GaN Enables Software-Defined Switch-Mode RF Power Amplifiers

May 18, 2018 by Paul Shepard

Eridan Communications' RF transmitter technology enables previously-unattainable single radio capabilities and allows for multiple radios to work in combination to extend bandwidth or power. Combining breakthroughs in GaN devices, radical amplifier circuit design, and novel system architecture, Eridan is enabling a new level of frequency agility, spectrum, and power efficiency, with applications from handsets to base stations and from Vehicle-to-Everything to IoT.

The company's MIRACLE Power Transciver is a digital-in, RF-out system that supports any modulation at any time. Custom or innovative signal types can be implemented easily in software.

A unique capability of the MIRACLE Power Transceiver is its ability to add - in power or in bandwidth - with other MIRACLE transceivers without the need for an isolator. This enables applications in bandwidth aggregation over disparate frequencies and phased-arrays.

Efficiency is the starting point of Eridan's design philosophy. Efficiency leads to longer battery life in portable applications, but it is also a prime contributor to size, cost, and reliability in stationary or vehicular applications.

Eridan starts its product designs from maximum efficiency circuitry, and makes that work with all signal types. MIRACLE uses a switch-mode architecture that is either fully on or fully off, and wastes little power in between. Historically this switch mode approach has presented intractable challenges. Eridan's innovations have solved these challenges through novel circuit topologies and device design.

Block diagram for the MIRACLE TxRx module showing the digital data & control inputs and the RF input/output (click on diagram to enlarge)

Switching Amplifiers Maximize Efficiency

Switching amplifiers, like switching power supplies, are the highest efficiency circuits.  This benefit is inherent to the approach, because switching circuits operate in the high efficiency regions of the current-voltage quadrant: they are either fully "ON," or fully "OFF".

In the "ON" state, they are designed to have extremely low resistance and generate very little electrical loss. In the "OFF" state they have extremely high resistance, pass no electrical current, and have almost no electrical loss. The secret to a switching amplifier is in jumping between these "ON" and "OFF" states extremely rapidly and spending as little time "in between" as possible.

Achieving High Efficiency AND Precision

High data rate signals (like 4G LTE or 3G WCDMA) require exquisite precision in the generation and amplification of the transmitted signal so the intended recipient can decode the data with no errors. Even as MIRACLE transceivers achieve maximum efficiency with an inherently "nonlinear" switching amplifier, they also achieve high precision when generating the full range of industry standard communications protocols.

This breakthrough in combined efficiency and precision is made possible through an integration of analog and digital design. First, the switch mode amplifier features a dynamic range >80dB.

The precision of this dynamic range enables a pure feed-forward 1:1 mapping between the target output and amplifier control signals. This eliminates the potential hidden efficiency "cost" of complex correction schemes sometimes referred to as "feedback digital predistortion" (or DPD).

MIRACLE is a software-defined transmitter and is fully compatible with all legacy wireless protocols and is future-ready for advanced concepts in development.

Prototype Specifications

Eridan's existing V5.1 prototype delivers 5W peak RF output, rapidly tunes over a frequency range of 200-3000MHz, and meets the 3GPP specification for all communications protocols (both uplink and downlink) up to 10MHz instantaneous bandwidth (IBW). Most importantly, it delivers this capability with an average signal efficiency, ηRF(avg), of 40-60%, where ηRF(avg) = PRF(avg)/(PDC - PSYS),

PRF(avg) is the signal-average RF output power, and PSYS is the DC power consumed by the balance of system including processing of the digital baseband, all control functions, DACs, ADCs, and drivers (estimated to be <1W in the final product stage).

This breakthrough performance is made possible by Eridan's development of its GaN-based Switch-Mode-Mixer-Modulator (SM3) technology. The MIRACLE transmitter circuit architecture separately amplifies phase and amplitude signals and recombines them in a single, ultra-efficient, and wideband final SM3 stage.

The transmitter element delivers 3GPP specification-compliant LTE uplink and downlink performance including 47-50dB ACLR. The MIRACLE SM3 technology has also demonstrated unprecedented spectral efficiency, delivering a 14-bit, 16,384 QAM WCDMA signal with >40dB SNR and <0.5% EVM.

The pre-production prototype, currently in development and projected to be completed by Q3/18, will integrate the balance of system into a single 65-nm ASIC, extend the signal IBW to 40MHz, extend the frequency range over a larger portion of the 40-4200MHz available from the SM3, and size-reduce the module to a total of <1 × 2 × 0.2 inches. Configurations extending the peak power operation to 41dBm (12W) and 44dBm (25W) are in development.