Analysis of Gain Variation With Changing Supply Voltages in GaN HEMTs for Envelope Tracking Power Amplifiers

Abstract

Envelope tracking (ET) is a promising power amplifier (PA) architecture for current and future communications systems, that uses dynamic modulation of the supply voltage to provide high efficiency and potentially very wide bandwidth over a large dynamic range of output power. The dynamic nature of the supply voltage can lead to a problematic variation in transistor gain however, particularly in GaN HEMTs. This paper describes and analyses this behaviour and the detrimental effect it can have on ET PAs. Contributing factors and origins of gain variation are described in detail along with how, for the first time, meaningful comparisons can be made between different devices. Using these guidelines, gain variation is shown to be a widespread issue effecting most GaN HEMTs presented in literature. To allow an analysis of the intrinsic device behaviour, an extended transistor model is developed that takes the effect of gate and source field plates into account. This model is refined using measurement data and used to demonstrate the fact that the parasitic gate-drain capacitance (C GD ) is the main contributor to the small-signal gain variation; a significant part of the overall gain variation. Based on this knowledge, possible strategies to reduce gain variation at the transistor technology level are proposed, allowing the optimisation of GaN HEMTs specifically for ET PAs. One identified strategy involves reducing the length of the gate field plate, and is shown to be a viable approach to reduce the gain variation in GaN HEMTs, albeit at an increased RF/dc dispersion.

Modulated rapid load pull system for the emulation of envelope tracking power amplifiers

“…4. This measurement clearly shows a gain variation with applied drain supply voltage, a behaviour prevalent in many GaN HEMTs as shown in [10]. Three different tracking trajectories or shaping functions were designed that demonstrate the usefulness of the integrated system as demonstrated in [11].…”

Design of an efficiency enhanced wideband Doherty power amplifier based on synthesising of a modified harmonic‐control load modulation network

“…High efficiency [4–6], the wide operating band [7], extended peak‐to‐average power ratio (PAPR) [8–10], and high linearity [11] are considered the main challenges in wireless communication infrastructure. Several strategies can handle the efficiency enhancement in commercial base stations, such as envelope tracking, and the technique of out‐phasing along with their capabilities to achieve high PAPR [12–15]. Compared to the other techniques, the Doherty configuration is widely used in wireless applications because of its ease of implementation and achieving high efficiency at back‐off [16–39].…”

Design of a Wideband Doherty Power Amplifier with High Efficiency for 5G Application

“…In order to increase back-off efficiency, various techniques have been proposed, both at the PA and at the system level, such as the Doherty power amplifier [13,15] and the Chireix outphasing [16,17] PA, based on the load modulation concept [18], or Envelope Elimination and Restoration (EER) [19,20] and Envelope Tracking (ET) [21,22] techniques, based instead on supply modulation. A combination of both supply and load modulation can be also exploited to maximize the achievable back-off efficiency [23,24].…”