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X9470
State 3: PA Standby Mode, DC Bias Adjustment
[Compensation for V GS Drift and Temperature
Variation]
When the Power Amplifier is in Standby Mode the
X9470 allows for dynamic adjustment of the DC bias-
ing point to take into account both V GS drift and tem-
perature variation. Dynamic biasing is achieved with
the X9470 by using the CS, and SCL pins. For exam-
ple, the SCL pin can be a steady clock and the CS pin
can be used as a control signal to enable/disable the
Bias Adjustment Block.
Figure 6 illustrates how the X9470 can be used for
dynamic biasing. Upon the presence of an RF signal,
the CS pin is pulled LOW. This will prevent the X9470
from changing the V BIAS voltage during I DQ peak cur-
rents. Once the RF signal is no longer present, the CS
pin can be enabled (closed loop), SDA high and the
X9470 Bias Adjustment Circuit moves the V BIAS volt-
age (the gate voltage of the FET) to meet the average
I DQ bias point for optimum amplifier performance.
State 4: Power Off Mode
During power saving or power-off modes the X9470
can be shut down via the SHDN pin. This pin pulls the
output of the V BIAS pin LOW.
Figure 6. Dynamic Biasing Technique: Automatic DC Bias Operating Point Adjustment
RF signal
State 0
Monitor
Mode
State 1
Calibration
(no RF present)
State 2
RF present
State 3
Recalibrate bias
point for drift
and temperature
State 4
shut
down
V REF
Set Operating Range Scale for Bias Adjustment
CS
Bias Adjustment ON
Bias Adjustment ON
Bias Adjustment OFF
SCL
INC/DEC
Saves wiper position to
volatile memory
BiasLock
BiasLock
SHDN
V BIAS
3
4
6
Shut
down
R bias default is
zero point of R total
2
Latch R bias DC point
in calibration vs V REF
RF present
Turn off
Bias
Adjustment
R bias increase/decrease
after RF present due to
temperature increase &
V GS -threshold drift
5
IDQ vs. gate
voltage bias
optimized
1
Automatic Bias Adjustment
17
FN8204.0
March 8, 2005
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