Conclusion - Linear CMOS RF Power Amplifiers for Wireless Applications - page 122

signals at 2.4 GHz. The results confirmed the effectiveness of the dynamic supply
technique for CMOS RF PAs.
Chapters 5 to 7 focused on the development of a frequency-tunable RF power
amplifier. The definition of such a system was presented as being a PA whose op-
eration is adaptive with respect to the frequency of operation. Normally, such a PA
would require different impedance matching networks to be implemented for the
different frequencies of operation. The proposed system uses coupled inductors as
the means for obtaining this tunable operation and obviating the need of different
matching networks.
The coupled inductors work as a variable inductor that is placed in the output
impedance matching network of the power amplifier. A control circuit is used to
change the phase and amplitude of the current that flows through the secondary
winding of the coupled inductors, thereby changing the inductance seen by the cir-
cuit connected to the primary winding. The variation of the inductance allows one
to maintain the transformation factor of the matching network when the frequency
is changed.
A CMOS prototype was fabricated in a 0 . 11 µm technology. The measurement
results did not allow us to prove that the principle works. Hence, we implemented
a hybrid system composed of an integrated CMOS power amplifier (designed for
testing purposes), a discrete transformer, and a discrete RF bipolar transistor. With
this hybrid implementation, we showed that a frequency-tunable RF PA can be built
using coupled inductors in its output matching network. However, the frequency
of operation had to be decreased to 200-300 MHz (TR

= 40%) from the targeted

3.7-5.2 GHz (TR 34%). The reason for reducing the frequency is that the com-

mercially available transformers were not able to operate at higher frequencies.
Furthermore, the high inductance values of these discrete transformers forced the
reduction of the maximum output power.
The measurement results from the hybrid system, the simulation results for the
complete CMOS solution, and the design methodology proposed form a set of in-
formation from which we can conclude that a frequency-tunable RF PA based on
coupled inductors is a good solution for a multiband PA.

=

8.2 Main Contributions

8.2.1 Efficienc Enhancement

The effectiveness of the dynamic supply technique had already been demonstrated
in a system using an integrated CMOS modulator and a discrete bipolar RF power
amplifier operating at 1.9 GHz for CDMA applications [ 3 ]. Hence, the main contri-
bution of Chaps. 2 to 4 of this topic is to show that the dynamic supply technique
can also be successfully applied in a system where both the modulator and the RF
power amplifier are monolithically integrated in a deep-submicron CMOS technol-
ogy. Furthermore, the system was designed for a higher frequency band, 5.2 GHz,