Matching Network Design Criteria for Wideband High-Frequency Amplifiers P. Colantonio, F. Giannini, L. Scucchia Department of Electronic Engineering, University of Roma “Tor Vergata”, via Del Politecnico n.1-00133 Roma, Italy Received 29 October 2004; accepted 3 May 2005 ABSTRACT: A powerful design approach for a broadband high-frequency amplifier is developed. In particular, the new matching network presented here, based on an extension of the dual-frequency transformer, allows us to overcome the inherent restriction of previously proposed structures. Design relationships are provided and discussed through a numerical example. Finally, the proposed structure is successfully applied to the design of a Ka-band (26 – 40 GHz) linear amplifier with a minimum return loss of 18 dB. © 2005 Wiley Periodicals, Inc. Int J RF and Microwave CAE 15: 423– 433, 2005. Keywords: transmission line; transformer; impedance matching; dual frequency; amplifier design I. INTRODUCTION The design of microwave amplifiers implies the syn- thesis of a suitable passive matching network in order to fulfill specific requirements (for example, the con- ditions of noise, power, gain, and so forth). Moreover, severe requirements with regard to the return loss, in order to assure a wide bandwidth, will increase the design efforts, while high-fre- quency applications normally claim a transmission- line (TL) approach, which becomes mandatory in order to reduce losses, as compared to lumped solutions. When facing the problem of design a matching network operating in a wide frequency band, the required impedance transformations are usually performed by continuously tapered transmission lines, by microstrip or coplanar technology, or by TLs with stepped characteristic-impedance values [1–3]. On the other hand, the matching-network synthesis very often results in the product of a blind-optimiza- tion procedure rather than a solution based on a well- defined design criterion. Recently, new TL impedance-transformation de- sign formulas, assuming purely resistive load trans- formation, have been presented. In particular, numer- ical approximated solutions of transcendental equations have been suggested in [4] for a dual- frequency transformer operating at frequency f o and its first harmonic 2f o . Moreover, an extended solution based on an exact analytical approach, assuming two different and not related frequencies, has been pre- sented in [5], but maintaining the constraint on the transformation of purely real impedances limits the power of the approach. Finally, a significant improvement has been pre- sented in [6], where the impedance transformation between complex conjugate loads at two given fre- quencies f 1 and f 2 has been demonstrated. In the present article, this last criterion is fully exploited and a new approach for a broadband matching-network design, based on the dual-frequency transformer, is developed and its usefulness is demonstrated through circuit examples. Correspondence to: L. Scucchia; e-mail: scucchia@uniroma2.it. DOI 10.1002/mmce.20107 Published online 21 July 2005 in Wiley InterScience (www. interscience.wiley.com). © 2005 Wiley Periodicals, Inc. 423