Study of innovative GaAs FET amplifiers
Student thesis: Doctoral Thesis
Related Research Unit(s)
Amplification is one of the most basic but important operation in today’s radio frequency/microwave circuits. The performance of an amplifier can be accessed by many parameters such as linearity, bandwidth, power handling, efficiency and size. Linearity is one of the major concerns in the design of RF amplifiers. In order to fulfill the requirements of linearity, backing-off the amplifier is a common solution since it can be easily achieved without a re-design process. However, this solution is at the expense of efficiency which is not preferred in mobile units as this would significantly shorten the battery life. Several linearization techniques such as pre-distortion and post-distortion can be employed to improve the amplifiers’ linearity and efficiency. In this thesis, we will discuss a linearizer that compensates the non-linearity of gate-source capacitance yielding a remarkable reduction in intermodulation distortion (IMD) for FET amplifiers. Volterra series analysis is used to explain the improvement in IMD between the original amplifier and the linearized amplifier. A broad bandwidth operation seems to be the trend for today’s wireless communication products. For example, most mobile phones are now required to cover multiple bands. Therefore, it is reasonable to predict that in the future, amplifiers will need to operate over an even wider bandwidth. Distributed amplifiers are well-known for their wideband characteristic so it should be a good choice for our application. Although it can give wideband amplification, it suffers from IMD problem that restricts its application in commercial products. Previous works on distributed amplifiers focus on its bandwidth performance but seldom consider the possibility of applying linearization to improve the IMD. Therefore, a central-feed distributed amplifier is proposed to reduce IMD without sacrificing bandwidth. Besides wideband operation, the size of amplifiers is also an important factor as the trend for current mobile products is towards smaller and smaller form factor. In the past decade, MMICs, or Monolithic Microwave Integrated Circuits, have enabled a significant size reduction. In this thesis, a compact front-end transceiver is proposed that would be suitable for applying to MMIC. This is achieved by using a switchless bi-directional distributed amplifier which can avoid the degradation of the system performance encountered with the insertion loss due to the switch. Also, the size is more compact than the traditional one as the switch is omitted. Furthermore, when it is combined with a distributed structure in the front-end, it can provide identical amplification for both transmission and reception over the current 2G/3G frequency range of 0.8-2.2 GHz.
- Transistor amplifiers, Gallium arsenide semiconductors, Field-effect transistors