Analysis of third-order intermodulation distortion in common-emitter BJT and HBT amplifiers

Abstract

This transactions brief presents an electro-thermal Volterra model for calculating third-order intermodulation distortion (IM3) in common emitter (CE) bipolar junction transistor (BJT) RF amplifiers. The model includes nonlinearities caused by input-output cross products, which previous studies have tended to overlook, in spite of their significance for RF devices. The nonlinear I-V and Q-V sources of the model are presented also as functions of temperature to analyze how distortion is affected by dynamic temperature variations inside the device. The model is organized to facilitate the recognition of different IM3 components, especially those arising from out-of-band second-order distortion voltages. In addition, this transactions brief presents a technique for characterizing the nonlinearity coefficients of a RF power BJT and studies the behavior of intermodulation distortion as a function of bias point and of out-of-band impedance matching. Optimum bias and matching points are established for the test amplifier, and a good correlation is demonstrated between the calculated and measured data. Finally, this transactions brief shows that some serious memory effects cannot be seen when simulated using the traditional Spice BJT model, but can be detected using the polynomial Volterra model.