A 2.6GHz Band 537W Peak Power GaN HEMT Asymmetric Doherty Amplifier with 48% Drain Efficiency at 7dB

Hiroaki Deguchi, Naoki Watanabe, Akihiro Kawano, Norihiro Yoshimura, Norihiko Ui and Kaname Ebihara

Sumitomo Electric Device Innovations, Inc. Yamanashi-Plant, 409-3883, Japan

Abstract — A 537W saturation output power (Psat)

asymmetric Doherty amplifier for 2.6GHz band was successfully developed. The main and peak amplifiers were implemented with Psat of 210W and 320W GaN HEMTs. The newly developed 320W GaN HEMT consists of a single GaN die, both input and output partial match networks and a compact package. Its output matching network was tuned to inverse class F and a single-ended 320W GaN HEMT achieved higher than 61.8% drain efficiency from 2.4GHz to 2.7GHz. The 210W and 320W GaN HEMTs asymmetric Doherty amplifier exhibited 57.3dBm (537W) Psat and 48% drain efficiency with -50.6dBc ACLR at 50.3dBm (107W) average output power using a 4-carrier W-CDMA signal and commercially available digital pre-distortion system. These excellent performances show the good suitability for 2.6GHz band basestations.

Index Terms — power amplifier, Doherty, GaN HEMT, DPD, W-CDMA, LTE, WiMAX.

I. INTRODUCTION

Recently, Doherty amplifiers are widely applied in the basestation transmitter system (BTS), since peak-to-average power ratio (PAR) of modern digital wireless communication signal, like W-CDMA, LTE and WiMAX, is around 6 to 8dB and Doherty amplifiers are suitable to operate in such back-off output power region [1]-[2]. Si-LDMOS transistor has been the dominant technology for the BTS application owing to its sufficient performance with reasonable price for a longtime [3]-[4]. On the other hand, GaN HEMT has high output impedance and is capable of high power operation with high efficiency due to high current density and high breakdown voltage. Many investigations have reported superior performances of GaN HEMT for BTS application [5]-[9]. Consequently, GaN-HEMT is gradually penetrating the

market, especially, high frequency BTS application, e.g., 2.1GHz and 2.6GHz band. Meanwhile, high output power BTS, like higher than 80W antenna output power, are requested to cover suburban area widely with minimized cost. In such case, instantaneous peak power of transmitter amplifier has to be higher than 500W in order to obtain a well-linearized modulated signal. However, commercially available output power range of a single transistor for BTS application is up to around 200W at the highest, and especially it is difficult to find such a high power device in 2.6GHz band. Therefore, appropriate power combining technique is required to realize 500W class amplifier.

In this paper, we report a newly-developed 320W inverse class F GaN HEMT in a compact package for Doherty peak amplifier use. We also demonstrate an asymmetric Doherty amplifier accompanied with a 210W GaN HEMT (Sumitomo Electric, SGN27C210I2D) and is capable of 537W saturation output power (Psat) at 2.655GHz. We show well-linearized ACLR and excellent back-off efficiency using a commercially available digital pre-distortion (DPD) system.

II. 320W GAN HEMT

Basically, it is difficult to design large dimension GaN HEMT due to the limitation of thermal treatment and strength against die crack. Meanwhile, the power consumption of peak amplifier in Doherty configuration is much lower than main amplifier. Therefore, thermal resistance design of GaN HEMT die for Doherty peak amplifier can be higher than that of main amplifier, which means that a narrower pitch of comb-shaped fingers is possible. In this work, we designed a 320W single die GaN HEMT and adopted 38% narrower gate to gate pitch than the current device series, based on the consideration of Doherty amplifier operation. TABLE I shows the calculated junction temperature (Tj) of asymmetric Doherty amplifier using 210W and 320W GaN HEMTs. The thermal resistance of 320W GaN HEMT was designed to be 1.3deg.C/W. Practically, power consumption of peak amplifier is less than half of main amplifier for BTS use, however, we estimated the worst-case condition of power consumption ratio for main and peak amplifiers as main : peak = 6 : 4. This GaN HEMT series has MTTF of 1 million hours at Tj=200deg.C, and from the calculation, Tj maintains the criterion. Figure 1 shows a

Doherty Operating Condition

Case Temperature [deg.C] 90 Output Power [W] 110 Estimated Drain Efficiency [%] (worst) 42

Main Peak

Calculated Value

Power Consumption of Main PA* [W] 91 - Power Consumption of Peak PA* [W] - 61 Thermal Resistance [deg.C/W] 1.1 1.3 Delta Tj [deg.C] 100 79 Tj [deg.C/W] 190 169

*) Estimated power consumption ratio; MainPA : PeakPA = 6 : 4(worst)

TABLE I Calculated Tj for the worst-case

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photograph of the developed 320W GaN HEMT die with total gate width of 81.2mm. Figure 2 shows an assembled 320W GaN HEMT with both input and output partial match networks for 2.6GHz band. An inverse class F circuit was implemented at output side with a simple single-pole circuit [8]-[9]. Figure 3 shows the power characteristics of single-ended 320W GaN HEMT tuned with external output matching network in a printed circuit board (PCB). The tuning condition was broadband tuning for 2.4GHz to 2.7GHz. The 320W GaN HEMT obtained Psat of greater than 305W and drain efficiency of higher than 61.8% in this 300MHz band.

