Stability Analysis of a Microwave Power Amplifier using Pole Zero Identification Method

This thesis demonstrates the use of pole zero identification method to stabilize a 2.32 - 2.37 GHz class AB power amplifier. In addition, the thesis presents a procedure to obtain values of stabilization components. Power amplifiers are used to drive transmitting antennas with high power in applications such as RADAR’s, cellular base stations and RF-driven lighting. They operate at large signal level to achieve these high output power levels. This increases the risk of potential oscillations in power amplifiers, which are undetectable using conventional linear stability factors. The oscillations degrade amplifier’s performance and may cause interference and transistor burnout. Non-applicability of linear stability factors for large-signal operation has led to development of new stability analysis methods such as Ohtomo, NDF and AG. Ability to detect oscillation due to large-signal has been the priority of these methods. A drawback is that, they are either complex to use or not fully complete in stabilizing a power amplifier. A recent method based on pole zero identification is shown to be rigorous and simple to use. However, this method requires a special additional software STability ANalysis(STAN) to identify poles and zeros of power amplifiers. This work utilized a simulation template in ADS 2011 to design the 2.32-2.37 GHz class AB power amplifier. The template requires measured load pull data of the transistor used in the amplifier design. The realized design has met maximum performance in first trial with little optimization. This design approach is useful to circumvent modeling problems in power transistors. However, the stability analysis is highly dependent on the transistor model accuracy. The designed and constructed class AB power amplifier in this work is unconditionally stable for small-signal operation and potentially unstable for large-signal operation. The amplifier is able to deliver an output power of 140 Watts with 15.2 dB gain and 42.08% efficiency at 2.345 GHz. Load pull measurements (peak power, peak gain and peak efficiency) of the amplifier and transistor used to design the amplifier are compared for design evaluation