    Equivalent Circuit Model of GaAs MMIC-Coupled Planar Spiral Inductors G. Avitabile, A. Cidronali, C. Salvador Dipartimento di Ingegneria Elettronica, Via S. Marta, 3, I-50139 Florence, Italy; e-mail: miclab@ingfi1.ing.unifi.it Recei  ed 30 May 1996; re ised 28 September 1996 ABSTRACT: The article describes a lumped component model which accounts for coupling between inductors in GaAs MMIC design. The model was developed using a full-wave approach. The validity of the proposed circuit has been successfully tested. Experimental results are reported to validate the model.  1997 John Wiley & Sons, Inc. Int J Microwa  e Millimeter-Wa e CAE 7: 318–326, 1997. Keywords: spiral inductors; computer-aided design; microwave passive elements INTRODUCTION Spiral inductors are used extensively in GaAs MMICs for both bias circuit decoupling and matching network design. The use of these com- ponents is almost mandatory in wideband applica- tions. Many problems are solved by MMIC foundries which furnish models of spiral induc- tors in one or more standard geometries andor configurations. The main problem in a MMIC design based on spiral inductors is the large chip area that these components require. In fact, space and thus cost reduction, is the prime goal of the MMIC de- signer. The straightforward approach to chip size reduction is to diminish the spacing between the various sections of the circuit, which may include spiral inductors. Unfortunately, a close spacing between large structures, like the inductors, intro- duces unwanted couplings, which in turn can sub- stantially modify the expected circuit perfor- mance. More generally, the MMIC designer often has to address problems which cannot be ap- proached using standard component libraries.   Full-wave analysis techniques 1  7 are commonly used to gain insight in these cases. Several CAD Correspondence to: G. Avitabile Ž full-wave tools are commercially available Son- net, HP-EES of HFSS and Momentum, Compact . Explorer . These tools, while allowing a high de- gree of accuracy in predicting passive component behavior, require long computational times to perform full-wave analyses. Moreover, their inter- action with commercial microwave CAD is gener- ally good but the automatic optimization proce- dure is time consuming. Both circumstances dis- courage the MMIC designer from performing it- erative design optimizations. This study introduces a wideband lumped-com- ponent model for coupled spiral inductors, which can be used either to minimize the coupling be- tween the sections of the MMIC or to refine the CAD design of the MMIC when a close spacing between inductors occurs. The model is deter- mined by first choosing a suitable circuit topology, which accounts for the physical behavior of the coupled inductors, and then extracting circuit ele- ments with the help of a full-wave analysis. To preserve the generality of the method, the proce- dure is repeated for several mutual positions of the inductors and thus, the model component values are expressed as frequency-independent functions of the geometry and of the substrate characteristics.  1997 John Wiley & Sons, Inc. CCC 1050-182797040318-09 318