III. ASYMMETRIC DOHERTY AMPLIFIER

Figure 4 shows a schematic of the asymmetric Doherty amplifier. Load impedance of the main amplifier is modulated

between 20Ω and 125Ω. The asymmetric Doherty amplifier was designed by small signal simulation, and optimum impedance (Γopt) for Psat or efficiency at the package lead plane was obtained by load-pull measurement. Figure 5 shows the consistency of Γopt and simulated load impedances of designed matching networks. Figure 6 shows a top view of developed asymmetric Doherty amplifier.

IV. MEASUREMENT RESULTS

Figure 7 shows the power characteristics of the developed asymmetric Doherty amplifier at Vds=50V and f=2.655GHz. The measured data were obtained under the condition of pulsed RF with duty cycle ratio of 10%. We obtained a Psat of 57.3dBm (537W) and a linear gain of 13.5dB. Figure 8 shows drain efficiency and PAE versus backoff power level under CW operation at 2.62GHz, 2.655GHz and 2.69GHz. We obtained drain efficiency of greater than 50% at 7dB backoff

13.2

mm

21mm Fig. 2. Top view of assembled 320W GaN HEMT.

Fig. 1. Developed 320W GaN HEMT die. 5.6mm

0.84

mm

Fig. 3. Power characteristics of single-ended 320W GaN HEMT

εr=3.3t=0.8mm.

Fig. 6. Top view of the asymmetric Doherty amplifier.

95mm (3.7”)

95m

m (

3.7”

)

320W

210W

Fig. 5. Schematic of the asymmetric Doherty amplifier.

Fig. 4. Schematic of the asymmetric Doherty amplifier.

978-1-4673-1088-8/12/$31.00 ©2012 IEEE

power in the 70MHz frequency band. In order to investigate the adaptability of the asymmetric

Doherty amplifier to DPD, commercially available DPD chipset (NetLogic Microsystems OP6180) was employed. The modulated signal condition was W-CDMA1111 4-carrier signal with PAR of 6.2dB (4-carrier total) at 0.01% probability (CCDF). Figure 9 shows ACLR and drain efficiency with DPD operation at 4-carrier center frequency of 2.655GHz. A high drain efficiency of 48% and a well-linearized ACLR of -50.6dBc were achieved at the average output power of 50.3dBm (107W, 7dB backoff power).

V. CONCLUSION

We have demonstrated a 537W Psat Doherty amplifier using 210W GaN HEMT and newly developed single die 320W GaN HEMT. We achieved a drain efficiency of 48% and an ACLR of -50.6dBc at the average output power of 50.3dBm (107W) with a 4-carrier W-CDMA signal using commercially available DPD. These results show GaN HEMT Doherty amplifier can realize a high output power 2.6GHz BTS with high efficiency.

REFERENCES

[1] Steve C. Cripps, RF Power Amplifiers for Wireless Communications. Norwood, MA: Artech House, 1999.

[2] Frederick H. Raab, et al., “Power Amplifiers and Transmitters for RF and Microwave”, IEEE Trans. Microwave Theory Tech., Vol. 50, pp. 814-826, Mar. 2002.

[3] M.Bokatius, et al., “A Linear 250 Watt Doherty Power Amplifier Based on Two-Stage Power ICs for 1.8GHz Single and Multi-Carrier GSM Applications”, 2009 IEEE MTT-S Int. Microwave Symposium Digest.

[4] J.H.Qureshi, et al., “A Wide-Band 20W LDMOS Doherty Power Amplifier”, 2010 IEEE MTT-S Int. Microwave Symposium Digest.

[5] N.Ui, et al., “A 80W 2-stage GaN HEMT Doherty Amplifier with -50dBc ACLR, 42% Efficiency 32dB Gain with DPD for W-CDMA Base station”, 2007 IEEE MTT-S Int. Microwave Symposium Digest.

[6] H.Sano, et al., “A 40W GaN HEMT Doherty Power Amplifier with 48% Efficiency for WiMAX Applications”, 2007 IEEE Compound Semiconductor Integrated Circuit Symposium Digest.

[7] H.Deguchi, et al., “A 33W GaN HEMT Doherty Power Amplifier with 55% Drain Efficiency for 2.6GHz base stations”, 2009 IEEE MTT-S Int. Microwave Symposium Digest.

[8] N.Ui, et al., “Inverse class F GaN HEMTs for Doherty and Envelope Tracking”, 2010 IEEE MTT-S Int. Microwave Symposium WorkshopWSF.

[9] N. Yoshimura, et al., “A 2.5-2.7GHz Broadband 40W GaN HEMT Doherty Amplifier with higher than 45% drain efficiency for multi-band Application”, 2012 IEEE Radio and Wireless Week Digest.

Fig. 7. Power characteristics of the asymmetric Doherty amplifier, f=2.655GHz, Pulsed RF with duty cycle ratio of 10%.

Fig. 9. ACLR and drain efficiency vs. average output power of the asymmetric Doherty amplifier, fc=2.655GHz.

Fig. 8. Drain efficiency and PAE vs. backoff power level of the asymmetric Doherty amplifier.

978-1-4673-1088-8/12/$31.00 ©2012 IEEE