Digital Signal Processing Reference
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which showed results very close to the experimental values in our case. In this first
design, both the PA and the transformer should be integrated. This first integration
allows the characterization of the RF power amplifier and the coupled inductors
separately. Then, in a second design, the complete system would be integrated fol-
lowing the schematic of Fig.
7.26
.
Although the fully-integrated version of the frequency-tunable PA did not work,
the hybrid implementation showed that the principle underlying the technique is
valid and can be used in tunable power amplifiers. When considering using the
technique, the designer should however take into account the important points listed
below.
•
Required output power: it determines the choice of
R
opt
and, hence, of the match-
ing network components.
•
Required frequency bands: it affects the inductance tuning range and the values
of
C
1
,
L
1
and
C
2
. The higher the frequency, the lower the inductor value for fixed
C
1
and
C
2
.
•
Required optimum resistance: it depends on the required output power. It affects
C
1
,
L
1
and
C
2
. For fixed values of
C
1
and
C
2
, the higher is
R
opt
, the higher is the
value of
L
1
and the lower is the attainable output power for a given VDD.
•
Value of the integrated inductor (
L
1
): it must be within a range that can be inte-
grated. Hence, it is limited by the required
R
opt
and by IC technology constraints.
•
Values of
C
1
and
C
2
: for a given
R
opt
, the higher are
C
1
and
C
2
, the higher is
Q
0
and, hence, the lower is the difference between the achieved
R
opt
at different
frequencies when keeping
C
1
and
C
2
constant and varying just
L
1
. However,
the higher
Q
0
is, the lower is
L
1
—in this case,
L
1
can attain values that are
impossible to be implemented.
•
Required inductance tuning range: it affects the required
α
and
β
variation. The
higher
α
and
β
variation is, the more difficult is the implementation of the control
circuit. The higher is
α
, the higher is the power consumption of the control circuit.
•
Coupling factor: it affects the attainable inductance tuning range. Layout opti-
mization must be done to increase the coupling factor.
7.7 Conclusion
This chapter presented the measurement results for the frequency-tunable RF power
amplifier including individual results for each functional block (RF PA and coupled
inductors) as well as the results for the complete tunable PA. Since the complete
integrated system did not work as expected, a hybrid circuit using the stand-alone
RF PA together with a discrete transformer and a discrete bipolar RF transistor to
implement the control system was realized. The simulation results of the integrated
tunable PA at 3.7 and 5.2 GHz presented in Chap. 6, and the measurement results of
the hybrid implementation at 200 and 300 MHz discussed in this chapter, showed
that a frequency-tunable impedance matching network based on coupled inductors
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