Design of Bandwidth-Enhanced Doherty Power Amplifiers for Handset Applications

Abstract

A quarter-wavelength impedance transformer as well as a number of other factors limit the bandwidth (BW) of Doherty power amplifiers (PAs). We utilize the lower Q of a quarter-wavelength transformer and propose a phase compensation circuit and an additional offset line to be incorporated into the matching networks for an enhanced BW of the Doherty PA. The quarter-wavelength transformer and the final output circuit have the same Q. Input dividing networks are also analyzed for operation of broad BW. The Doherty PA for long term evolution (LTE) applications is integrated into a 1.4 x 1.4 mm(2) die using an InGaP/GaAs heterojunction bipolar transistor (HBT) process. For an LTE signal with a 7.5-dB peak-to-average power ratio (PAPR) and a 10-MHz BW, the PA with a supply voltage of 4.5 V delivers a power-added efficiency (PAE) of 36.3% and an adjacent channel leakage ratio (ACLR) of -32 dBc with an average output power of 27.5 dBm at a frequency of 1.85 GHz. Across frequencies from 1.6-2.1 GHz, the PA performs with a PAE of more than 30%, a gain of more than 28 dB and an ACLR of less than -31 dBc at an average output power of 27.5 dBm while satisfying the standard spectrum mask. These figures verify that the proposed bandwidth enhancement techniques are effective for handset Doherty PAs.