Broadband Inherent Linear Transmitters based on Load Modulation
Abstract
Approved for Public ReleaseThe load modulated balanced amplifier (LMBA) is a recently developed power amplifier (PA) architecture that offers potential benefits in terms of both RF bandwidth and back-off efficiency compared to conventional architectures. The architecture is based on a balanced amplifier (BA), but has an additional control amplifier (CA) that drives the isolation port of the output hybrid coupler, generating active load modulation of the BA. The recent literature has explored basic design choices like how to bias and drive the BA and CA, and octave-bandwidth demonstrations have been published. The PIs group has pioneered the use of a passive RF structure to produce an octave-bandwidth LMBA operating from a single RF input. While preliminary work on the LMBA is promising, there are fundamental questions that must be explored to understand its capabilities or limitations. This work proposes to perform a comprehensive study in theory and experiment, focusing on at least octave bandwidth operation around X-band (6-12 GHz). In particular, the work will focus on three key aspects of the LMBA:1) Combining structure. In published analyses to date, the combining structure is assumed to be an ideal hybrid coupler. In practice, imperfect isolation in this coupler will produce load modulation on the CA, which theoretically drives a fixed load. A thorough study of isolation requirements is needed, particularly at the band edges where isolation degrades. Understanding the effects of imperfect combining will not only allow us to define the bandwidth limitations of the LMBA, but to potentially exploit combiner nonidealities to improve performance. 2) Inherent linearity of the LMBA. While large RF bandwidth and high efficiency are important design goals, the transmitter linearity is ultimately the critical factor inmeeting link budgets. By controlling the second harmonic termination of the BA constituent amplifiers, the AMPM response of the LMBA can be controlled.In this work we will evaluate these design tradeoffs, focusing specifically on the ability to transmit multiple concurrent signals within the octave RF bandwidth. As part of the theoretical framework for this analysis, we will analyze the effects of load modulation on amplitude (LMAM) and on phase (LMPM).3) LMBA architecture extensions. A modular approach to hardware will enable architecture studies including: (1) injecting a second control signal at the isolation port of the input hybrid coupler, (2)exploiting the periodicity of the hybrid coupler for additional signal injection, and (3) exploring baseband terminations in the BA.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- May 05, 2021
- Source ID
- N000142112465
Entities
People
- Taylor Wallis Barton
Organizations
- Office of Naval Research
- Regents of the University of Colorado
- United States Navy