Frequency-Tunable Capability - Linear CMOS RF Power Amplifiers for Wireless Applications - page 62

5.1.1 Scenario

Several wireless communication standards are available today. The frequency bands
used are subject to different regulations. The consumer industry is demanding multi-
standard, multifrequency RF PAs. Such devices are realized with either a wideband
PA covering all the frequency bands of interest or with a narrowband PA whose
center frequency can be adjusted while switching to a different standard. The later
option is the underlying principle of the frequency-tunable RF PA.
The scenario can be visualized by imagining a person traveling with his/her Per-
sonal Digital Assistant (PDA) from Country A to Country B. When in the airport
in Country A, the PDA must comply with Standard A so that it can have access to
Internet. This standard has a set of requirements, one of them being Frequency A.
After a long flight to Country B, the person would like to access his/her e-mails
already in the airport. However, there, the wireless communication is regulated by
Standard B, which operates at Frequency B. In order that the PDA can connect to
Internet in these two situations, it must be equipped with a PA that can work satisfac-
torily at Frequency A and Frequency B. This is where a multifrequency PA plays its
major role. However, in the case where such a device is not available the PDA must
count on two PAs, one operating at Frequency A and the second at Frequency B. It
is obvious that if a multifrequency PA exists, it is preferred because of the silicon
area saving with a single PA and the simplicity of the system that does not need to
switch from one PA to another.

5.2 Definition

It is worth clarifying the meaning of the word tunable that is used hereafter. We
mean by frequency-tunable power amplifier a PA whose main characteristics are
adaptive with frequency. Ideally, the objective is that the power amplifier operates
optimally in each of the frequency bands for which it is designed. We also employ
the word tunable to qualify the element that enables this feature: the tunable inductor
in the output impedance matching network. In this case it means that its inductance
can be varied with frequency.

5.2.1 Tuning Range

To compare the performances of different tuning techniques, we define in this sec-
tion the Tuning Range (TR) as a figure of merit. In the context of the frequency-
tunable amplifier that we are presenting, the TR can be related to the center fre-
quency ( f c ), the transformation factor ( m ), and the inductance ( L ). In the literature,
different definitions are used although no explicit formula is given. In this topic, we
define the TR as the relative variation of a parameter with respect to its center value:

x
x c ;

TR = 100 × (5.1)