Frequency-Tunable Capability - Linear CMOS RF Power Amplifiers for Wireless Applications - page 66

•

Variable inductors
- Ferromagnetic Inductors [ 56 ]
- Micro-Electro-Mechanical System (MEMS) Inductors [ 34 , 65 ]
- Active Inductors [ 31 , 35 ]
- Saturable Reactors [ 45 ]
- Coupled Inductors [ 6 , 7 , 13 , 17 , 18 , 32 , 41 , 47 , 51 , 57 ]

• Switching

- MEMS Switches [ 8 , 40 ]
- P-Intrinsic-N (PIN) Diodes [ 61 , 62 ].

5.3.2.1 Varactors

The Britannica Encyclopedia online defines the term varactor as follows [ 9 ]:
The varactor (variable reactor) is a device whose reactance can be varied in a controlled
manner with a bias voltage. It is a p-n junction with a special impurity profile, and its
capacitance variation is very sensitive to reverse-biased voltage. Varactors are widely used
in parametric amplification, harmonic generation, mixing, detection, and voltage-variable
tuning applications.
In the literature, the term varactor has not been exclusively used for varactor
diodes, as defined by the Brittanica Encyclopedia, but also for other types of tunable
capacitors whose reactance is controlled by a bias voltage. This is in agreement with
the first phrase of the definition above and will be employed throughout this chapter.
Therefore, we classified the tunable BST and MOS capacitors under varactors.

BST Capacitors Tunable circuits can be built using a BST capacitor [ 1 , 38 ]. The
dielectric material used is a ferroelectric ceramic, (BaSr)TiO3, that exhibits an elec-
tric field-dependent dielectric constant ( ε )[ 38 ]. Its capacitance can be adjusted with
a DC voltage. Tunable matching networks using BST capacitors [ 55 ] and varac-
tors [ 49 ] have been reported. Vicki Chen et al. in [ 55 ] presented a tunable matching
network with transformation factors from 3.8 to 1.7 over a frequency range from
420 to 500 MHz. However, as this method relies on a specific technology not avail-
able in standard processes, it was not considered as an option for the design of the
frequency-tunable CMOS amplifier described in this topic.

MOS Capacitors A MOS transistor can be used as a capacitor by connecting
together its drain and source terminals and applying a voltage V c between the gate
terminal and these two terminals. If V c is positive, the MOS capacitor works in
inversion mode and if it is negative in accumulation mode. In between these two
regions (refer to [ 60 , Fig. 10.10]), close to the transistor threshold voltage ( V T ),
the capacitance undergoes a high variation with the applied voltage. By varying the
applied voltage close to V T the MOS capacitor can work as a varactor [ 60 , Chap. 10].
In [ 50 ], a MOS varactor is used in the interstage matching of an integrated CMOS
LNA tuning its operating frequency from 1.8 to 2.4 GHz (TR 28 . 6%). In [ 43 ],

a discrete MOSFET is used as a MOS capacitor in the design of a tunable class E
PA operating in a frequency range of 19-31 MHz (TR

=

= 48%).