Design and W-CDMA Characterization of a Wideband AlGaN/GaN HEMT Power Amplifier for Future 3G Multiband Base Station Applications D. Wiegner1, U. Seyfried1, W. Templ1, T. Naß1, S. Weber1, S. Wörner1, I. Dettmann2, R. Quay3, F. van Raay3, H. Walcher3, H. Massler3, M. Seelmann-Eggebert3, R. Reiner3, R. Moritz3, and R. Kiefer3 1 Alcatel SEL AG, Holderaeckerstrasse 35, 70499 Stuttgart, Germany (specification, board assembly and characterization), fax: +49 711 821 32455, e-mail: Dirk.Wiegner@alcatel.de 2 Institute of Electrical and Optical Communication Engineering at University of Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany (board design and simulation) 3 Fraunhofer Institute of Applied Solid State Physics, Tullastrasse 72, 79108 Freiburg, Germany (GaN technology, modelling and packaging) Abstract— A single stage wideband amplifier based on an AlGaN/GaN HEMT power cell with a total gate-width of 16 mm matched to 50 Ohm has been successfully developed and characterized by use of a single-carrier W-CDMA signal. A very wide bandwidth of more than 1.7 GHz covering several mobile radio frequency bands within L- and S-band with peak output power levels up to 44 dBm has been demonstrated with meeting 3GPP ACLR requirement in the complete measured frequency range. The maximum measured output power level was 45.8 dBm translating into a power density of 2.4 W/mm, which is superior compared to that of established technologies like LDMOS. The presented power amplifier shows the impressive potential of AlGaN/GaN HEMT technology regarding output power and bandwidth, supporting the development of future multiband/ multistandard capable base stations. Keywords- multiband/multistandard, AlGaN/GaN HEMT technology, wideband amplifier, W-CDMA test signal I. INTRODUCTION Since the number of mobile radio standards (GSM, UMTS, WiMAX, etc.) and frequency bands (900 MHz, 2100 MHz, 2500 MHz, etc.) increased in mobile communication in recent years, the demand for multiband/multistandard capable base stations arose , e.g. in order to reduce system manufacturers’ product diversity and to support the flexibility of mobile operators. Additionally linearity and efficiency requirements of future base stations increase, due to new modulation schemes with peak-to-average ratios (PAR) of e.g. 10.5 dB for single carrier W-CDMA signals, which are used in order to improve data rate, necessary to allow new applications like music or video download. These requirements of future mobile base stations directly translate into the power amplifier module, which is located in the transmit path of a base station and which is one of the most challenging components when realising a multiband/multistandard capable base station. Established technologies like LDMOS e.g. provide high power and good linearity but only a limited bandwidth especially for high output power levels. New wide bandgap semiconductor device technologies like GaN HEMT promise an improved power and broadband capability, due to superior physical properties like high breakdown voltage and high carrier mobility. A high breakdown voltage allows to apply higher supply voltages, resulting in an increased power density and thus increased impedance level of the power transistor itself. An increased impedance level facilitates realisation of broadband matching circuit to the 50 Ohm in- and output of the amplifier module. First reported promising results of assembled wideband power amplifiers [2,3], covering several hundreds of MHz and providing power levels up to 100 W, show a high potential of GaN technology for future wideband/multiband amplifier applications. II. DEVICE TECHNOLOGY The AlGaN/GaN HEMT technology is based on multiwafer MOCVD growth on 2-inch s.i. SiC substrates using an Aixtron 2000 multiwafer reactor. The T-gate technology is e-beam defined with a gate length of lg = 300 nm. The basic sub cells with a gate-width of Wg = 2 and 4 mm yield a maximum drain current density of > 900 mA/mm and a transconductance of > 200 mS/mm at VDS = 35 V. The current gain cut-off frequency fT of the 2 mm device is beyond 30 GHz. Currently the maximum operational voltage is VDS = 35 V with a corresponding on-state breakdown voltage of BVDS > 70 V. The basic cell of 2 mm yields a saturated output power density of 6 W/mm and a linear gain of 23 dB at VDS = 35 V and 2 GHz single carrier operation. Single carrier power added efficiency (PAE) can be as high as 58 % with proper second harmonic tuning. The devices are diced and soldered by Au/Sn process. They are packed in standard LDMOS packages optimised for broadband operation without internal prematching. Thus, special care is taken