# Highly Linear 2.6 GHz Band InGaP/GaAs HBT Power Amplifier IC Using a Dynamic Predistorter
## 1 Introduction  
In recent years, as wireless communication systems have advanced, the need for higher data rates and more efficient management of increasing mobile traffic has become crucial. Small cells, designed to provide reliable coverage in confined areas, are now densely deployed to achieve high spatial and spectral efficiency at increased data rates, making them an essential component in 5G and future wireless networks [1]. For these networks, orthogonal frequency division multiplexing (OFDM) is utilized for both uplink and downlink communications. The OFDM-based modulated signal, characterized by high PAPR, imposes stringent linearity requirements on PAs, especially in small-cell applications [2]. Additionally, the PA consumes a significant amount of power; hence, high efficiency is crucial. To enhance power efficiency, PAs with low quiescent current such as class-AB have been utilized [3; 4]. However, operating with low quiescent current may lead to nonlinear distortion in both AM\(-\)AM and AM\(-\)PM. To achieve the required linearity specifications, particularly for modulated signals with high PAPR, the PA operates in a large Output Power Back-Off (OBO) region. As OBO increases to improve linearity, the PAE decreases. Consequently, there is a need for research into linearization techniques for small-cell PAs with low quiescent current to enable more efficient operations. Accordingly, various linearization techniques have been investigated, and external approaches such as digital predistortion (DPD) and analog predistortion (APD) have been widely applied [5; 6; 7; 8; 9]. While DPD has become the mainstream solution in recent years, it requires high-speed data converters, complex digital signal processing, and significant power consumption, which may limit its applicability in compact and low-power systems. In contrast, APD offers several advantages, including lower power consumption, real-time operation without latency, and simplified hardware implementation. The APD techniques include feedback [5], feedforward [6], and predistortion [7; 8; 9]. The feedback technique offers advantages in linearity and stability but results in reduced gain. Feedforward linearization achieves significant improvements in linearity but requires an additional error amplifier, complicating its integration as an IC. Predistortion linearization typically provides moderate linearity enhancements at a relatively low additional cost and benefits from its compact size, low complexity, and enhanced stability. As depicted in Figure 1, predistortion linearization operates by flattening the overall AM\(-\)AM and AM\(-\)PM characteristics through a predistorter, which possesses characteristics opposite to those of a nonlinear PA. Consequently, this results in linear signal amplification at the output. Generally, since PAs tend to exhibit gain compression characteristics, designing a predistorter with gain expansion characteristics is necessary to counteract the PA's gain compression [7; 8; 9]. However, when PAs operate with low quiescent current to boost efficiency, they exhibit a more dynamic response in their amplitude response (AM\(-\)AM) where gain expansion appears before gain compression. A similar dynamic is noticeable in the phase response (AM\(-\)PM). Specifically, the InGaP\(/\)GaAs HBT PA displays dynamic nonlinear characteristics that vary based on the configuration of its active bias circuits [3; 4]. The AM\(-\)PM characteristics also change depending on the harmonic impedance at the matching network and the active bias circuits. Therefore, a predistorter capable of compensating for these dynamic AM\(-\)AM and AM\(-\)PM characteristics of the InGaP\(/\)GaAs HBT PA is essential. Figure 1: Operation principle of the predistortion linearization technique. This paper proposes a predistorter that dynamically compensates for the nonlinear characteristics of the two-stage InGaP/GaAs HBT PAIC used in 5G small-cell applications.