Digital Signal Processing Reference
In-Depth Information
The asymptotic frequency response of the magnitude of the transfer function of
such a system is characterized by a horizontal line at 0 dB at low frequencies and by
a straight line having a slope of
−
40 dB per decade at high frequencies. These two
asymptotic lines crosses at
ω
ω
n
and at the vicinity of this frequency a resonant
peak occurs. The damping ratio determines the magnitude of this peak.
In the case of the LC filter for
t
he dynamic supply PA, we are interested in a
critically damped response (
ξ
=
√
2
/
2) for which the step response is well damped
and the 3 dB cutoff frequency coincides with the undamped natural frequency. From
(
3.4
) and (
3.5
) we can write
ω
n
as
=
1
√
LC
,
ω
n
=
(3.6)
and the damping ratio as
L
C
1
2
1
R
p
.
ξ
=
(3.7)
Rearranging (
3.6
) and replacing into (
3.7
), we obtain
1
2
Lω
n
1
R
p
=
1
2
1
Cω
n
1
R
p
,
ξ
=
(3.8)
which, rearranged, yields
1
2
ξR
p
ω
n
.
C
=
(3.9)
Replacing (
3.9
)in(
3.6
) and rearranging it, we obtain
1
Cω
n
L
=
.
(3.10)
=
√
2
/
2, and the desired 3 dB cutoff frequency, the LC filter
can be designed. However, as these equations depend on the load
R
p
, the values of
L
and
C
will be determined in Sect.
3.4
after that the simulation results for the RF
PA are presented.
With (
3.9
), (
3.10
),
ξ
3.3 RF Power Amplifie
As explained in the previous chapter, the PA for a dynamic supply system must be
linear. Hence, it can operate in class A, AB, or B. This section treats the design
of a class A amplifier to be employed in a dynamic supply system. The reason for
this choice is that an efficiency-enhancement technique has the side effect of dete-
riorating the linearity of the amplifier. Therefore, selecting the most linear of the
classes should result in a better overall linearity performance of the whole system.
Although class AB PAs can have superior linearity performance, it depends on spe-
cial bias conditions [
14
] that may be not repeatable and, hence, must be used with
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