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

Abstract

One of the most important requirements that RF and microwave power amplifiers designed for radiocommunication systems must meet is an optimum power added efficiency (PAE) or an optimal combination of PAE and linearity. A harmonic active load-pull system which allows the control of the first three harmonic 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 "nonlinear network" analyzer. For the first time, optimized current/voltage waveforms for maximum PAE of microwave field effect transistors (FET's) have been measured. They confirm the theory on high efficiency microwave power amplifiers. The proposed load-pull setup is based on the use of three separated active loops to synthesize load impedances at harmonics. The measurement of absolute complex power waves is performed with a broadband data acquisition unit. A specific phase calibration of the setup allows the determination of the phase relationships between harmonic components. Therefore, voltage and current waveforms can be extracted. The measurement results of a 600 gate periphery GaAs FET (Thomson Foundry) exhibiting a PAE of 84% at 1.8 GHz are given. Such results were obtained by optimizing the load impedances at the first three harmonic components of the signal coming out of the transistor. Optimum conditions correspond to a class F operation mode of the FET (i.e., square wave output voltage and pulse shaped output current). A comparison between measured and simulated current/voltage waveforms is also presented.

Waveform Measurements - The Load-Pull Aspect

“…In [12], some examples of waveform-load-pull measurements of GaAs power FETs in class-F biasing are described. First, the authors calculate the optimum fundamental and harmonic loading impedances for maximum fundamental output power from a nonlinear FET model.…”

“…Recently, a few researchers started to investigate the combination of time-domain and load-pull measurements [11][12][13][14]. Classical load-pull setups can be subdivided into two groups which use passive or active tuning components, respectively.…”

An analytical approach to obtain optimum load impedance using X-parameters^®

“…Load pull measurements can be used directly for power amplifier design to achieve maximum output power delivered or maximum PAE, by determining the optimum impedance terminations required for device operation. When designing power amplifiers in nonlinear operating classes based on shaping and controlling waveforms, it is necessary to measure harmonic load-pull data to obtain optimum impedances at harmonic frequencies [3,4].…”

An active load-pull setup for broadband signals using digital baseband processing for the active loop

“…To overcome losses of the setup and/or to increase the range of realizable load impedances active loads [2][3][4] are often a must. Those systems synthesize a certain load by terminating the forward traveling wave coming from the DUT and generating a signal with modified phase and magnitude (to simulate a certain load impedance), which is injected by an auxiliary amplifier (the loop PA) as the backward traveling wave.…”