Measurement and control of current/voltage waveforms of microwave transistors using a harmonic load-pull system for the optimum design of high efficiency power amplifiers

One of the most important requirements that RF and microwave power amplifie rs designed for radiocommunication systems must meet is an optimum power ad ded efficiency (PAE) or an optimal combination of PAE and linearity. A harm onic active load-pull system which allows the control of the first three ha rmonic frequencies of the signal coming out of the transistor under test is a very useful tool to aid in designing optimized power amplifiers, In this paper, we present an active load-pull system coupled to a vectorial "nonli near network" analyzer. For the first time, optimized current/voltage wavef orms for maximum PAE of microwave field effect transistors (FFT's) have bee n measured. They confirm the theory on high efficiency microwave power ampl ifiers. The proposed load-pull setup is based on the use of three separated active loops to synthesize load impedances at harmonics. The measurement o f absolute complex power waves is performed with a broadband data acquisiti on unit. A specific phase calibration of the set-up allows the determinatio n of the phase relationships between harmonic components. Therefore, voltag e and current waveforms can be extracted. The measurement results of a 600 gate periphery GaAs FFT (Thomson Foundry) exhibiting a PAE of 84% at 1.8 GH z are given. Such results were obtained by optimizing the load impedances a t the first three harmonic components of the signal coming out of the trans istor. Optimum conditions correspond to a class F operation mode of the FET (i.e., square wave output voltage and pulse shaped output current). A comp arison between measured and simulated current/voltage waveforms is also pre sented.