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1200MHz to 2500MHz Adjustable
RF Predistorter
Phase Expansion Circuitry
Figure 1 shows a typical PA ’s phase behavior with
respect to input power. For input powers less than the
breakpoint level, the phase remains relatively constant.
As the input power becomes greater than the break-
point level, the phase begins to compress and deterio-
rate the power amplifier’s linearity. To compensate for
this AM-PM distortion, the MAX2009 provides phase
expansion, which occurs at the same breakpoint level
but with the opposite slope. The overall result is a flat
phase response.
Phase Expansion Breakpoint
The phase expansion breakpoint is typically controlled
by a digital-to-analog converter (DAC) connected
through the PBIN pin. The PBIN input voltage range of
0V to V CC corresponds to a breakpoint input power
range of 3.7dBm to 23dBm. To achieve optimal perfor-
mance, the phase expansion breakpoint of the
MAX2009 must be set to equal the phase compression
breakpoint of the PA.
Phase Expansion Slope
The phase expansion slope of the MAX2009 must also
be adjusted to equal the opposite slope of the PA’s
phase compression curve. The phase expansion slope
of the MAX2009 is controlled by the PFS1, PFS2, PDCS1,
and PDCS2 pins. With pins PFS1 and PFS2, AC-coupled
and connected to a variable capacitor or varactor diode,
the PFS1 and PFS2 pins perform the task of fine tuning
the phase expansion slope. Since off-chip varactor
diodes are recommended for this function, they must
be closely matched and identically biased. A minimum
effective capacitance of 2pF to 6pF is required to
achieve the full phase slope range as specified in the
Electrical Characteristics tables.
As shown in Figure 2, the varactors connected to PFS1
and PFS2 are in series with three internal capacitors on
each pin. By connecting and disconnecting these inter-
nal capacitors, a larger change in phase expansion
slope can be achieved through the logic levels present-
ed at the PDCS1 and PDCS2 pins. The phase expan-
sion slope is at its maximum when both V PDCS1 and
V PDCS2 equal 0V. The phase tuning has a minimal
effect on the small-signal gain.
Gain Expansion Circuitry
In addition to phase compression, the PA also suffers
from gain compression (AM-AM) distortion, as shown in
Figure 3. The PA gain curve remains flat for input pow-
ers below the breakpoint level, and begins to compress
at a given rate (slope) for input powers greater than the
breakpoint level. To compensate for such gain com-
pression, the MAX2009 generates a gain expansion,
which occurs at the same breakpoint level with the
opposite slope. The overall result is a flat gain response
at the PA output.
PA PHASE
COMPRESSION
BREAKPOINT
SLOPE
P IN (dBm)
MAX2009
PHASE EXPANSION
P IN (dBm)
IMPROVED
PHASE DISTORTION
P IN (dBm)
Figure 1. PA Phase Compression Canceled by MAX2009 Phase Expansion
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