A pulsed load modulation (PLM) power amplifier with 3-level envelope delta-sigma modulation (EDSM)

An X-band power amplifier (PA) based on the pulsed load modulation (PLM) architecture is developed. The PLM technique leverages on the load modulation characteristics incurred for modulated pulses with different duty cycles as the consequence of the interactions between switching PAs with the high-Q output filter. Though a high drain efficiency is promised by the PLM amplifier with a binary envelop input, the low coding efficiency of the 2-level pulse modulations including the 2-level Envelope Delta- Sigma Modulation (EDSM) reduces the effective gain of the PA and thus decreases the overall power added efficiency. The great amount of quantization noise also needs to be rejected with high-Q filter which adds a great burden to the output filter design. In this paper, it is proposed to feed the PLM PA with a 3-level EDSM signal for input with higher effective gain and lower quantization noise. The concept is demonstrated with an X-band PLM PA fabricated with discrete 0.25μm GaN HEMT. The proposed transmitter delivered 35.6 dBm output power with 40.5 % drain efficiency for 5 MHz WCDMA signal with 5.3 peak-toaverage ratio at 9.2 GHz.

I. Introduction

Wireless communication systems require highly efficient electronics to extend the life spans of batteries and to reduce the cost of cooling systems. Power amplifiers (PAs) play a critical role in determining the overall efficiency of a wireless transmitter, as they consume the largest portion of the DC power. Various schemes have been introduced to improve the power efficiency including polar amplifiers, Doherty amplifiers, and envelope tracking amplifiers [1], [3]. Fig. 1:

Block diagram of a multi-level envelope delta-sigma modulation (EDSM) scheme.

Fig. 2:

Power spectrum density of 2-level and 3-level EDSM for 5 MHz WCDMA signal.