Polar Modulation And Bipolar RF Power Devices

Abstract
In this article, a brief survey of polar transmitter architectures is followed by a discussion of circuit fundamentals required for intentionally compressed power amplifiers. The application of bipolar power transistors is examined with respect to these circuit fundamentals, resulting in a list of advantages and disadvantages. A call is made to the bipolar power transistor community toward addressing these identified disadvantages.

Introduction
To specifically address the problem of low transmitter power amplifier (PA) efficiency with envelope varying signals, the use of polar modulation techniques has been proposed and used as early as 1915. While polar modulation techniques generally do increase overall PA efficiency, they have also generally provided reduced output signal quality compared to contemporary good-practice linear circuit approaches. For applications that do not require demanding signal quality, such as early amplitude modulation (AM) broadcast stations, this efficiency gain along with increased signal distortion was an acceptable tradeoff. However, as the performance requirements on radio signals have become ever more demanding, the use of these early polar techniques waned in favor of newer and improved linear techniques.

Modern wireless systems have adopted signal types that provide high bandwidth efficiency, such as orthogonal frequency division modulation (OFDM). Use of these signals in battery powered mobile devices has again made improving of RF power amplifier efficiency a serious market demand, and polar techniques are experiencing a renaissance in interest toward addressing this problem.

© Earl McCune 2005. All Rights Reserved. By Earl McCune, CTO and Co-founder, Tropian, Inc.