Abstract
In a radio frequency (RF) system, it is possible to use variable inductors for providing tunable or selective frequency range. Variable inductors can be implemented by the microelectromechanical system (MEMS) process or by using transistors as switches to change the routing of coils or coupling quantities. In this paper, we investigated the design method of a variable inductor by using MOS transistors to switch the main coil paths and the secondary coupled coils. We observed the effects of different metal layers, turn numbers, and layout arrangements for secondary-coupled coils and compared their characteristics on the inductances and quality factors. We implemented two chips in the 0.18 μm CMOS process technology for each kind of arrangement for verification. One inductor can achieve inductance values from about 300 pH to 550 pH, and the other is between 300 pH and 575 pH, corresponding to 59.3% and 62.5%, respectively, inductance variation range at 4 GHz frequency. Additionally, their fine step sizes of the switched inductances are from 0.5% to 6% for one design, and 1% to 12.5% for the other. We found that both designs achieved a large inductance tuning range and moderate inductance step sizes with a slight difference behavior on the inductance variation versus frequency.
Highlights
The simplest way is using CMOS transistors as switches to short or open an inductor that is in series with another inductor [5]
CMOS transistors as switches to change the path of the main coils and/or the secondary coupling coils
When Sc is open-circuited, the simulated main coil inductances are almost the same regardless of the metal layer used in the secondary pling coil, we generated a similar layout and performed an electromagnetic (EM) simulation to obtain the mutual coupling between the two tinds of coils
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Besides MEMS approaches, variable inductors can be implemented by using transistors as switches to change routing paths of coils or coupling quantities. The simplest way is using CMOS transistors as switches to short or open an inductor that is in series with another inductor [5] This method occupies a larger area for using two inductors, and the inductances can be changed by only two discrete values. Proper routing [7] may resolve the area issue, but they can still not provide flexible inductance tuning Another possible mechanism for changing the inductance is to vary the mutual coupling between coils [8].
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