Biography
Enrollment Date: 2014
Graduation Date：2017
Degree：M.S.
Defense Date：2017.05.25
Advisors：Hanjun Jiang
Department：Institute of Microelectronics，Tsinghua University
Title of Dissertation/Thesis：Power Management Technologies for Envelope Tracking Power Amplifiers
Abstract:
Mobile communication technology is developing rapidly, and it is widely used in multimedia, big data, Internet of things and other fields. In order to improve the data rate on the basis of limited spectrum resources, non-constant envelope modulation is usually used. RF transmitters with non-constant envelope modulated signal must use linear power amplifiers. Traditional linear power amplifiers use fixed power supply. With the incensement of peak to average power ratio, a large amount of energy is dissipated in the form of heat. Envelope tracking technology is a power amplifier efficiency enhancement technology. Based on the input signal envelope, Envelope tracking system adjusts power amplifier supply voltage dynamically, thereby reducing power loss.
In this paper, based on the structure of traditional wireless transmitter, envelope signal calculation, shaping table, supply modulator and other modules are added to form an envelope tracking system. The power supply modulator needs to provide watt-level output power, while its bandwidth and efficiency must be high enough. Hence, the supply modulator is the focus of this paper. The modulator designed in this paper combines an efficient switching regulator with a broadband linear amplifier. With a voltage control loop and a control current loop, the modulator is able to meet the design requirements of envelope tracking power amplifier.
In this paper, the loss of supply modulator is deduced in detail. Hysteresis voltage calibration and MOSFET size adjustment scheme are proposed. The efficiency of the switching circuit is optimized according to the signal frequency and output power. A DC-DC converter is introduced at the power supply of the linear amplifier to improve linear efficiency. In order not to introduce an additional inductor, single-inductor dual-output (SIDO) structure is applied to the envelope tracking power modulator. SIDO converter design is difficult for unbalanced load. In this paper, two outputs are linear amplifier and switching amplifier, their load current varies greatly. In order to reduce the mutual influence between the two outputs, this paper adopts pseudo-continuous conduction mode (PCCM), which can control the output power and efficiency by controlling the freewheel time and freewheel current. Then, a hybrid control mode is proposed, which allows the two sub-converters to operate in the peak current mode and the comparator-based pulse skipping mode respectively, thus further enhancing the light load efficiency.
The power modulator designed in this paper is designed with UMC 0.18μm CMOS process, and the chip is still in the production stage. In order to verify the system performance, this paper uses ADS and Cadence joint simulation. Using the ADS to generate FDD-LTE test signals, and conduct system-level simulation. Then use Cadence to perform transistor-level simulation of the supply modulator. According to the gain characteristics of the power amplifier, fixed-gain shaping table is designed, which can simultaneously guarantee the system efficiency and linearity. The traditional power amplifier is compared with the envelope tracking power amplifier. The envelope tracking system designed can improve the average efficiency from 23.56% to 32.73%, while the EVM from 3.84% to 2.15%.