Digitally Equalized Doherty RF Front-End Architecture for Broadband and Multistandard Wireless Transmitters

This paper introduces a new architecture of frequency-agile Doherty power amplifier (PA)-based RF front-end. This architecture incorporates a baseband equalizer that is implemented using finite-impulse-response (FIR) digital filters to improve the performance of Doherty PAs when driven with large bandwidth and multiband wireless radios. Depending on the center frequency and bandwidth of the input signal, the FIR filters of the proposed equalizer are synthesized to compensate for the nonideal frequency behavior of the RF building blocks of the Doherty PA. It is shown that the proposed architecture enables the Doherty PA to operate with significantly improved power efficiency and linearity performance beyond its nominal frequency of design, which is suitable for multistandard applications. As a matter of fact, when driven with a 140-MHz bandwidth long-term evolution (LTE) signal centered around 2.30 GHz, the average efficiency of a 2.14-GHz Doherty PA prototype with the proposed digital baseband equalizer is enhanced from 33.5% to 44.7%. Moreover, its linearity performance in terms of adjacent channel leakage ratio (ACLR) is improved from -21 to -26 dB. In addition, when operated in concurrent dual-band mode with two 20-MHz bandwidth LTE signals centered at 1.96 and 2.34 GHz, the proposed Doherty PA enabled a reduction in dc power consumption by nearly 15%. Furthermore, it is demonstrated that the integration of the proposed baseband equalizer does not compromise the linearizability of the Doherty PA. To the best of the authors' knowledge, this is the first demonstration of the use of digital equalization for performance enhancement of RF Doherty PAs.