Multilayer Pyramidal Symmetric Inductor

Abstract

In most of the integrated circuits like amplifiers, mixers, oscillators, etc., the differential topology is preferred because of its less sensitivity to noise and interference. There are mainly two categories of differential inductor design found in the literature. The first one is a pair of asymmetric planar inductors connected together in series [1] as shown in Fig. 1.9 symmetric (differential). Since the currents always flow in opposite direction in these two inductors, there must be enough spacing between them to minimize electromagnetic coupling. As a result, the overall area occupied is very large. The second one is the planar symmetric inductor of [2] as shown in Fig. 1.10 which is realized by joining coupled microstrip from one side of an axis of symmetry to the other using a number of cross-over and cross-under connections. An intermediate metal layer is dedicated for the underpass of the cross coupled connections. The center-tapped idea was proposed in [3] for balanced circuits and this type of winding of the metal trace was first applied to monolithic transformers [4]. The symmetrical inductor under differential excitation results in a higher quality factor and self resonance frequency. It also occupies less area than its equivalent pair of asymmetrical inductors. Since these structures are planar, the area is still large. Minimization of inductor area is equally important as enhancing the performance to reduce the production cost. A multilevel symmetric inductor can be realized by stacking two differential inductor of [2] as shown in Fig. 3.1. The structure is a natural extension of the planar differential inductor. This structure is referred hereafter as multilayer conventional symmetric inductor. Realization of cost effective symmetric inductor structures with minimum area without performance degradation is addressed in this chapter.