for the stabilisation of the devices with respect to low- and high frequency oscillations without compromising the broadband operation. The optimisation of the bond wires within the package is found useful for that purpose. Further, odd mode suppression is achieved by various means, such as the introduction of additional losses and additional bond wires. III. VDrain VGate RFC RFC RL Zopt /in Pavs Input Matching Output Matching /out Pout Figure 1. Power amplifier module To obtain the maximum output power, the reference impedance Z0 (typically 50 9) must be transformed to the optimum output impedance Zout_opt of the transistor. Zout_opt can be derived from load-pull measurements, simulation or by the load line method . With the load line method the optimum resistance Ropt inside the transistor can be approximately calculated to 1 V - VK Ropt = × DS 8 Pout Gate LS C in POWER AMPLIFIER DESIGN Fig.1 shows the typical block diagram of a power amplifier module. The circuit consists of an input and output matching block and a bias circuit for the gate and drain of the transistor. While the output matching network is designed for highest output power, the input matching network is designed for flat gain and high bandwidth. The bias circuits also guarantee the stability of the amplifier. The bias point was found by power sweep simulation over different bias points. At the optimum bias point the gain is nearly constant over the power sweep. RS Z0 2 (1) where VK is the knee voltage, VDS the drain voltage and Pout the output power of the transistor. For high voltage power transistors as GaN transistors, Ropt is closer to the reference impedance Z0 compared to e.g. a LDMOS transistor with the same output power. However, due to the parasitic capacitances of the transistor and the housing Ropt is transformed to Zout_opt with a value well below 50 9. The parasitic output capacitance limits the bandwidth of the transistor. Since the output capacitance of a GaN transistor is smaller than e.g. that of an LDMOS device with comparable output power, GaN transistors are more attractive for broadband applications. At the input of the transistor a matching network is required to obtain a flat gain characteristic over the required frequency range. The input circuit of the transistor can be modelled as a series LC resonance circuit in series with the reference impedance Z0 as shown in Fig.2. Figure 2. Simplified input circuit of the transistor To get a high bandwidth, Z0 must be transformed to low values. The required matching network should have a low quality factor Q in order not to limit the bandwidth performance of the transistor and to get a flat gain characteristic. In addition, matching networks should also guarantee low reflection coefficients at the input and output of the amplifier. However, both high bandwidth and low reflection can not be fulfilled at the same time for a single transistor amplifier at the input, as bandwidth and reflection coefficient are coupled by Fanos limit . At the output the reflection coefficient is determined due to the necessity of matching the reference impedance Z0 to Zout_opt. Therefore input and output reflection coefficients are secondary design goals for a broadband single transistor power amplifier. Since a large signal model has been available from FhGIAF Freiburg, Zout_opt has been found by load pull simulation. At 2000 MHz, Zout_opt of the GaN power cell has a value of (3.7 + j1.2) 9. The change of Zout_opt over the required frequency range is sufficiently small. The output matching network has been designed using the Smith chart. Capacitors in the output matching network for matching purposes have been avoided since they introduce losses in this network, which degrades the overall efficiency of the amplifier. The matching network consists of five line sections. The quality factor Q of the network is kept low enough in order not to affect the large signal bandwidth of the transistor over the required frequency range. To obtain a large small signal bandwidth and a flat gain characteristic, the input matching network transforms the 50 9 reference impedance to 1.5 9. A multisection transformer at the input has been designed using the Smith chart. The multisection transformer consists of line sections with varying line impedances. Each line section is terminated by a shunt capacitor. Using shunt capacitors instead of e.g. quarter-wave transmission lines reduces the overall size of the matching network and allows tuning the network after production. Three sections have been used to get a sufficiently low Q factor of the network. The bias networks at the input and the output of the