RFMicrowave Circuit Design for Wireless Applications by Rohde Ulrich L. & Rudolph Matthias

Author:Rohde, Ulrich L. & Rudolph, Matthias [Rohde, Ulrich L. && Rudolph, Matthias]
Language: eng
Format: epub
Publisher: Wiley
Published: 2012-11-13T23:00:00+00:00

Figure 3.71 Spiral inductor realized in a RFCMOS process (from Ref. [30], reprint with permission). Courtesy W.A. Debski.

The equivalent-circuit model shown in Figure 3.71b provides a first-order model for the spiral inductance. It is quite similar to the model of the parallel-plate capacitor, at least regarding the impact of the lossy substrate that is modeled through Cox1,2, Rsub1,2, and Csub1,2. The parasitic capacitance turns out to be in parallel to the lossy inductance, which yields a parallel resonance circuit. There is, therefore, a maximum achievable inductance, depending on frequency and technology, since adding winding will increase the parasitic capacitance and loss resistance together with the inductance.

Variations and refinements of the equivalent circuit are possible, for example, the symmetrical structure shown in Figure 3.71c. It intends to enhance the validity of the equivalent circuit model somehow toward higher frequencies, by somehow distributing the coupling to the lossy substrate. However, even such a model fails close to or beyond the resonance frequency of the structure.

Transformers are important building blocks in many CMOS-integrated circuits. These can be realized by designing concentric spiral inductors. An example is shown in Figure 3.72. This transformer consists of one turn on the primary side and two turns on the secondary side.

Figure 3.72 Transformer realized in a RFCMOS process (from Ref. [31], reprint with permission). © 2006 Andriy Vasylyev.

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