortion cancellation occurs. This is a complex analog circuit design task and represents a major issue when cost reductions and increased volume production is to be considered.
Figure 1 Feed Forward Amplifier Topology
In contrast, digital predistortion is a baseband signal processing technique that eliminates the analog manufacturing complexity of feed forward amplifiers. Furthermore, because the error amplifier is eliminated, the efficiency of the system is dramatically improved because a single class AB amplifier is required. Importantly, volume production issues are eased because the digital manufacturing environment is significantly more reliable than the integration and alignment of analog signal processing elements.
2．Amplifier Linearization via Digital Predistortion
Figure 2 illustrates the principles of operation of a digital predistortion system. The objective is to numerically generate, in the real time digital complex baseband signal processing domain, a nonlinearity that has a complimentary characteristic to that exhibited by the amplifier. If the baseband non-linearity is correctly constructed, then the overall system response to a signal that flows serially through the cascade of the baseband non-linearity and the amplifier will be that of a linear gain response. A linear gain response is highly desirable because it implies that distortion and spectral regrowth will not occur.
Figure 2 Basic Principles of Predistortion
Figure 2 is utilized to explain the basic principles of digital predistortion. Unfortunately, the simplified non-linear amplifier characteristic that is illustrated is not representative of a practical class AB amplifiers. Ordinarily, radio engineers are predominately concerned with both AM-AM and AM-PM distortion. These distortion mechanisms are referred to as memoryless and correspond to the belief that the instantaneous distortion observed at the output of the amplifier can be directly mapped to the instantaneous amplitude of the signal driving the amplifiers input. This distortion mechanism represents the bulk of the amplifier’s distortion characteristic. However, eliminating this bulk distortion mechanism is not sufficient to entirely eliminate all spectral regrowth generated by the amplifier because small, residual non-linear memory effects are present.
The exact definition of a non-linear memory effect is often subject to debate. However, a practical working definition is that the current output of the amplifier is affected by current and previous input stimuli. Moreover the relationship between the current output and the current and previous input stimuli is not restricted to being linear. In practice power amplifiers exhibit several distinct non-linear memory characteristics, which are distinguished by substitutionally different time constants.
Following the basic principles of predistortion, if a linear system is to be constructed from a cascade of non-linearities and the amplifier is classified as a weak Volterra kernel then the complimentary non-linearity will also require the construction of a Volterra kernel. This is the very essence of the PMC-Sierra PALADIN Predistortion product family. Figure 3 illustrates the amplification of a 1x, 2x, 3x and 4x WCDMA carrier system’s occupying 5 MHz of signal bandwidth per carrier by a system employing a raw class AB amplifier and a system utilizing memoryless and enhanced memory based predistortion approaches. Clearly the spectral regrowth performance as measured by the adjacent channel power ratio measurements, see Table 1, indicates memory based predistortion provides a significant advantage over traditional basic predistortion approaches. This is particularly advantageous when considering 20 MHz systems.
Table 1 Summary of ACLR Performance
Figure 9 Comparative Linearization Performance of 1x, 2x, 3x and 4x Carrier systems
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