LG GB108, Service Manual

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Power Amplifier Module(SKY77517/ SKY77518)
3.2.1 GB100/GB100b/GB102/GB105/GB105b/GB108 (SKY77518)
Figure.3-2-1 SKY77518 FUNCTIONAL BLOCK DIAGRAM
The SKY77518-21 is a transmit and receive front-end module (FEM) with Integrated Power Amplifier Control
(iPAC™) for dual-band cellular handsets comprising GSM900 and DCS1800 operation. Designed in a low
profile, compact form factor, the SKY77518-21 offers a complete Transmit VCO-to- Antenna and
Antenna-to-Receive SAW filter solution. The FEM also supports Class 12 General Packet Radio Service (GPRS)
multi-slot operation.
The module consists of a GSM900 PA block and a DCS1800 PA block, impedance-matching circuitry for 50
Ω input and output impedances, TX harmonics filtering, high linearity and low insertion loss PHEMT RF
switches, diplexer and a Power Amplifier Control (PAC) block with internal current sense resistor. A custom
BiCMOS integrated circuit provides the internal PAC function and decoder circuitry to control the RF switches.
The two Heterojunction Bipolar Transistor (HBT) PA blocks are fabricated onto a single Gallium Arsenide
(GaAs) die. One PA block supports the GSM900 band and the other PA block supports the DCS1800 band. Both
PA blocks share common power supply pads to distribute current. The output of each PA block and the outputs
to the two receive pads are connected to the antenna pad through PHEMT RF switches and a diplexer. The
GaAs die, PHEMT die, Silicon (Si) die and passive components are mounted on a multi-layer laminate substrate.
The assembly is encapsulated with plastic overmold. Band selection and control of transmit and receive modes
are performed using two external control pads. Refer to the functional block diagram in Figure.3-2-1 below. The