Frequency-Tunable Capability - Linear CMOS RF Power Amplifiers for Wireless Applications - page 63

where x is the tunable parameter, x is the difference between the highest ( x hi )
and the lowest ( x lo ) value that x can take, and x c is the center value between them.
Hence, the tuning range can be rewritten as

2 x hi − .

x lo

TR = 100 × (5.2)

x hi + x lo

For instance, a PA whose center frequency ( f c ) can be tuned from 2 to 2.5 GHz
has a TR of 22.2%. 2

5.3 Overview of the Existing Frequency-Tunable Techniques

This section covers the most important works found in the literature that represent
the state of the art with respect to the techniques that can be applied in the develop-
ment of a frequency-tunable power amplifier. We classify these works into two dif-
ferent groups: broadband and narrowband techniques. Broadband techniques have
the objective of enlarging the bandwidth of an amplifier without changing its center
frequency. They are presented in Sect. 5.3.1 . Narrowband techniques have the goal
of adjusting the center frequency of the amplifier while keeping a narrow bandwidth
around this frequency. They are described in Sect. 5.3.2 . The technique used in the
frequency-tunable RF power amplifier described in the following chapters falls into
the second group and, hence, is also introduced in Sect. 5.3.2 .

5.3.1 Broadband Techniques

5.3.1.1 Lossy Matching

The use of lossy matching networks is a common technique employed in MESFET
amplifiers in order to achieve a flat gain and a low Voltage Standing Wave Ratio
(VSWR). By using a resistive matching network, the gain roll-off of the amplifier is
compensated at the expense of a lower power gain and, hence, a lower efficiency.
Arell and Hongsmatip in [ 3 ] and Zhu et al. in [ 64 ] designed GaAs RF power
amplifiers with an operating frequency from 2 to 6 GHz. The output power is in the
order of 40 dBm (10 W) with a PAE > 20%, and a VSWR < 2.
Bahl in [ 4 ] presents a MESFET PA operating from 5 to 8.5 GHz and providing
more than 33 dBm (2 W) output power, 15 dB gain, 31% PAE, and a VSWR better
than 2.
The design equations for the implementation of lossy matching networks can be
found in [ 25 ].

2 f c _ c

= 2 . 25 GHz, f c _ lo = 2 GHz, f c _ hi = 2 . 5 GHz, and f c = 0 . 5 GHz.