A Wideband Analog Linearizer Design Method Based on Frequency-Dependent Gain and Phase Characteristics of Amplifiers

This article presents a design method of analog predistorter (APD) for Doherty power amplifiers (DPAs) based on minimum frequency-dependent gain and phase deviation to improve the output adjacent channel leakage ratio (ACLR). To prove this method, the gain and phase characteristics of APDs and PAs under the excitation of a broadband-modulated signal are analyzed by short-time Fourier transform (STFT). The condition necessary to accurately compensate for nonlinearities at wideband is obtained to validate and guide the high-performance wideband APD design. Furthermore, according to this necessary condition, the normalized deviation that integrates the frequency information is derived in conjunction, and used as the core to optimize the APD design and linearized performance prediction platform. An APD consisting of two T-networks in a series is designed as an example of an existing DPA. It has low circuit complexity and low power consumption, which can effectively improve the amplitude modulation (AM)–AM and AM-phase modulation (PM) of the DPA after optimization. At 3.55GHz, modulated signal measurements are conducted to compensate for the example DPA with a saturation power of 43.1 dBm. Under the excitation of an 80 MHz, 64-QAM modulated signal, the results show that the ACLR of the DPA output is improved by nearly 8.8/11.5 dB with the proposed APD, from −36.3/−32.6 dBc to −45.1/−44.1 dBc, at 35.1dBm power level. Moreover, the proposed APD still demonstrates good linearization performance at different output power back-off (OPBO) levels.