GaN HEMT Based 40W Doherty Amplifier with Digital Pre-Distortion Correction for WiBro Applications

*Jun-Chul Park1,2, Dongsu Kim1, Chan-Sei Yoo1, Woo Sung Lee1, Jong-Gwan Yook2, Sang-Hyun Chun3,

Jong-Heon Kim3 and Cheol Koo Hahn

1Korea Electronics Technology Institute

1

#68 Yatap-dong, Bundang-gu, Seongnam-si, Gyeonggi-do, 463-816, South Korea

2Department of Electrical and Electronics Engineering, Yonsei University #262 Seongsanno, Seodaemun-Gu, Seoul 120-749, Korea

3Department of Radio Science & Engineering, Kwangwoon University #447-1 Wolgye-dong, Nowon-ku, Seoul 139-701, Korea

*e-mail : spiffyjohn@naver.com, Tel : +82-31-789-7231

Abstract — This paper presents a high power Doherty

amplifier for 2.345 GHz wireless broadband (WiBro) application using Nitronex 125-W (P3dB) GaN high electron mobility transistor (HEMT). The main and peaking amplifiers are symmetrical structure and biased as class AB and class C, respectively, so that the output power and the drain efficiency of the amplifier are satisfied at the highly back-off region for WiBro signal, which has a peak-to-average ratio of about 9.5 dB at 0.01 % for the probability on CCDF. From the measurement results for a center frequency of 2.345 GHz, the proposed Doherty amplifier attains a high P3dB of 51.5 dBm, a gain of 12.5 dB and a power-added efficiency (PAE) improvement of about 16 % compared to a single class AB amplifier at 6-dB back-off power region from P3dB. For a WiBro OFMDA signal, the Doherty amplifier shows an adjacent channel leakage ratio (ACLR) at 4.77-MHz offset is -33 dBc at an output power of 42 dBm, which is 9.5 dB back-off power region from P3dB. By employing digital pre-distortion (DPD) technique, ACLR of the Doherty amplifier is improved from -33 dBc to -48 dBc.

Index Terms — Doherty amplifier, gallium nitride high electron mobility transistor (GaN HEMT), WiBro, digital pre-distortion

I. INTRODUCTION

In recent years, commercial wireless communication systems are required to transmit high data rate signals for numerous multimedia communications, which have a large signal bandwidth and high peak-to-average ratio (PAR). In this current trend, the wireless broadband (WiBro) system, an official name of portable internet system in Korea is on the way of commercial service. The WiBro is a homegrown variant standard of IEEE 802.16e and designed to provide seamless mobile connectivity over the 2.3 GHz spectrum at

ground speed up to 60 kilometers per hour with an average bandwidth of 16 Mbps [1]. WiBro is based on orthogonal frequency division multiplexing access (OFDMA), which typically has peak-to-average envelope excursions of 8-12 dB or more. For that reason, the power amplifiers have to operate at a large back-off region although the drain efficiency of the power amplifier is dropped. Nowadays various kinds of efficiency enhancement techniques are examined for high PAR applications, such as the Doherty amplifier, envelop tracking (ET), EER, LINC and so on [2]. Among them, the Doherty amplifier has been widely investigated due to circuitry simplicity, easy of configuration and wide bandwidth compared with other techniques [3].

According to an increase of operating power and frequency band, the performance operating limits of the Si lateral double-diffused MOSFET (LDMOSFET) are reached. The GaN high electron mobility transistor (HEMT) has been developed recently as a replacement for Si LDMOSFET because of its high breakdown voltage, high electron saturation velocity, high frequency and high power performance. Many varieties of studies for power transmitter using GaN HEMT have been performed [4]-[6].

In this paper, the high power Doherty amplifier using GaN HEMT device for 2.345 GHz WiBro application are designed, fabricated, and tested. The Doherty amplifier has provided high P3dB and improvement of efficiency compared to a single class AB power amplifier. In addition, digital pre-distortion has been applied to the amplifier in order to improve the linearity performance of the proposed Doherty amplifier.

II. DESIGN & FABRICATION OF HIGH POWER DOHERTY AMPLIFIER FOR WIBRO APPLICATION

A schematic of the high power Doherty amplifier for 2.345 GHz WiBro application is shown in Fig. 1. This is a structure of the conventional Doherty amplifier using symmetric transistor. The basic operation principle and efficiency analysis of the Doherty amplifier (load modulation) have been well described in previous literature [7]-[10]. The input signal is divided into the two amplifiers by a 90 deg hybrid coupler. The gate voltages of the carrier amplifier and the peak amplifier are determined to be biased as class AB and class C, respectively. The main and peaking amplifiers use the same drain voltage, 28 V and matching circuit so that the high power and the improvement of efficiency are obtained. Agilent advanced design system software was used for the simulation of the Doherty amplifier. A GaN HEMT, which is NPT25100 by Nitronex corporation is used as the carrier amplifier and the peak amplifier. The NPT25100 is a 125 W P3dB peak envelop power, pre-matched, 36 mm gate periphery GaN HFET mounted into an air-cavity CPC package. Matching circuits were designed by load-pull datasheet of the NPT25100. A single class AB amplifier was designed and fabricated in advance. Fig. 2 and Fig. 3 show the measured characteristics of a single class AB amplifier for a continuous wave and for a WiBro OFDMA signal at 44 dBm (P1dB), respectively. The efficiency of the amplifier is not enough at a large back-off power region.

