Low cost, low delay UMTS power amplifier using digital-controlled adaptive analog predistortion

A low cost power amplifer (PA) is described that is suitable for UMTS applications comprising single carrier or two adjacent carriers where the available input signal is RF and the allowable insertion delay is low (less than 30 ns). Digital-controlled adaptive analog predistortion, used to linearize the PA, is implemented at minimal cost by exploiting analog computing and high levels of integration found in commercial parts. Memoryless distortion is corrected by adapting a small number of coefficients with the goal of making the AM-AM and AM-PM curves flat across the range of input envelope levels. The prototype amplifier is shown to meet the UMTS spectral mask for an output power level of 30 Watts. The measured distortion cancellation is 17 dB at the 4 MHz offset from the carrier. The predistortion function has a polynomial behavior implemented indirectly using the product of first-order functions. This novel structure requires less dynamic range than a direct polynomial making it suitable for analog computing.