transistor have been designed by using quarter-wave transformers. The length of the lines are determined by the upper frequency limit. This guarantees an inductive behaviour of the bias network over the whole frequency range. The impedance of the bias network is several times higher than the impedance of the matching network over the whole frequency range and therefore the bias network has no impact on the matching network. A proper termination of the bias network for both high and low frequencies is important. The impedance of the bias network should be constant from DC to the modulation bandwidth. This reduces nonlinearities when the amplifier is driven near the compression point . IV. PROTOTYPE ASSEMBLY V. CHARACTERIZATION RESULTS First characterization measurements at a supply voltage of VDS = 35 V have been performed by using a single-carrier WCDMA test-signal with a peak-to-average ratio (PAR) of approximately 10.5 dB which means no reduction of signal dynamic. Furthermore, no linearisation was used for the measurements. The amplifier has been characterized with respect to 3GPP Adjacent Channel Leakage Ratio (ACLR) specification, which requires a signal suppression of - 45 dBc at 5 MHz frequency offset and - 50 dBc at 10 MHz frequency offset. Fig.5 illustrates the frequency characteristic of the assembled amplifier within a wide measured frequency range from 1000 MHz up to 2800 MHz at a peak input power level of approximately 32.5 dBm. Single Carrier W-CDMA Performance vs. Frequency @ approx. 32.5 dBm peak input power Power [dBm], Gain [dB], The designed wideband GaN amplifier has been assembled as a first amplifier demo-board by Alcatel SEL model workshop. In- and output matching has been realised by microstrip lines and lumped elements. The realised amplifier prototype is illustrated in Fig.3. Figure 4. Packaged GaN power cell (TGW = 16 mm) Pout (peak) Pout (average) Gain ACLR -5 MHz -15+5 MHz ACLR 55 50 45 40 -25 35 30 -35 25 20 -45 15 10 -55 5 0 -65 900 1100 1300 1500 1700 1900 2100 2300 2500 2700 2900 ACLR [dBc] Stability is an important issue for power amplifiers. Since power transistors show a very high transconductance gm, they tend to become instable due to parasitic feedback capacitances. Inband stability is guaranteed by the design of the matching networks. However, to achieve outband stability additional loss must be included. This can be realised either in the output or in the input matching network. However, it is not recommendable to add loss in the output matching network because this will degrade the efficiency of the amplifier. In many cases stability problems occur at frequencies near the lower frequency limit of the amplifier pass band. At these frequencies the matching network is blocked by the decoupling capacitors and the transistor sees mainly the impedance of the bias network. Inductive behaviour of the gate and drain bias network at these frequencies will potentially cause oscillation. Adding a small series resistor in the gate bias network improves the stability of the amplifier. Placing a series resistor in the drain bias network is not possible, since a high current is flowing through the drain bias line. Therefore a small shunt resistor decoupled by a capacitor is placed in the bias network. The design and simulation of the wideband amplifier has been done at the Institute of Electrical and Optical Engineering at University of Stuttgart. Frequency [MHz] Figure 5. Frequency characteristic at nearly constant input power Figure 3. 16 mm GaN wideband amplifier prototype The GaN power bar with a total gate-width (TGW) of 16 mm housed in a single standard LDMOS package is depicted in Fig.4. The power bar itself consists of eight basic GaN power cells with a TGW of 2 mm, each. Packaging and bonding was done at FhG-IAF Freiburg. The GaN HEMT power cells themselves are based on a FhG-IAF Freiburg proprietary AlGaN/GaN HEMT technology. In order to reduce the source inductance and to support stabilisation of the power cells, bonding was done over an additional substrate. Due to the current stress on drain side, a plurality of bond wires has been used. The effective ripple of the presented final amplifier stage can be seen by means of the gain curve. The gain is about 10 dB over a wide frequency range. At 32.5 dBm peak input power, the amplifier achieves a maximum measured peak output power of approximately 43 dBm at 1200 MHz and is above 40 dBm within the complete frequency range. The related average output power is above 30 dBm for the used W-CDMA signal. Since measured ACLR is clearly better than the required - 45 dBc for wide frequency ranges, an improved output power