LG 512W, Service Manual

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3. TECHNICAL BRIEF
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Figure 3-7. Power Amplifier and its Control Part Circuits.
D. PA Circuit and Control
The power amplifier control circuit ensures that the RF signal is regulated to the required limits of
operation.
RF power is controlled by driving the power control pins of power amplifier and sensing the
resultant RF output power via a directional coupler (U305). The RF sense voltage is peak detected
using an on-chip Schottky diode inside of AD8315(U304). This detected voltage is compared to the
DAC voltage at the VSET pin of APC IC (U304) to control the output power.
An internal input signal(TXRAMP) from AD6521(U2) is applied to the positive input of the APC
IC(U304) during the TXEN mode and a directional coupler near the antenna feeds a portion of the
RF output signal back to the APC IC(U304) and peak detector converts this signal to a low
frequency feedback signal that balances the amplifier when this signal equals the RAMP input
signal level.