The Pa Circuit And Its Control - LG -200 Service Manual

2. TECHNICAL BRIEF

2-3. The PA Circuit and its Control

1. The PA Circuit and its Control
The power amplifier control circuit ensures that the RF signal is regulated to the required limits of
operation.
Power-control is carried out according to the measurement of the power-output of the P.A. (RF_Detect,
see Fig.: P.A Circuit Control). A voltage drop, which is converted from the RF power through a Diode-
Detector, is then integrated by the operational amplifier (U21) to give an error voltage regulates the
TXP signal.
The TXP input signal to the power control circuit is generated by the U1 #90. TXP provides a limit for
the frequency spectrum caused by burst modulation.
VDD_TXBUF (2.85V) is the main power supply for power control circuit. An op-amp, U21, which
outputs a control signal, but is not enough to drive the P.A. to the maximum output. Q20 and Q26 are
an extra amplifier and buffer stage that use Vbat (more than 3.5V) in order to drive the control signal to
a sufficient level for P.A.
The power control circuitry has two different loop gains for increasing the dynamic power control range
and it is switched by U54 that is controlled by the control supply, PCL_SW.
The control supplies, PA_900_EN_B and PA_1800_EN_B control the switches Q28 and Q29 to make
sure the control signal goes to the right PA, VRAMP_900 and VRAMP_1800.
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