Single-chip Dual-Mode Power Amplifier MMIC using GaAs E-pHEMT for WiMAX/WLAN Applications

WiMAX (World Interoperability for Microwave Access) IEEE 802.16 technology in the wireless communication systems grows up quickly. The advantages of WIMAX are its wireless coverage is larger than WLAN and does mobile internet access (IEEE 802.16e). With WLAN technology and commercial applications maturing, we can use internet with PC or notebook around fixed hotspots. In coming trend, there will be a solution for dual-mode of WiMAX/WLAN. A user can receive WiMAX/WLAN signal by a dual-mode wireless card at the same time, and it can switch from WLAN to WiMAX mode when move to outdoor without WLAN signal for surfing internet anywhere and anytime.

The integration of wireless communication systems with different frequency bands has been investigated for optimizing the size and power consumption. Conventional multi-mode RF circuits that duplicate the front-end components for each frequency band have separated transmitter and receiver for each operating band [1- K. Matsuge, S. Hiura, M. Ishida, T. Kitahara, T. Yamamoto, “Full RF module with embedded filters for 2.4 GHz and 5 GHz dual band WLAN applications,” IEEE MTT-s Volume 2, 2004.]. Recently, concurrent architectures [2- H. Hashemi and A. Hajimari, “Concurrent Multiband Low noise Amplifiers-Theory, Design, and Applications,” IEEE Transaction on MTT, VOL. 50, NO. 1, January, 2002.]-[3- S.-F.R. Chang, Wen-Lin Chen, Shuen-Chien Chang, Chi-Kang Tu, Chang-Lin Wei, Chih-Hung Chien, Cheng-Hua Tsai, J. Chen, A. Chen, “ A dual-band RF transceiver for multistandard WLAN applications,” IEEE Transaction on MTT ,Volume 53, Issue 3, Part 2, March, 2005.] that integrate the RF transceivers of various frequency bands have been proposed. The novel dual-band transceivers make dual-band RF front-end circuits, such as filter and power amplifier, that simultaneously operate at two frequency bands promising components for future wireless applications. Single-in and single-out power amplifiers with dual-band performance have been studied extensively. A broadband matching circuit for single chip MMIC was designed to cover the both bands [4- M. R. DeHaan, M. Jones, G. Wilcox, J. Mcleod and S. C. Miller, “ A 15-Watt dual band HBT MMIC power amplifier,” IEEE MTT-s, 1997.]-[5- Y. S. Noh and C. S. Park, “PCS/W-CDMA dual-band MMIC power amplifier with a newly proposed linearizing bias circuit,” IEEE journal of solid- state circuits, Volume 37, NO 9, Sep., 2002.]. One of more straight approach is the design of dual-band matching circuits that transform the system impedance to prescribed impedance for optimizing power amplifier performance.

In this paper we simulated the matching circuit and large signal analyses of power amplifier by using ADS (Advantaged Design System) .And we present one fully input and inter-stage matching network integrated chip of class-AB power amplifier MMIC [6- S. C. Cripps, RF Power Amplifier for Wireless Communications, Artech House, Inc., 1999.] that could be applied to 2.45GHz and 3.5GHz two frequency bands, and only need to design an adaptive bias choke circuit and dual-band output-matching network with specific frequency on the printed circuit board, and it can meet the linearity requirements of WIMAX/WLAN applications.

MMIC CIRCUIT DESIGN

Linearity and efficiency are the key parameters for WLAN and WiMAX applications. E-pHEMT technology provides excellent power performance with good temperature stability and reliability. So the dual-mode power amplifier is designed with the GaAs pHEMT process , using 0.5um E mode pHEMT process of WINs Corp.. And E mode pHEMT transistor is biased with positive voltage that is proper for the PA single bias design.

The proposed dual-mode PA is a power amplifier operated for 2.45GHz and 3.5GHz. The device is manufactured by WINs Corporation with GaAs E-pHEMT MMIC process. To achieve over 25dB of gain and 30dBm of P_1dB at 3.5GHz, a two-stage topology was selected. The two-stage PA is composed of two power transistors of different fingers and integrated broadband input and inter-stage matching networks. The driver stage is sixteen of fingers and 100um of width transistor cells. The driver stage can have 20dB of gain at 20dBm of Pout. The power stage is thirty-two of fingers and 100um of width and can achieves to 32dBm of Pout and 10dB of gain. The external dual-band output-matching is designed for matching to Zopt of 2.45GHz and 3.5GHz network based on low-pass filter cascade high-pass filter structure by lump elements. To estimate loss from inductor on output RF path, the series inductor is replaced to capacitor as Fig. 1. And Fig.2 is the comparion with simulation and measurement of output matching network on ADS data display screen .

