Frequency Analysis and Multiline Implementation of Compensated Impedance Inverter for Wideband Doherty High-Power Amplifier Design

In this paper, we present a frequency analysis of a compensated impedance inverter and its effective implementation in the context of high-power Doherty amplifier (DPA) design. On the basis of an idealized DPA model, we calculate the expected operative bandwidth and the corresponding maximum drain efficiency as a function of the modulated signal statistics. In addition, we introduce an effective technique for the implementation of the compensated impedance inverter, suitable for preserving its broadband characteristics in the presence of large device's output capacitance. The implementation technique is based on the generalized equivalent transmission-line equivalence principle, which considers both shortening and lengthening the equivalent transmission lines. We demonstrate that, by the proposed approach, parasitic absorption is possible maintaining a low Q-factor of the equivalent network. The technique is validated by the development of a silicon laterally diffused metal-oxide-semiconductor DPA with optimized peak power and efficiency for applications in the 650-950-MHz band. The fabricated prototype is characterized by an LTE signal with a 20-MHz bandwidth and peak-to-average-power ratio of 7.5 dB and features an average drain efficiency between 37% and 47%, with an average power of 49 dBm across 37.5% of the bandwidth.

I. Introduction

The mainstream about the development of advanced Doherty power amplifiers (DPAs) considers the improvement of energy efficiency across wide signal dynamics [1]–[3], the assessment of the linearity-versus-peak-power tradeoff [4], [5], as well as the development of design techniques to overcome the operational bandwidth constraints [6]–[8]. In this context, the present paper addresses the topic of the enhancement of the operative bandwidth of a DPA by introducing a new design technique for the development of a DPA output combiner. The technique is based on the innovative use of the equivalent transmission-line equivalence concept and consists of the inclusion of large parasitic device’s elements in the equivalent transmission line, even in the more general case of peak and main devices with different output capacitances.