The wireless communication industry has evolved over the years from an era of simple,low-power,low-data rate applications and systems to today’s systems with multiple-carrier transmissions,complex modulation schemes and higher power requirements to achieve greater spectral efficiency for numerous applications across various fields.RF power amplifiers(PAs),which form the most critical sub-components of transmitting systems,must therefore be able to effectively cope with varying signal characteristics while ensuring cost-effective and reliable transmissions to intended recipients.The main topic following in this research is listed below:1)Various types of power amplifier including linear and nonlinear classes are investigated in Chapter 2.The linear PA schemes are first explained in detail including voltage and current waveforms with their power efficiency and their power linearity performances.In Chapter 3 the design procedure of broadband,high-efficiency,high-power RF PA with a low gain variation over 100-500 MHz covering more than one octave bandwidth is proposed.This procedure is practically validated by using a 25-W LDMOS transistor from NXP Company as a case study,to credit to the proposed design procedure.The design procedure is systematically explained,and therefore,it can be used for design of the other linear PA classes.2)The design of nonlinear PA classes require to access to the intrinsic drain instead of virtual drain to investigate drain voltage and current waveforms certainly.For this reason,the nonlinear model of GaN HEMT is explored in detail in Chapter 4 and a novel method for extracting intrinsic and parasitic elements of a packaged transistor is proposed.In this model,channel length modulation is carefully modeled to be provided an accurate GaN HEMT model.By using this nonlinear model,the design on nonlinear PA such as Class-F/F-1 and Class-J will be facilitated.3)Class-F and Class-J are two kinds of nonlinear scheme of PAs which have investigated in the today’s system more than ever due to their proper compromise between power efficiency and linearity performance.Class-F PA with only third harmonic trap circuit is able to theoretically provide close to 88%power efficiency while Class-J is theoretically able to provide 78,5%of power efficiency by using two early harmonic components.A novel design procedure for a broadband Class-J PA is proposed which is able to provide high-efficiency operation with simultaneous high linearity ones over 1.6 GHz-2.6 GHz.This is practically validated using a CGH40006F HEMT.