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Efficiency improvement in base station power amplifiers

Azad, Ehsan 2023. Efficiency improvement in base station power amplifiers. PhD Thesis, Cardiff University.
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Power amplifiers (PAs) are critical components in mobile base stations and are responsible for a significant portion of the overall power consumption. Due to the stringent design requirements and the high peak-to-average power ratio (PAPR) of modern communication signals, PAs are often energy inefficient, with a significant portion of the consumed power being dissipated as heat rather than being utilised for signal transmission. To address this issue, various efficiency enhancement techniques, such as Doherty, envelope-tracking, and load-modulated balanced amplifiers, have been developed. At present, Doherty PAs are the prevalent topology employed in mobile base station applications. In recent years, system level energy-saving techniques for base stations have attracted much attention. One such technique is average power tracking, which dynamically adapts the PA’s supply voltage and power output based on the number of users, as opposed to maintaining a static configuration at all times. This approach effectively reduces PA consumption during low traffic periods by operating in a lower power mode and increases output during peak traffic periods by operating in a higher power mode, resulting in more efficient energy utilisation. This thesis presents the design and development of a Doherty PA for tracking average power in applications with varying supply voltages at frequencies around 3.5 GHz. This work focuses on the characterisation and modelling of the behaviour of transistors in response to changes in supply voltage, while acknowledging the challenges in fully implementing this method, such as the implementation of a supply modulator and advanced signal processing. Load-pull measurement are widely used within the PA design community to characterise the large-signal behaviour of the transistors. In this work, load-pull measurement at different supply voltage were performed on 3W GaN HEMT die device. The collected load-pull data was used to develop a new DC-dependent Cardiff behavioural model which was capable to accurately interpolate the load-pull data with regard to the DC supply voltages. In addition, the model was verified against the load-pull data of 10W and 25W packaged GaN HEMTs which they were then used to design the multi-bias Doherty PA with a DC supply range of 30 V to 50 V. The main challenge in utilising a behavioural model for the design of Doherty PAs is modelling the dynamic characteristics of the auxiliary stage in both ON and OFF states. To overcome this, a modified version of the Cardiff model was implemented which incorporates input drive variation. Additionally, a technique utilising a hyperbolic tangent activation function was employed to seamlessly switch between small-signal and large-signal device responses. This enabled the design of a dual-input Doherty PA, whose performance was validated through measurement and simulation results. Furthermore, a single-input version of the Doherty PA was fabricated and evaluated, displaying excellent back-off performance at 30V and 50V supply voltage, with a 100 MHz 5G test signal. The efficiency of this device exceeded 47% and it exhibited an adjacent channel leakage ratio (ACLR) of less than -45 dBc when the digital predistortion (DPD) was applied. The thermal operating conditions and their effect on the performance of semiconductor devices are a critical consideration in the development of nonlinear models. In this thesis, a new temperature-dependent Cardiff model is developed using load-pull measurement data of two GaN HEMT dies over a temperature range of 25 ◦C to 100 ◦C. The validity of the model was verified by designing a class AB power amplifier based a 10 W packaged device. Comparison of the simulating and measurement results of the PA verified the accuracy of the model in predicting the device’s behaviour under varying temperature conditions. With the development of advanced efficiency enhancement techniques, it is now common to have multiple stages of amplification in a PA. Conventional methods for distributing the input signal to multiple stages involve the use of passive splitter components. However, providing separate inputs for each stage can offer increased flexibility for optimising PA performance. This thesis presents an unconventional application of the Cardiff model to predict the simulated output response of a dual-input load modulated balanced amplifier (LMBA). Initially, a smaller dataset with varying input trajectories was used to extract the model coefficients, which were then tested on a much larger dataset. The device response at different input trajectories was used to identify the optimal combination of input signals, considering their magnitude and phase, to achieve optimal performance. This demonstrates the capability and flexibility of the Cardiff model in predicting the response of multi-port nonlinear devices, such as the dual-input LMBA, and its ability to be used in system-level simulations.

Item Type: Thesis (PhD)
Date Type: Submission
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: 1) Power Amplifier 2) Behavioural Modelling 3) GaN HEMT 4) Load-pull 5) Supply Modulation 6) Transistor Characterisation
Date of First Compliant Deposit: 23 February 2024
Last Modified: 23 Feb 2024 16:56

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