A Method for Deriving Large-Signal Input Impedance of Microwave Devices Using a Modified Source and Load Pull Technique

Microwave power-amplifier technology is no longer dominated by class-A FET designs. For example, GaAs HBTs offer high-efficiency microwave performance in class-B and class-C operation. When operated at lower conduction angles, device input impedance is often a strong function of input-signal amplitude. Knowing Zin for the expected bias and drive conditions is essential to first-pass success of input- and interstage-matching circuits. One widely-used, yet incorrect, method of estimating large-signal input impedance optimizes the input tuner for maximum power transfer. Assuming that this produces a complex-conjugate impedance match, the designer assigns the device under test an input impedance equal to the complex conjugate of the tuner´s s22. This procedure has triggered numerous unnecessary design iterations to correct the frequency responses of high-efficiency amplifiers. The authors present a technique for accurately calculating Zin of a device under test from tuner data obtained in a modified source and load-pull configuration. The new technique inherently improves the robustness of tuner-optimization algorithms, and also allows an input tuner to function for higher device reflection coefficients than previously possible.