Radio Frequency Integrated Circuit Design - Webs

The Use and Design of Passive Circuit Elements in IC Technologies
oscillators (VCOs) or power amplifiers using on-chip inductors, to have limited
performance compared to designs using off-chip components. The use of offchip
components adds complexity and cost to the design of these circuits, which
has led to intense research aimed at improving the performance of on-chip
inductors [5–19].
Traditionally, due to limitations in modeling and simulation tools, inductors
were made as square spirals, as shown in Figure 5.9. The wrapping of the
metal lines allows the flux from each turn to be added, thus increasing the
inductance per unit length of the structure and providing a compact way of
achieving useful values of inductance. Square inductors, however, have less than
optimum performance due to the 90° bends present in the layout which add
to the resistance of the structures. A better structure is shown in Figure 5.10
[7, 18]. Since this inductor is made circular, it has less series resistance. This
geometry is more symmetric than traditional inductors (its S parameters look
the same from either side). Thus, it can be used in differential circuits without
needing two inductors to get good symmetry. Also, bias can be applied through
the axis of symmetry of this structure if needed in a differential application
(i.e., it is a virtual ground point).
Figure 5.9 A conventional single-ended inductor layout.
107