Quasi-Load Insensitive Doherty PA Using Supply Voltage and Input Excitation Adaptation

Abstract

This article presents a quasi-load insensitive (QLI) Doherty power amplifier (DPA). The proposed theory makes the amplifier load insensitive in terms of output power, while its efficiency is slightly degraded for complex loads. The load insensitiveness is achieved by dynamically changing the supply voltages ( <formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex>${V_{DD}}$</tex> </formula> ) and the input power splitting for both the carrier and peaking transistors. The optimal, load-dependent, <formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex>${V_{DD}}$</tex> </formula> values are theoretically derived from back-off (BO) and full power conditions using load line theory. The optimal input excitation signals for the carrier and peaking devices are also derived for these variable <formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex>${V_{DD}}$</tex> </formula> conditions. A 3.6-GHz QLI DPA was designed, and a complete system, composed of the DPA output stage, a two-channel medium power driver, an adaptive input driving stage, and a load sensing system, was implemented. The laboratory measurements have been performed for loads distributed inside a 2.0 maximum voltage standing wave ratio (VSWR) circle and show an output power variation between 43.8 and 42.6 dBm and a BO efficiency between 50% and 35%. Under modulated signal excitation, for the worst case loads, the peak output power capability of the DPA is improved from 41.7 to 43.1 dBm, and the average efficiency is increased from 32.6% to 43%.