characteristic can be achieved by adjusting the input power level considering 3GPP ACLR conformance, as shown by Fig.6. Since the amplifier is intended to cover frequency bands within the frequency range from 1800 MHz up to 2700 MHz, comprising GSM, UMTS and WiMAX bands, the power adaptation has been performed within this limited frequency range. Pout (peak) Pout (average) Gain Pin (peak) -15-5 MHz ACLR ACLR +5 MHz Single Carrier W-CDMA Performance vs. Frequency 55 50 45 40 35 30 25 20 15 10 5 0 1700 -25 -35 -45 ACLR [dBc] Power [dBm], Gain [dB] @ Adjusted Peak Input Power Considering 3GPP ACLR Conformance -55 1900 2100 2300 2500 2700 Figure 8. Single-carrier W-CDMA spectrum at 2.0 GHz and 43.4 dBm peak output power -65 2900 Frequency [MHz] Figure 6. Improved frequency characteristic by adjusted input power level The achieved peak output power level is now above 43 dBm, with a related average output power level of approximately 34 dBm over a very wide frequency range. Maximum achieved peak output power is 45 dBm at 1800 MHz. 3GPP ACLR specification is met within the complete measured frequency range. Fig.7 shows the related power sweep measurement at 2000 MHz by use of a single-carrier W-CDMA test-signal. 1 dB compression point can be found approximately at a peak output power level of 44 dBm. 3GPP ACLR at 5 MHz offset will be met up to a peak output power of nearly 45 dBm, ACLR at 10 MHz is well below the required - 50 dBc and thus completely uncritical. CONCLUSION This work presents promising results of a GaN HEMT based wideband power amplifier matched to 50 Ohm for base station applications in 3G mobile communication. The used design approach in order to achieve a wideband characteristic has been discussed and the assembled amplifier has been characterized with respect to 3GPP ACLR specification. By use of a single-carrier W-CDMA signal, the presented wideband amplifier covers a very wide frequency range from 1000 MHz up to 2800 MHz, meeting 3GPP ACLR. By input power adaptation, a peak output power level of approximately 43 dBm can be achieved over a wide frequency range. Maximum measured output power at 2000 MHz is 45.8 dBm translating into a power density of 2.4 W/mm, which exceeds power densities of established technologies like e.g. LDMOS or GaAs. mean power Gain ACLR@5MHz -5 ACLR@10MHz P1dB -15 -25 -35 -45 ACLR [dBc] Output Power [dBm] 1-Carrier W-CDMA Signal, PAR approx. 10.5dB 50 45 40 35 30 25 20 15 10 5 0 ACKNOWLEDGMENT peak power Output Power vs. Input Power @ 2000 MHz The authors wish to acknowledge the support of the German Ministry of Education and Research (BMBF) under contract 01BU382 and 01BU386 in the context of the ‘Leitinnovation “Mobile Internet”’. The continuous support of the epitaxial and technology department at Fraunhofer IAF as well as the support of the radio department and model workshop at Alcatel R&I Stuttgart is gratefully acknowledged. -55 REFERENCES -65 22 24 26 28 30 32 34 36 38  Peak Input Power [dBm] Figure 7. Power sweep measurement The measured gain is approximately 10.8 dB for low input power levels, when the applied test-signal is not driven into compression. Fig.8 shows the spectrum of a single-carrier WCDMA signal directly at the output of the 16 mm wideband GaN amplifier, at 2000 MHz. The shape of the spectrum at 2000 MHz suggests that 3GPP ACLR requirements at 5 MHz offset (required: - 45 dBc, measured: - 47.48 dBc, - 46.66 dBc) as well as at 10 MHz offset (req.: - 50 dBc, meas.: - 59.08 dBc, - 59.55 dBc) are clearly met at a peak output power of approx. 21.8 W (43.4 dBm). The corresponding average output power is approx. 2.2 W (33.4 dBm).      W. Koenig, S. Walter, U. Weiss, D. Wiegner, “A multiband frontend for a medium range base station - an important step towards SDR”, 3rd Karlsruhe Workshop on Software Radios, WSR’04, March 17/18, 2004, Karlsruhe. A. Maekawa, M. Nagahara, T. Yamamoto and S. Sano, “A 100W high efficiency GaN HEMT amplifier for S-band wireless system”, European Microwave Conference, October 03.-07., 2005, Paris, France. D. Wiegner, T. Merk, U. Seyfried, W. Templ, S. Merk, R. Quay, F. van Raay, H. Walcher, H. Massler, et. al. “Multistage broadband amplifiers based on GaN HEMT technology for 3G/4G base station applications with extremely high bandwidth“, European Microwave Conference, October 03.-07., 2005, Paris, France. Steve C. Cripps, “RF power amplifiers for wireless communications”, pp. 24, Artech House, 1999. David M. Pozar, “Microwave engineering”, John Wiley and Sons Inc., 1998. Joel Vuolevi and Timo Rahkonen, “Distortion in RF power amplifiers”, Artech House, 2003.
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