A predictive model describing the upconversion of 1/f noise into AM sideband noise in HBTs

Abstract

An analysis of AM noise in AlGaAs-GaAs HBT amplifiers is presented. A predictive model is introduced which allows calculation of AM noise sideband power alongside carrier signals in HBT amplifers as a result of the upconversion of low frequency noise. These sidebands are an unavoidable result of the nonlinear components in microwave oscillators and power amplifiers which causes upconversion of intrinsic low frequency 1/f noise into amplitude modulated (AM) and phase modulated (PM) sidebands about the carrier. AM and PM sidebands degrade the spectral purity of the carrier which ultimately limits system performance in communication systems. Many works have described the upconversion process in microwave oscillators with its dominant phase noise, but less attention has been paid to the AM noise of power amplifiers. This work presents the theory of such noise in AlGaAs-GaAs HBT amplification and details a predictive model that allows calculation of AM noise sidebands for any HBT device.