Browsing by Author "He, Jianqing"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
- Design of Class-E Radio Frequency Power AmplifierAl-Shahrani, Saad Mohammed (Virginia Tech, 2004-02-06)Power amplifiers (PA) are typically the most power-consuming building blocks of RF transceivers. Therefore, the design of a high-efficiency radio frequency power amplifier is the most obvious solution to overcoming the battery lifetime limitation in the portable communication systems. A power amplifier's classes (A, AB, B, C, F, E, etc), and design techniques (Load-pull and large-signal S-parameters techniques) are presented. The design accuracy of class-A power amplifier based on the small-signal S-parameters was investigated, where compression in the power gain was used as an indicator for design accuracy. The effect of drain voltage variation on the power gain compression has been studied in this research. The class-E amplifier has a maximum theoretical efficiency of 100%. It consists of a single transistor that is driven as a switch and a passive load network. The passive load network is designed to minimize drain (collector) voltage and current waveforms overlapping, which minimize the output power dissipation. Two L-band class-E amplifiers are implemented in section 5.3. One of them is a lumped elements based circuit and the other is a transmission lines based circuit. Both circuits show good performance (60% PAD) over a wide bandwidth (1.0 GHz). In section 5.4, lumped elements and transmission lines based X-band class-E amplifiers are presented. Both circuits show good performance (62% PAD) over wide bandwidth (4.8 GHz). A new technique to improve the drain efficiency of the class-E amplifier has been proposed. This technique uses two passive networks. One of them is in a series with the shunt capacitor CS and the other is in a series with the transistor's source terminal. This technique shows improvement in the drain efficiency, which jumps from 62% to 82%. Last few years have seen an increase in the popularity of the wireless communication systems. As a result, the demand for compact, low-cost, and low power portable (Single-chip) transceivers has increased dramatically. Among the transceiver's building blocks is the power amplifier. Thus, there is a need for a low-cost power amplifier. A 900 MHz CMOS RF PA with one-watt output power and a high power added efficiency (68%) is presented in chapter 6. This PA can be used in the European standard for mobile communications (GSM) handset transmitter.
- Finite difference time domain simulation of subpicosecond semiconductor optical devicesHe, Jianqing (Virginia Tech, 1993-11-05)An efficient numerical method to simulate a subpicosecond semiconductor optical switch is developed in this research. The problem under studying involves both electromagnetic wave propagation and semiconductor dynamic transport, which is a nonlinear phenomenon. Finite difference time domain (FDTD) technique is used to approximate the time dependent Maxwell's equations for full-wave analysis of the wave propagation. The dynamic transport is handled by solving the balance equations using the energy and momentum relaxation time approximation. Based on the structure of the device, a physical semi-analytical model is also developed for preliminary analysis. Simulation results in the device's subpicosecond responses including nonlinearity and overshoot. The validity of the method is verified by comparing the simulation with the published experimental results. The method can be extended to other devices as well.
- Investigation of transport mechanisms for n-p-n InP/InGaAs/InP double heterojunction bipolar transistorsHe, Jianqing (Virginia Tech, 1989-01-05)A more complete model for InP/InGaAs Double Heterojunction Bipolar Transistors (DHBT) is obtained in this thesis by physically analyzing the transport process of the main current components. The potential distribution of the energy barrier constitutes a fundamental analytical concept and is employed for applying the diffusion, the thermionic emission, and the tunneling theories in investigating the injection mechanisms at the e-b heterojunction interface. The diffusion transport is considered first for electron injection from the emitter into the base. The thermionic emission is applied properly at the point of maximum potential energy as one of the boundary conditions at that interface. A suitable energy level is selected with respect to which the energy barrier expression is expanded for the calculation of the tunneling probability. The first "spike" at the conduction band discontinuity is described as the potential energy for the injected electrons to obtain kinetic energy to move into the base region with a substantially high Velocity. The electron blocking action of the second "spike" at the b–c junction is also analyzed by considering the transport Velocity with which electrons are swept out of that boundary. Based on the material parameters recently reported for both InP and InGaAs, computations of the nI current components are carried out to provide à characteristics in good agreement with the reported experimental results.