Design and analysis of ultra-broadband distributed power amplifiers for multi-platform communications
Thein, Than Tun
Date of Issue2014
School of Electrical and Electronic Engineering
Positioning and Wireless Technology Centre
The growth in wireless communication, the evolution of applications in smart mobile devices, the elimination of cord clutter from devices and different spectrum allocations around the world are calling for integration of multiple communication platforms in a single device. Implementing multiple arrow-band amplifier chips with corresponding matching networks and biasing circuitry to fulfill the requirements will be a complicated way. This indicates a need for ultra-roadband amplifier, which can operate over a wide range of frequency to eliminate various individual narrowband amplifiers. Moreover, one major trend in future applications is the increasing demand for more power over a very wide bandwidth. Hence, this thesis aims to analyze the necessities and develop a distributed power amplifier, which is often a constriction in the transmitter unit, for multi-platform systems. First of all, some of the existing bandwidth enhancement techniques in the literature are reviewed and their advantage and disadvantages are examined with detailed studies on the distributed amplification technique. The review is extended to contemporary device technologies, thermal issue, equivalent circuit model, active cell configuration and biasing. State-of-the-art techniques used to improve the distributed power amplifier (DPA) performances are also summarized. As the aim is to develop a broadband power amplifier, GaAs based hetero-junction bipolar transistor (HBT) is selected for this application. Poor thermal conductivity and higher power dissipation inside the HBT make thermal characterization unavoidable. Both steady state and dynamic thermal behaviors are analyzed in detail. Modern high speed communication systems use OFDM and QAM signaling for high system capacity resulting in large peak to average signal envelope and thus, dynamic self-heating effect become an important issue for PA. The short-falls of current thermal models are addressed and a new model together with numerical technique capable of predicting highly fluctuating dynamic junction temperature of a power HBT is developed. The new model can cover thermal cutoff frequency range of up to 30MHz, with the modulated carrier frequency having the pulse width as short as 10ns and power levels of up to 10dB higher than the power dissipation under class-A operation of the HBT under consideration. The numerical technique which uses frequency domain dynamic thermal analysis is applicable for a wide range of signal types, power levels, frequencies, substrate position and material properties. A multi-section thermal network model is proposed and verified for incorporation into compact simulation software such as Agilent's Advance Design Systems for transient thermal simulation. Unilateral model for the transistor is often used as a building block for DA design, particularly when design formulae are to be developed. The validity of using simplified and Miller's approximated unilateral model for the transistor in high frequency and high power amplifier is analyzed. It was found that the two unilateral models are not accurate enough and hence a modified unilateral model is introduced. The detail steps of parameter extractions for the modified transistor unilateral model are presented. The model is validated with measurement results at various biasing conditions; 3mA to 150mA collector currents with Vcc = 1V and V_cc = 5V. The model is able to accurately predict input reflection coefficient, forward transmission coefficient and output reflection coefficient of the transistor over 250MHz to 30GHz frequency range. The developed transistor unilateral modeling is then used to derive close form design formulae for artificial transmission lines incorporating the transistor as gain unit. The formulae take into account all intrinsic and extrinsic elements of the transistor. Using these formulae a highly efficient DPA is designed and fabricated using WIN Semiconductors Corp's H02U-41 InGaP/GaAs HBT technology. For thermal stability every finger in the transistor cells is ballasted at the base. Dual-feed technique is applied for power added efficiency (PAE) improvement. The DPA is biased from 4V supply and achieved average output power of 28.5dBm over 2GHz to 6GHz. Average PAE across the pass band is better than 30\% and the highest PAE is 43.5\% at 3.5GHz. However, the gain of the amplifier is relatively low and driver stage amplifier would be needed. This would compromise the amplifier PAE performance. Hence, an analysis is carried for high gain DA. Specifically, collector line attenuation is carefully examined and a technique is developed to control the output impedance using external components. The concept is realized through cascode-configuration of the transistors. Using this technique, the relatively low output impedance of the HBT cascode cell is increased 3 times. As a result, the attenuation through the collector line is significantly reduced. The redesigned DPA incorporating this technique has small-signal and large signal gain of more than 20dB over 500MHz to 6GHz bandwidth and 17dB over 500MHz to 10GHz bandwidth with very good reflection coefficients. Without any other optimization techniques, significant PAE improvement can be observed for this 28.5dBm output power amplifier.
DRNTU::Engineering::Electrical and electronic engineering::Wireless communication systems