13
To Duplexer
Battery
PA
_
+
_
R3
V1
R4
R5
R2
Vin
C1
R1
PA_ON
TX_ADC_ADJ
Baseband IC
TxIC
Vcontrol
Switch Circuit
Enable
Vdd1
Vdd2
Vcntl
Vbias
To Duplexer
Battery
PA
+
_
+
_
PA_ON
TX_ADC_ADJ
PDM1
Baseband IC
TxIC
Vcontrol
Switch Circuit
Enable
Vdd1
Vdd2
Vcntl
Vbias
3. Dynamic Bias Control Alternate Implementation
Phone designers can use TX_ADC_ADJ pin of the
baseband IC to get dynamic bias control with Vcntl
pin of PA. TX_ADC_ADJ is a PDM output pin produced
by the TX AGC subsystem and used to control the gain
of the Tx signal prior to the PA. The variable output
levels from two inverting operational amplifiers,
generated and compared by TX_ADC_ADJ, provide
dynamic control voltages for the Vcntl of 1.0V ~ 2.7V
with a 0.1V step.
Av = -(V1/Vin) = -R3/R2,
V1 = -(R3/R2)Vin,
Vo = -(R5/R4)V1= [(R5*R3)/(R4*R2)]*Vin
The using of combination of two pins, PDM1 and
TX_ADC_ADJ, is another method of realizing a dynamic
bias control scheme. The two OP Amps control the
Vcntl voltage levels with compared and integrated
circuits.