Fig. 1. Dual-band LP+HP output-matching network

Fig.2. Comparison with dual-band output-matching simulated and measured S-parameters

In order to reduce losses in the matching networks, 1mil bond wires connected with external high-Q quarter-wavelength RF bias chokes were uesd instead of on-chip spiral inductors. We can tune capacitor Ct to shorten the length of a quarter-wave length RF bias choke on a printed circuit board (PCB) to make the power amplifier MMIC module size smaller. The RF bias choke in the circuit is adaptive by using different scales of bypass capacitors Cs to fit optimum performance for 2.45GHz or 3.5GHz [7- Y.C.Hsu and C.C. Lin,”Single-chip Dual-band WLAN Power Amplifier using InGaP/GaAs HBT”2005 EUMC.]. The schematic of the dual-mode PA MMIC is shown in Fig.3.

The total chip size is 1090um X 810um within 6 I/O pins, and its photograph as shown in Fig.4.

Fig. 3. Schematic of the dual-mode power amplifier

Fig. 4. Photograph of the dual-mode PA MMIC

EXPERIMENTAL RESULTS

The dual-mode power amplifier MMIC is measured on a printed circuit board (PCB) with external quarter-wavelength RF bias chokes and dual-band output matching networks. In order to attenuate low frequency oscillations, dc bias paths are mounted big bypass capacitor about 10uF. The dual-mode PA evaluation PCB is shown in Fig. 5. We implement dual-band output-matching network by mounting 0402 size capacitors and inductor and compare with simulation and measurement as Fig. 6. The PA MMIC is bias DC voltage 5V to drain and 0.4V to gate respectively on PCB, and the quiescent DC current is about 260mA and 230mA at 3.5GHz and 2.45GHz. To achieve high linearity and efficiency requirements, the dual-mode PA MMIC operates in class AB mode. Figure 6 shows PA MMIC power sweep versus single tone input power in the 3.5GHz path. It could be easily observed that the power amplifier is 25.6dB of flat gain. In 2.45GHz WLAN mode, the PA achieves 21dB of gain as Fig.7.

Single tone performance measurement results show that original design targets were successfully achieved at the bands of 2.45GHz and3.5GHz respectively. The 25.6dB of gain and 25dBm of P1dB and about 40% of PAE are measured at the path of 3.5GHz. Fig.8 shows the 21dB of gain and 27dBm of P_1dB and operating total current 380mA at the band of 2.4GMHz.

Fig. 5. Photograph of the dual-mode PA MMIC on EVB

Fig. 6. Comparison with dual-band output-matching simulated and measured impedance versus frequency

Fig. 7. Measured Gain, Ids, Pout, and P.A.E. versus Pin at 3.5GHz

Fig. 8. Measured Gain, Ids, Pout, and P.A.E. versus Pin at 2.45GHz

Linearity performance was evaluated through EVM (Error-Vector Magnitude) measurements using an IEEE 802.16-2004 OFMA modulated signal. The PA EVB is soldered SMA connectors in I/O ports. The EVM measurements in this work have been performed using Agilent Vector Spectrum Analyzer system. WiMAX IEEE 802.16-2004 modulated signal (7MHz BW, 64QAM) and WLAN IEEE 802.11g (OFDM 64QAM/54Mbps) modulated signal were generated by Agilent E4438C SG, and using 89601A VSA software on PC to analysis as Fig. 8.and Fig.9. The EVM of 3.5GHz path and 2.45GHz path are 2.7% and 2.5% respectively at linear output power.

Table I. shows summary of the linear output power, gain, PAE, and EVM measured at 2.45GHz.

Fig. 9. Measured EVM at output power of 24.4dBm under WiMAX IEEE 802.16-2004 OFDM modulation

Fig. 10. Measured 64QAM/54Mbps modulated signal at output power of 19dBm at 2.45GHz

TABLE I
Measured performance of the WLAN PA

The single-chip dual-band WiMAX/WLAN power amplifier operates at a frequency of 3.5GHz under supply 5V bias with 30dBm of P1dB, and 25.6dB RF power gain, and over 15% P.A.Eff. at 24.4dBm. For 24.4dBm OFDM output power (802.16-2004 OFDM), it provides a low EVM of 2.7%, and consumes about 500 mA total current. While at the frequency of 2.45GHz, the PA module could be also applied to IEEE 802.11b/g standards with +27dBm of P1dB, 21dB of RF power gain, about 250mA of total current(CW), and 2.5% of EVM for +19dBm OFDM output power (64 QAM, 54 Mbps). The dual-mode PA is an ideal solution for high linearity, broadband power amplifier requirements for IEEE 802.16d WiMAX and 802.11b/g WLAN applications.