Frequency-Tunable Capability - Linear CMOS RF Power Amplifiers for Wireless Applications - page 71

the coupled inductors—capacitor C 1 acts like a DC block—generating the current
I RF . The drain current of M 1 is related to the input voltage through the transcon-
ductance g m1 . This current generates a voltage at the gate of transistors M 2 and M 3
when flowing through 1 /g m2 . The current through M 3 is related to its gate volt-
age through g m3 and is the same current flowing into the secondary winding of
the coupled inductors. Hence, the relationship between I ctrl and I RF depends on the
transconductances g m1 , g m2 , g m3 , and on the inductance L 1 . The correct adjustment
of these parameters leads to the cancellation of the real part of the impedance seen
bytheRFcircuit(referto[ 57 , (3)]). The circuit was implemented in a 0.18 µm
CMOS technology and quality factor values higher than 3000 were reported from
1.5to2.1GHz.
The coupled-inductor technique has been used in integrated tunable RF circuits
since its first demonstration in 1997. In [ 6 , 7 ], using three inductors with mag-
netic coupling between each two of them, the authors developed a tunable pass-
band CMOS RF filter. The filter attained a blocking dynamic range of 80 dB with
a center frequency with a TR of 11% around 1 GHz. Other applications using the
coupled-inductor technique in Q -enhanced RF filters have been reported [ 17 , 18 ,
32 , 51 ]. The application in tunable oscillators was described in [ 13 , 47 ]. It is worth
noting that the coupled-inductor technique has also been subject of research under
the name of Boot-Strapped Inductor (BSI), as found in [ 2 , 14 , 48 ].

5.3.2.3 Switching

Tunable matching networks can also have recourse to a switching strategy to vary
the capacitance or inductance of one or more network branches. Two kinds of inte-
grated switching strategies can be found in the literature and are explained below.

MEMS Switches Another use of MEMS in tunable matching networks, other than
the inductors described in Sect. 5.3.2.2 , is the MEMS switch. Bartlett et al. in [ 8 ]
use these switches within a bank of parallel inductors in the series branch and a bank
of parallel capacitors in the shunt branch of an L -type output matching network of a
PA. Zhou et al. in [ 63 ] implemented variable inductors that could achieve a tuning
range from 2.5 to 324.8 nH (TR

=

197%) in frequencies from 0.5 to 1.6 GHz using
four MEMS switches. In [ 40 ], the switches are used to activate stub tuners that can
be used in impedance matching networks. The tuners can operate from 10 to 20 GHz
to match loads with real parts between 1.5 and 110 and imaginary parts between
−

260 and 91 .

PIN Diodes PIN diodes have a structure in which a lightly doped (nearly intrinsic)
semiconductor separates a p + from a n + region. The device behaves as a voltage
controlled resistance that can be used as a switch in RF frequencies [ 24 , p. 904].
The resistance of a forward-biased PIN diode is controlled by the DC current flow-
ing through it. In [ 61 , 62 ], a PIN diode, four capacitors, and four inductors form a
π -matching network. The series inductance between the two shunt capacitors of the