Analysis and Design of a Doherty-Like RF-Input Load Modulated Balanced Amplifier

This paper presents an analysis and design technique for an RF-input load modulated balanced amplifier (LMBA) with Doherty-like efficiency enhancement characteristics. The LMBA is a recently introduced power amplifier (PA) architecture in which a control signal is injected into the isolation port of the output coupler of a balanced amplifier. This control signal modulates the apparent load impedance of the two main devices. In the RF-input LMBA presented in this paper, the control signal is generated from the single-modulated RF input using a class-C control amplifier, producing an overall Doherty-like response. A complete theoretical analysis of the RF-input LMBA is presented, by assuming generalized networks for the output combiner and input splitter which are then solved in terms of transistor parameters and boundary conditions for high efficiency. Based on this analysis, a generalized design technique is presented. The LMBA is, thereby, shown to have advantages over the Doherty PA, including the device periphery scaling between the main and control devices for a given back-off range. The design technique is demonstrated at 2.4 GHz using packaged GaN devices. The prototype has a peak CW output power of 45.6 dBm, with a power-added efficiency of 60% at peak power and 50% at 6-dB output back-off, including the power dissipated by the control PA. A long-term evolution signal at 2.4 GHz with a 7.5-dB peak-to-average power ratio is demonstrated with an unlinearized ACLR of -27 dBc and an average drain efficiency of 47% for a 38-dBm average output power.