The proposed Doherty amplifier was fabricated and measured. A photograph of the proposed Doherty amplifier is shown in Fig. 4. Peaking bias point and compensation line length were optimized to obtain an accurate load modulation so that the output impedance of the peak amplifier at the combining point is almost open at a low power region. The substrate is a takonic RF-35, which has a dielectric constant of 3.5, a substrate thickness of 0.762 mm and a dissipation factor of 0.0018 at 2.345 GHz. The 90 deg hybrid coupler, which is XC2500E-03S by Anaren is used in front of two amplifier.

Fig. 1. A schematic of the high power Doherty amplifier.

Fig. 4. Top view of Doherty amplifier.

Fig. 2. Measured output power, gain and PAE characteristics of single class AB amplifier for a continuous wave.

Fig. 3. Output spectrum of a single class AB amplifier for a WiBro OFDMA signal.

Fig. 5 shows the measured and simulated characteristics of the Doherty amplifier using 2.345 GHz continuous wave (CW). The measurement was done by using Agilent E8267D PSG Vector Signal Generator and E4440A PSA Series Spectrum Analyzer. The class AB amplifier, from CBP02064741 of CERNEX Inc., which has a gain of 51 dB, was used for drive amplifier. The Doherty amplifier can carry on a P3dB of 51.5 dBm and has a power gain of 12.5 dB. The measured result of the amplifier is a good agreement with the simulated result.

Fig. 6 shows the power-added efficiency (PAE) of the Doherty amplifiers and single class AB amplifier according to output power levels for a continuous wave signal. Peaking bias of the Doherty amplifier is tuned from -1.5 V (class AB) to -4.5 V so that optimum bias point was obtained. The Doherty amplifier displays the excellent efficiency of about 41 % at the 6-dB back-off power region, which is 16 % higher than that of single class AB amplifier, when the peaking bias is selected to -4 V.

Fig. 7 shows the measured error vector magnitude (EVM) spectrum of WiBro OFDMA signal at an average output power of 46 dBm. The linearity specification for the modulation signal is about 3 % of EVM. The proposed Doherty amplifier has 2.58 % of EVM and is fully satisfied this in-band-error specification.

III. DIGITAL PRE-DISTORTION FOR IMPROVEMENT OF LINEARITY

For a WiBro OFDMA signal, the proposed Doherty amplifier has a poor ACLR of -30 dBc at the 6-dB back-off region with the ±4.77 MHz offset frequency of WiBro standards. Accordingly, digital pre-distortion using the test bench as shown Fig. 8 has been applied to improve the linearity of the GaN Doherty power amplifier [11].

The test bench consists of a programmable signal generator (Agilent PSG 8267D), a device under test with a drive amplifier, a 40 dB attenuator, a performance spectrum

Fig. 5. Continuous wave (CW) output power and gain of a Doherty amplifier.

Fig. 8. The test bench for digital pre-distortion.

Fig. 6. Measured power added efficiency performance of a Doherty amplifier using a continuous wave at 2.345 GHz.

Fig. 7. Measured error vector magnitude of 2.345 GHz WiBro OFDMA signal at an average output power of 46 dBm.

analyzer (Agilent PSA E4440A) with 40 MHz analysis bandwidth, and a personal computer with the Agilent Advanced Design System 2008, the Agilent 89601 Vector Signal Analysis, and the Mathworks Matlab 2008. The PSA for receiver has analysis bandwidth better than at least 26.25 MHz for compensating the 3th order distortion, because the 1FA WiBro modulated signal has 8.75 MHz signal bandwidth.

The memory polynomial based PD (of 7th polynomial order with 3 memory terms) was used in order to minimize the AM-AM and AM-PM distortion of the GaN Doherty power amplifier.

Fig 9. shows the measured 1FA WiBro spectrum of a GaN Doherty power amplifier non-applied and applied digital pre-distortion at 42 dBm which is 9.5 dB output back-off power region from P3dB. At ±4.77 MHz offset frequency, a linearity improvement of 13.7 dBc and 16.7 dBc respectively was achieved. Table I shows the ACLR of GaN Doherty Power Amplifier non-applied and applied digital pre-distortion at ±4.77 MHz offset frequency.

Table I

ACLR of GaN Doherty Power Aamplifier non-applied and applied the DPD

ACLR(Lower) ACLR(Upper) Non-applied DPD 34.6 dBc 33.2 dBc

Applied DPD 48.3 dBc 49.9 dBc

IV. CONCLUSION

The high power Doherty amplifier using GaN HEMT device for 2.345 GHz WiBro applications have been successfully implemented. For a 2.345 GHz CW signal, the Doherty amplifier shows the high P3dB of 51.5 dBm, the gain of 12.5 dB and the PAE of 41.2 % at 6-dB back-off power region from P3dB. Because of the poor linearity characteristics

of GaN device, an ACLR of the Doherty amplifier was improved using digital pre-distortion technique for a WiBro OFDMA signal. Whereas the proposed Doherty amplifier has a peaking point at 6-dB back-off region, the OFDMA signal needs to have a 9.5 dB peak-to-average ratio. In future work, the Doherty amplifiers with higher back-off region such as unequal-cells-based Doherty amplifier or multistage Doherty amplifier will be considered for the operation at 9.5 dB back-off power region by incorporation with digital pre-distortion technique.

ACKNOWLEDGMENT

This work was supported by the IT R&D program of MIC/IITA. [2007-F-044-01, Development of GaN Power Amplifier for 4G Base Station]. Also, the authors would like to thank C. Y. Park and J. W. Park at RFHIC Inc. for their helpful discussion.

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Fig. 9. 1FA WiBro spectrum of a GaN Doherty PA non-applied and applied DPD.