A 1-W GaAs Class-E Power Amplifier with an FBAR Filter Embedded in the Output Network

Transcription

1 A 1-W GaAs Class-E Power Amplifier with an FBAR Filter Embedded in the Output Network Kyle Holzer and Jeffrey S. Walling University of Utah PERFIC Lab, Salt Lake City, UT 84112, USA Abstract Integration of a class-e power amplifier and a thin-film bulk acoustic wave resonator (FBAR) filter is shown to provide high power added efficiency in addition to superior out-of-band spectrum suppression. A discrete GaAs phemt transistor is implemented to operate as a class-e amplifier from MHz. The ACPF7041 compact bandpass FBAR filter is incorporated to replace the resonant LC tank in a traditional Class-E PA. To reduce drain voltage stress, the supply choke is replaced by a finite inductance. The fabricated PA provides up to 1 Watt of output power with a peak PAE of 58%. The improved out-of-band spectrum filtering is compared to a traditional Class-E with discrete LC resonant filtering. Such PAs can be combined with linearization techniques to reduce out-ofband emissions. Index Terms Ceramics, coaxial resonators, delay filters, delay-lines, power amplifiers. I. INTRODUCTION Switched-mode power amplifiers offer higher efficiency than their linear counterparts, however natively generate a rich spectrum of harmonics and intermodulation products. This paper presents an efficient Class-E power amplifier with an embedded thin-film bulk acoustic wave resonator filter (FBAR, Fig. 1a). The FBAR filter is incorporated into the Class-E output circuit, replacing the traditional LC resonant tank (Fig. 1b). This provides superior in-band frequency selectivity and out of band spectrum suppression. Piezoelectric materials have been used for microwave frequency filtering for decades. Thin film technologies including surface acoustic wave (SAW) and bulk acoustic wave (BAW) filters have proven advantages compared to traditional discrete inductor and capacitor derived counterparts due to the acoustic wavelength at a given frequency being several orders of magnitude shorter than the analogous electrical wavelength in a comparable electromagnetic filter medium. The FBAR builds on BAW filter technology with the advantages of improved performance and improved ease of integration with standard planar device technologies. FBAR filters do not require exotic materials typical of both SAW and BAW filters such as quartz, lithium niobate, ZnO and others. FBAR (1) can be fabricated in the same process as a typical high performance amplifier, enabling the integration of a power efficient, and spectrally pure, switching amplifier design; an integrated extension of the discrete design presented in this paper. II. DESIGN DETAILS The amplifier implementation is fabricated including a custom printed circuit board with chip-on-board construction as shown in Fig. 2. The small size of this implementation shows future potential for fully integrated PA dies with an FBAR either integrated into the process technology or bonded directly to the die in a system-in-package implementation. A. Class-E Amplifier Design A broadband TGF2080 GaAs phemt device is chosen as the power transistor for this switching power amplifier. This device offers low input capacitance and moderate optimal impedance load magnitude across the target design frequency. 1

2 A high pass dual element conjugate match is implemented at the input to minimize insertion loss and reduce component count. The optimal Class-E output shunt susceptance is achieved with a combination of the device s drain capacitance, C 1 and L 1 as shown in Fig. 1 (2). The device s output capacitance is larger than required; hence, finite L 1 is used to offset the additional shunt capacitance. Finite inductance in the drain has also been shown to reduce the voltage swing at the drain to as low as 2.5 V DD(3). Breakdown voltage for this 0.25um GaAs process is in excess of +20V enabling continuous Class-E operation. Series inductance (L 2, Fig. 1) selection optimizes transition from the Class-E output into the load impedance as derived in (4). ωωωω RR = (1) The series inductance aligns the voltage and current incident to the load. B. Class-E with Embedded FBAR Intrinsic to the Class-E amplifier output tuning circuit is a series resonant circuit to filter undesired harmonic content that arises as a result of the periodic pulsed operation of the amplifier. In the classic implementation this series resonant circuit can be designed with any quality factor (Q) and performs well even for relatively low Q. This series resonant circuit can be replaced by any bandpass filter or matching network, and can be comprised of more than two components. Hence, the implementation is a trade-off between the required out-of-band (OOB) filter rejection and the number of filter poles. A higher number of filter poles increases both the amplifier size and its insertion loss. In this design we replace these traditional LC tanks with an FBAR filter that increases the OOB rejection significantly, while minimizing the insertion loss associated with a high-order LC filter. The size is comparable to that of a single LC tank filter while providing significantly better OOB rejection. Due to the operating voltage and desired output power, the 50 Ω FBAR filter provides the real part of the impedance termination for the PA; hence no additional matching circuits are required. Using the previous equation, L 2 is optimized using the FBARs input impedance, R=50Ω. Subsequently in the case where a traditional LC tank is used, the selection of L 2 does not change for operation at the resonant frequency. Given the antenna input impedance closely matches 50Ω. C. Class-E with Traditional LC Tank For comparison to the embedded FBAR resonator a single LC tank resonator is designed for the center frequency of operation. High frequency multi-layer ceramic components are chosen to maximize the realizable network Q. To aid the comparison between the two amplifiers, no impedance transformation is used hence the Q of the LC-tank is limited by the load resistance, which is 50Ω, and yields a 20% fractional bandwidth. D. Amplifier Input Match Optimization Due to amplifier reverse isolation, S 12, the input match can be affected with changes to the load impedance. This was verified through simulation of input impedance comparing a traditional LC tank and the FBAR filter resonant elements. The previously designed high pass dual element input amplifier match provides sufficient in both cases. E. Chip-on-Board (COB) Implementation 2

