# A Review of Advanced CMOS RF Power Amplifier Architecture Trends for Low Power 5G Wireless Networks
## Advanced RF PA Architectures
### Envelope Tracking RF PA (ET/EER PA)  
Dynamic supply, or envelope tracking (ET), is an efficiency enhancement technique based on the older envelope elimination and restoration (EER) architecture that was proposed by [PERSON] in 1952, incorporating a modulator for shaping the PA power supply according to the low-frequency (baseband) envelope. A generalized diagram of ET/EER PA is presented in Figure 3.  
The overall efficiency of the ET PA system is roughly the product of the envelope amplifier efficiency and the RF power amplifier drain efficiency, which can be expressed as  
\[\eta_{\text{overall}}=\eta_{\text{Envelope amp}}\cdot\eta_{\text{RF PA}} \tag{1}\]  
Therefore, the design of the high-efficiency envelope amplifier is critical to the overall efficiency of the ET PA system. The envelope amplifier provides a dynamically changing supply to the RF PA to keep its efficiency higher in the back-off region. Traditionally, the supply modulator is implemented in the form of a linear regulator (LDO). However, since the linear topology has a wide bandwidth and little output ripple, but lacks efficiency it is therefore not well-suited for modern handheld wireless devices. The basic LDO regulator contains three main components--a differential amplifier with its output connected to a power transistor as well a negative feedback circuit to the amplifier. The power transistor acts as a variable resistor which limits the voltage at the PA based on the signal envelope. An alternative to the LDO modulator is a switching (DC-DC) one, forming a switching ET architecture. The efficiency of the latter architecture is high (over 80%), but additional noise is induced due to its switching nature and the architecture needs a high switching frequency to be used in high data-rate wireless devices [18]. The linear and switching ET architectures are two different but still traditional approaches, which paved the way for different architecture derivatives. These hybrids are intended to overcome the bandwidth limitation of the switching regulator and poor efficiency of the LDO at back-off. A hybrid regulator can be constructed either by a parallel or a series linear and switching regulator connection, providing a desirable combination of wide bandwidth, low ripple, and high efficiency. Other reported supply modulation methods, such as adaptive bias and multimode supply, can also been included in the ET/EER family; although they are not considered as mainstream ET/EER implementation techniques. Figure 3: Simplified ET/EER architecture diagram. A summarized comparison between the reported ET/EER architecture variations is presented in Table 1 with the architectures analyzed in detail in [19]. Other reported supply modulation methods, such as adaptive bias and multimode supply, have also been included, although they are not considered as the main ET/EER implementation techniques. The hybrid parallel architecture is one of the most popular variations of the ET modulator across multiple papers [19; 20; 21] as it provides different approaches to efficiency, linearity and noise improvements. A summary of papers reporting CMOS and BiCMOS ET/EER PA research results and parameter improvement solutions, utilizing all architecture variations mentioned in Table 1, is presented in Table 2. The latter summary reveals that ET/EER architecture PAs, similar to classic DPAs, are narrowband. Even if the PA itself is wideband (ex. hundreds of megahertz), the overall bandwidth is limited to the supply modulator, which becomes a bottleneck. It can be seen that, at frequencies below 1 GHz, the reported signal bandwidth can reach 20 MHz or even 40 MHz. With the increase of carrier frequency, signal bandwidth (\(BW\)) drops to 5 MHz. The output power and supply voltages are in the range of portable device specs with the overall system power added efficiency (_PAE_) of 22-48%. Many papers have reported the use of switched converters in both EER and ET configurations improving the efficiency of the PA in the range 5-20% compared with the classical amplifiers.