Design of a wideband microwave power amplifier using non-linear device model

In this MSc. Thesis, a design metodology of a wideband microwave power amplifier using non-linear device model is proposed in details over a design example. And design steps are explained as could be as in details to be usefull for microwave power amplifier designers who encounter with similar issues. It can be considered as a motivation point that usage of a non-linear device model which is also take part on the name of this study. Because modelling and studying such a non-linear device can not be possible via linear equation sets and most of the time optimization algorithms can not convergence on the solution. In addition if the desired bandwidth is wide, it will be more difficult and nearly impossible to solve equations and usage of the optimization algorithms will be useless due to the convergance problems. For these reasons, such a study is done using a computer-aided and user-driven method and design systems and design steps are explained in details as could be as possible. The Simplified Real Frequency Tecchnique (SRFT) which is popular in literature with its ability of broadband matching solutions also implemented in the design steps. And at the final of the work, a wideband microwave power amplifier is designed and prototyped in success. On the path of production, post layout simulation called "cosimulation" is implemented and realistic vendor models are used in the cosimulation to observe realistic effects. The simulation results of the designed circuit are seen sufficiently satisfying. After ideally designed and simulated with cosimulation tool, the circuit is prototyped and measured. Measurement results are in aggrement with simulation results which shows the prototyp circuit works well. Classical microwave circuit design relies on the linear network parameters such as s-parameters due to their capability of characterizing the behavior of any linear circuit successfully. Furthermore, the classical design and analysis of linear microwave transistor based circuits are based on the simple analytical approaches which utilize the transistor s-parameters. So that it is not complicated to provide an analytical solution for the input and output variables and analytically determining fundamental parameters as the stability factor, the power gain contours or the input-output matching conditions. But all these happens when the microwave devices operates in their linear regions. S parameters depends on super position principle and only passive microwave circuits such as filters, couplers, power dividers, etc., and active microwave devices such as amplifiers that operates small signal operating points can be characterized in accuracy with them. If the amplifiers input signal level increases over its linear region, super position principle becomes no more useful, distortions such as gain compression, harmonic production and entermodluation begins to form. For the last few decades, the linear system theory was utilizing in success on the design of microwave circuits. This linearity approach was modelling the linear microwave devices succesfully and characterizing them consistently with the support of vectorel network analyzers (VNA). Nevertheless, todays systems of space and aeronautics, social media and mobile personal communication devices make it inevitable to operate the active network device in its non-linear region to improve efficiency and bandwidth. So on this path, usage of linear theory gives no more use and in becomes a neccesity to study the non-linear parameters and characterizing such devices with them. Some examples of these kind of non-linear applications can be found on RF power amplfier studies which requires high power and mostly operates its non-linear region. It is important to design of microwave power amplifiers with high power level and it depends to the transistors non-linear model to low cost design and high efficiency. One of the main problem in the microwave circuit design with large signals is the difficulty of the design with complex, non-linear devices to achieve desired delivered power level, desired efficiency and good reflectances. Since the non-liear device can not be modeled with linear equation sets, several design steps are required to achieve desired criterias on the input and output regions. And these processes take importance for time efficiency and performance. For this reason it is important that to select a suitable optimum design methodology for the desired circuit. In the study desired design criterias are achieved for the microwave power amplifier circuit which contains non-linear device model. On the design process, different kind of design techniques in the literature are used to solve different problems encountered on the different steps of the design. The thesis contains six main chapters. In the first part, the introduction and problem definition take place. The second chapter of the study takes a brief look to the non-linear design models and discuss about why they are important and how they can be obtained comparing them small signal parameters. In the third chapter, power amplifiers and their design techniques are introduced. Some important parameters about power amplifier designs are explained in the chapter three. Also in this chapter, broadband matching problems and some common matching techniques are explained. The fourth chapter, which is the longest part of the study is the section where the design of broadband microwave power amplifier with non-linear device model started. In this section, the matching techniques are implemented, reflectance levels of the circuit are reduced, flat gain is obtained and all desing criterias are achieved over the ideal case and the ideal design is completed with high success. In the fifth chapter, the designed ideal circuit in chapter four is prepared for the prototype production. The layout design is made and cosimulations are performed. According to the results, the post layout optimizations are done and also good results are obtained for the post layout design. Also in this chapter, the prototyped circuit and its measurement results are given. It is observed that the measurement results are in a good agreements with simulation results. The last section, chapter six is the conclusion part. In this section, the completed study is overviewed and required discussion and future works are given. In the final of the work, a microwave power amplifier with a ultra-wide bandwidth from ~0.8GHz to ~3.2GHz, and has nearly 4-6Watt output power on the operating bandwidth is achieved. The harmonic performance shows also good results. Circuit is includes Cree's Gan HEMT power transistor and designed with it's large signal model. Input and outpt matching networks are designed with using several methods in combination. All the design steps are detailed in related section. However the design of a wideband microwave power amplifier using non linear device model is explained over a design example. The study and techniques that used can be generalized for different design criterias. And it is hoped that this study will help power amplifier designers challenging with wide bandwidth and non-linear devices.