Analysis and Design of Doherty Power Amplifiers for Digital Pre-distortion Linearizer

In this paper, we propose the optimum design of microwave Doherty power amplifier for digital pre-distortion system. The design concept for the load modulation matching and its experimental optimization procedure were presented. The output matching circuit consists of a short lambda/4 microstrip line located between the carrier and peaking amplifier offset transmission lines and a 35 Omega transmission line for broad power matching. Two 2.14GHz Doherty amplifiers have been implemented using silicon LDMOS FETs. The RF performance of the Doherty amplifier-AB (a combination of a class AB carrier amplifier and a bias-tuned class C peaking amplifier) has been compared with the Doherty amplifier-B(a combination of a class B carrier amplifier and a bias-tuned class C peaking amplifier). The input signal applied in both case was WCDMA signal (34DPCH, 3-carrier, 6.5dB PAR). A combination of digital pre-distortion along with highly linear Doherty amplifier achieved an ACLR of -55 dBc at plusmn5 MHz offset frequency and -57 dBc at plusmn10 MHz offset frequency for a 3-carriers WCDMA signal. Thus the Doherty amplifier with digital pre-distortion gives an increased linearity as well as efficiency.