Analysis and optimization of third order intermodulation distortion mechanisms in AlGaAs/GaAs heterojunction bipolar transistors

The third-order intermodulation distortion (IMD3) mechanisms of HBTs (heterojunction bipolar transistors) are analyzed using Volterra series theory. The third-order nonlinear currents generated by the device nonlinearities are evaluated for this purpose. Second-harmonic loading is addressed in view of IMD3 optimization while, at the same time, maintaining high gain through conjugate matching at the fundamental frequency. It is shown that IMD3 depends on a complex process involving interactions between various nonlinear elements and is highly sensitive to C/sub bc/ generated nonlinear current. The interaction of the latter with the other HBT elements significantly affects the IMD3. Optimum IMD3 occurs at high second-harmonic reflection coefficients corresponding to open load conditions. An IMD3 improvement of up to 27 dBm can be obtained by proper loading.<<ETX>>