IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 5, MAY 1999 645 mA, – V) and from 0.5 to 25.5 GHz. Simulated data calculated with the HBT model 1 variant showed good fits to measurements. Such fits were superior to those obtained with an SGPM developed for the same device. These results manifest the superiority of the HBT model 1 model compared to the SGPM. Large-signal data were also employed to further validate the extracted models. The HBT’s were measured at 840 MHz under four different load/source impedances and continuous wave (CW) exci- tation using a passive load–pull setup. The quiescent bias condition was mA, V. Table II lists the different terminations together with their second and third harmonic impedances. Fig. 4 shows the measured and simulated transducer gain, power-added efficiency, and collector current as a function of the available input power, respectively. A good agreement is observed for all four cases. V. CONCLUSIONS In this paper, the base–collector bias dependence of a recently published HBT large-signal model was modified in order to prop- erly reflect the operational characteristics of power HBT’s. The minority-charge bias dependence is determined by employing op- timization of all transit-time-related model parameters using the effective base–collector capacitance and the cutoff frequency as simultaneous optimization goals. The modified model resulted in good correspondence between measured and simulated small- and large-signal device characteristics. ACKNOWLEDGMENT The author would like to thank Dr. C. Farley for his support and Prof. P. Asbeck, University of California at San Diego, for the useful discussions on the formulation of the HBT model. REFERENCES [1] G. Massobrio and P. Antognetti, Semiconductor Device Modeling with SPICE, 2nd ed. New York: McGraw-Hill, 1993, pp. 45–130. [2] L. H. Camnitz and N. Moll, “An analysis of the cutoff-frequency behavior of microwave heterostructure bipolar transistors,” in Com- pound Semiconductor Transistors: Physics and Technology, S. Tiwari, Ed. Piscataway, NJ: IEEE Press, 1993, pp. 21–46. [3] K. Lu, P. A. Perry, and T. J. Brazil, “A new large-signal AlGaAs/GaAs HBT model including self-heating effects, with corresponding parameter extraction procedure,” IEEE Trans. Microwave Theory Tech., vol. 43, pp. 1433–1445, July 1995. [4] Q. M. Zhang, J. Hu, J. Sitch, R. K. Surridge, and J. M. Xu, “A new large-signal HBT model,” IEEE Trans. Microwave Theory Tech., vol. 44, pp. 2001–2009, Nov. 1996. [5] C.-J. Wei, J. C. M. Hwang, W.-J. Ho, and J. A. Higgins, “Large- signal modeling of self-heating, collector transit-time and RF-breakdown effects in power HBT’s,” IEEE Trans. Microwave Theory Tech., vol. 44, pp. 2641–2647, Dec. 1996. [6] P.C. Grossman and J. Choma, Jr., “Large signal modeling of HBT’s including self-heating and transit time effects,” IEEE Trans. Microwave Theory Tech., vol. 40, pp. 449–464, Mar. 1992. [7] C. M. Snowden, “Large-signal microwave characterization of Al- GaAs/GaAs HBT’s based on a physics-based electrothermal model,” IEEE Trans. Microwave Theory Tech., vol. 45, pp. 58–71, Jan. 1997. [8] L. H. Camnitz, S. Kofol, Tom Low, and S. Bahl, “Large-signal high frequency model for GaAs HBT’s,” in GaAs IC Symp. Dig., 1996, pp. 303–306. [9] D. R. Pehlke and D. Pavlidis, “Evaluation of the factors determining HBT high-frequency performance by direct analysis of -parameter data,” IEEE Trans. Microwave Theory Tech., vol. 40, pp. 2367–2373, Dec. 1992. [10] L. H. Camnitz, S. Kofol, T. Low, and S. R. Bahl, “Using IC-CAP in the development of an accurate large-signal model for GaAs HBT’s,” HP Characterization Solutions, vol. 2, no. 3, pp. 1–2, 8–11, 1997. [11] A. Samelis, D. R. Pehlke, and D. Pavlidis, “Volterra series based nonlinear simulation of HBT’s using analytically extracted models,” Electron. Lett., vol. 30, no. 13, pp. 1098–1100, June 23, 1994. A Novel Wide-Band Tunable RF Phase Shifter Using a Variable Optical Directional Coupler K. Ghorbani, A. Mitchell, R. B. Waterhouse, and M. W. Austin Abstract—We present a novel RF phase-shifter design with a usable bandwidth of 80 : 1. The design is verified through demonstration of a proof of concept device, consisting of a readily available voltage variable optical coupler fabricated from LiNbO , combined with an optic-fiber delay line. The design is analyzed theoretically and measurement of the device confirms the predicted range of operation. Methods of extension of this range of operation are discussed. Index Terms—Optical beamforming, optical directional coupler, phase array, true time delay. I. INTRODUCTION Phased-array antennas with highly directional steerable beams are becoming key components of many military, navigation, and satellite communication systems. Requirements for these devices to be small, lightweight, immune to electromagnetic interference, and most importantly, very broad band, have led to the development of phasing elements involving photonic components [1]. The use of optical delay lines in photonically controlled phased arrays has been established as a successful means of providing broad-band phasing between the elements [2]–[4]. However, many systems based on this technique require tunable lasers and/or large numbers of optical switches and, hence, incur considerable expense and complexity. In this paper, we present a novel broadband electronically tunable RF phase-shifter design. The bandwidth of the design is competitive with true-time delay devices, but has the advantage of a contin- uously tunable phase shift with a single device. This reduces the number of devices and complexity required for antenna phasing and allows for a continuously steerable beam. Our design is verified through the production and demonstration of a proof-of-concept device utilizing readily available photonic components. The scale of the prototype limited operation to below 80 MHz; however, as predicted, multioctave-bandwidth tunable operation is demonstrated. Section II of this paper describes the operation of the phasing unit and presents a simple theoretical analysis of its behavior. This Manuscript received March 18, 1998; revised October 30, 1998. The authors are with the Department of Communication and Electronic Engineering, Royal Melbourne Institute of Technology, Melbourne, Vic. 3000, Australia (e-mail: rwaterhou@rmit.edu.au). Publisher Item Identifier S 0018-9480(99)03138-5. 0018–9480/99$10.00  1999 IEEE Authorized licensed use limited to: RMIT University. Downloaded on December 20, 2009 at 20:31 from IEEE Xplore. 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