Automatic control of constant highly reflective loads in active load-pull systems for the characterization of compression effects and IM3 in SSPAs

Optimization of trade-offs between power added efficiency and third order intermodulation (IM3) in transistors is the most critical point encountered in the design of communication power amplifiers. Power transistors are generally highly mismatched components. The characterization of such devices by load-pull techniques is a very difficult task. We have recently proposed a solution to this problem. It consists in using appropriate mismatched power sources to drive the device under test (DUT) during the measurement process [1]. Prediction of IM3 can be achieved by computing the measured single tone complex gain of the DUT for any load impedance of interest. Nevertheless, in active load-pull systems the load impedance doesn´t remain constant when the power level driving the input of the DUT varies. Therefore, it is necessary to compute measurement data files in order to extract the complex envelope gain function of the DUT. The modified active load-pull set-up proposed in this paper allows the control of any constant load impedance of interest. Any desired load impedance is automatically kept constant independently of the input driving signal of the DUT. In consequence, complex gain can be directly extracted from measurements and IM3 quickly predicted.