When a power amplifier (PA) is presented with a multi-tone signal at its input, it will amplify the desired signal as well as generate unwanted intermodulation (IM) terms (Figure 1a). This non-linear distortion increases as the PA approaches its saturation point and will vary in nature based on operating conditions and from PA to PA. To achieve the desired linearity at the PA output (without predistortion), the PA must be operated with significant backoff from its saturation point (PSAT(3dB) in Figure 2a). Operation in backoff means that the PA's maximum output power level must be reduced so that the entire signal is within the linear region of the PA transfer curve. However, the PA's efficiency (PA's ability to convert DC supply power into RF energy) decreases as the PA's operating point is lowered further away from its saturation point (Figure 1b). Efficiencies of 8% or less for a Class AB PAs are not unusual in order to accommodate the signal’s peak-to-average ratio (PAR) and the additional backoff required to meet the system linearity requirements.
Figure 1a. Intermodulation terms generated by PA
Figure 1b. Relationship between output power, efficiency and distortion
Considering that the most popular linearization method by far for Class A/AB PAs transmitting 20W average power and below is operation in backoff, for these applications active linearization can provide very compelling benefits. Active linearization techniques, including digital predistortion (DPD) or RF predistortion (RFPD), allow the transmitter to operate close or even slightly above its PSAT - PAR operating point (Figure 2b). Both use predistortion techniques where a correction signal is injected at the PA’s input in order to reduce the overall distortion at the output of the PA (Figure 3a and Figure 3b).
Sidebar: Note that no predistortion solution can correct signals whose peaks extend much past the PA's saturation point as the information becomes more difficult, or even impossible, to recover as the amount of signal clipping increases. Pushing the PA past its saturation point is ultimately a system design decision that is based on many factors including margin to the adjacent channel leakage ratio (ACLR) specification, spectral emissions mask (SEM) specification and/or error vector magnitude (EVM) requirements for example.
Figure 2a. Unlinearized performance of PA with no predistortion (operation in backoff)
Figure 2b. Linearized performance of PA with predistortion enabled
Figure 3a. PA output characteristics without linearization
Figure 3b. PA output characteristics with predistortion linearization
Using active linearization, a Class AB PA can achieve 3 dB to 6 dB of additional (linearized) output power and improve its efficiency by 2X to 4X. Compared to operation in backoff, active linearization enables the final stage amplifier, power supply, cooling elements and operating costs to be reduced by half or more. Maxim's RF PA linearizer efficiency calculator clearly demonstrates that for all but the lowest average output power Class A/AB PAs, the cost of the predistortion solution can be easily recovered in less than 2 years of operation just due to lowered electricity usage. Considering the added benefits of lowered PA device cost, lowered power supply cost and lowered cooling costs, predistortion offers a very compelling value proposition.
In systems requiring wide signal bandwidth like LTE, or in wideband multi-carrier/multi-protocol systems, active linearization is the only option if the PA cannot reach the desired linearity regardless of the amount of backoff applied. In these systems, active linearization is required to pass regulatory radiated emission testing and meet project requirements.
Finally, systems requiring an improvement in efficiency (beyond what is achievable using a Class A/AB PA) will use more advanced PA topologies like a Doherty configuration. The advanced topologies depend on predistortion solutions to meet their system linearity requirements.