Frequency-Tunable Capability - Linear CMOS RF Power Amplifiers for Wireless Applications - page 67

Varactor Diodes Varactor diodes are commonly used in Voltage-Controlled Os-
cillators (VCOs) [ 60 , Chap. 2]. In the design of tunable matching networks, discrete
implementations such as [ 23 ], and SiGe HBT implementations such as [ 36 ], have
been reported. In [ 36 ], the power amplifier delivers 27 dBm (500 mW) output power
in the 900, 1800, 1900, and 2100 MHz bands. In [ 23 ], the tunable matching network
allows a complex impedance of magnitude from 6.3 to 1120 to be matched at
1 GHz.

5.3.2.2 Variable Inductors

Several possibilities for inductor variation have been recently reported. These alter-
natives are analyzed in the following sections.

Ferromagnetic Inductors FerroMagnetic (FM) materials can be used in induc-
tor cores to form tunable inductors. By injecting a DC current in the inductor, the
permeability of the core changes and so does the inductance. Vroubel et al. in [ 56 ],
presented a tunable inductor comprising a planar solenoid with a thin film ferro-
magnetic NiFe core. With the inductance ranging from 1 to 150 nH, the authors
demonstrated that a relative inductance variation ( L/L ) from 20% at 2 GHz up
to 85% at 100 MHz could be achieved. The drawbacks are the low quality factor
( Q< 2) and high DC current consumption.

MEMS Inductors The advances in MEMS devices in the last decade have al-
lowed their use in a number of applications. The field of tunable RF devices is con-
tinuously experiencing the benefits of such advances [ 16 ]. A tunable inductor can be
built with MEMS technology as reported in [ 34 , 65 ]. Zine-El-Abidine et al. in [ 65 ]
proposed an inductor whose value was tunable by controlling the magnetic coupling
between two inductors. This magnetic coupling contributes to the total inductance
and is controlled by varying the distance between the two inductors using thermal
MEMS actuators. An inductor with a TR of 12.5% in a frequency range of 2-5 GHz
could be achieved. In [ 34 ], the angle between two inductors is controlled with some
kind of self-assembling technique allowing 18% inductance variation (TR =

20%),
over 15 GHz Self-Resonant Frequency (SRF) and a quality factor higher than 13.
Okada et al. in [ 39 ] presented an inductor whose value could be tuned by the move-
ment of a metal plate controlled by a MEMS actuator. However, the actuator itself
was not implemented and the movement of the plate was made manually by a mi-
cromanipulator.

Active Inductors Active inductors are another possible implementation of tun-
able inductors, but high consumption, complexity, noise, and nonlinearity [ 56 ]im-
peded their widespread. Active inductors are mainly designed based on the gyrator
principle, like in [ 58 ], or Generalized Impedance Converter (GIC) topologies, as
in [ 22 , 31 , 35 , 59 ]. Kobayashi et al. in [ 31 ] proposed an active inductor using the
common-gate, cascode-FET feedback topology [ 22 ] to implement a GaAs VCO-