3 To minimize parasitics associated with packaging, the amplifier die is COB bonded directly to the PCB. This provides additional advantages of smaller size compared to packaged surface mount devices as well as lower junction to board thermal conductivity. III. MEASUREMENT RESULTS The PA was fabricated and its performance characteristics measured. A TQP3M9035 driver amplifier is included on the printed circuit to lower the required power level for input test signals. In order to validate the FBAR tuned class-e PA (Fig. 2a), measurements are included for a Class-E with LC tank PA (Fig. 2b). Both amplifiers use the same TGF2080 device and PCB with only modifications to the output network components (e.g., FBAR, LC tank). The peak output power of 29.95dBm is achieved by the Class-E FBAR PA at 2.64GHz (Fig. 3). The passband ripple is a characteristic of the FBAR filter. The Class-E LC tank PA has the highest average output power across the passband due to the small insertion of the single LC tank compared to 2.5dB insertion loss for the FBAR. The PAE of each amplifier configuration is plotted vs. frequency in Fig. 4. The low insertion loss of the single LC tank clearly yields the highest PAE. The peak PAE for the Class-E FBAR PA is only moderately lower at 58%. The PAE can be significantly improved with integration onto a single die due to a combination of lower insertion loss through the FBAR filter optimized for target filter rejection and minimized signal transition losses between component interfaces. The distinct advantage of FBAR integration is the superior filtering capability in a small footprint. The three amplifier configurations harmonic content are measured and plotted vs. frequency in Fig, 5. The Class-E LC tank PA that offers a PAE advantage is also unable to adequately suppress harmonic content. The FBAR filter shows similarly strong harmonic rejection for both the linear mode and the Class-E mode, as expected. The PA output is plotted over one octave of spectrum in Fig. 6 and demonstrates the adjacent channel rejection afforded by the FBAR filter, in this case nulling among others the adjacent ISM 2.4GHz band used by WLAN and Bluetooth. The LC tank offers virtually no spectrum shaping. This application of harmonic filtering has a performance vs. size advantage over alternate approaches such as tuned distributed element filters (5). Active harmonic suppression (6) require considerable stability optimization for mitigated harmonic improvement. In order to provide linear operation, the class-e PA must be linearized via external means. One example is Envelope Elimination and Restoration (EER); to accurately recreate the amplitude envelope modulation at the amplifier load (7). The optimal amplifier load impedance will vary with the drain voltage. A drain voltage of 8V was used for device design and optimization. The performance over a wider range of voltages is plotted in Fig. 7. Increasing the drain voltage beyond 8V offers minimal effect on output power as the drain current is at saturation. Decreasing the drain voltage reduces the output signal swing and power as expected without significantly affecting load match performance. IV. CONCLUSIONS An integration of a Class-E power amplifier and an FBAR filter is shown to provide high PAE in addition to superior out of band spectral content suppression. The power amplifier is designed using a broadband discrete GaAs phemt device configured for 3

4 Class-E operation in concert with the ACPF7041 FBAR filter ( MHz). The power amplifier provides up to 1 W of output power with a peak PAE of 58%. Superior out of band spectrum filtering is shown in comparison to discrete LC resonant filtering. ACKNOWLEDGEMENTS The authors wish to acknowledge Triquint Semiconductor for their donation of the PA driver and transistor dies and Avago Technologies for their donation of the FBAR Filters. We also wish to thank L-3 Communications for their measurement support. REFERENCES 1. Morkner H, Ruby R, Frank M, Figueredo D. An Integrated FBAR filter and PHEMT switched-amp for wireless applications. MTT-S International Microwave Symposium Digest p Sokal NO, Sokal AD. Class E-A new class of high-efficiency tuned single-ended switching power amplifiers. IEEE J Solid-State Circuits. 1975;10(3): Yoo C, Huang Q. A common-gate switched 0.9-W class-e power amplifier with 41% PAE in 0.25um CMOS. IEEE J Solid-State Circuits. 2001;36(5): Andrei Grebennikov. RF and Microwave Power Amplifier Design. McGraw-Hill Professional; Lim J-S, Jeong Y-C, Ahn D, Lee Y-T, Cho H, Nam S. Size-reduction and harmonic-rejection of microwave amplifiers using spiraldefected ground structure. 33rd Eur Microw Conf. 2003; Park JW, Razavi B. A Harmonic-rejecting CMOS LNA for broadband radios. IEEE J Solid-State Circuits. 2013;48(4): Walling JS, Allstot DJ. Linearizing CMOS switching power amplifiers using supply regulators. IEEE Trans Circuits Syst II Express Briefs. 2010;57(7): FIGURES Fig. 1 Schematic of the (a) class-e PA with embedded FBAR filter and (b) traditional class-e PA. 4

5 (a) (b) Fig. 2 PCB layouts for the (a) class-e PA with FBAR filter and (b) class-e PA with LC tank. Fig. 3 Measured output power vs. amplifier configuration Fig. 4 Measured PAE vs. amplifier configuration 5

6 Fig. 5 Measured harmonics vs. amplifier configuration: ( ) = class- E FBAR Harmonics 2-6, (--) = class-e LC Tank Harmonics 2-6 Fig. 6 Measured class-e PA output spectrum comparison Fig. 7 Measured class-e PA FBAR Pout vs. VDD (+ 5V, O 6V,* 7V,x 8V) 6