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3.2
Power Amplifier Module (SKY77518)
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
selects between GSM and DCS modes of operation. The transmit enable (TX_EN) pad controls
receive or transmit mode of the respective RF switch (TX = logic 1). Proper timing between transmit
enable (TX_EN) and Analog Power Control (VRAMP) allows for high isolation between the antenna
and TXVCO while the VCO is being tuned prior to the transmit burst.
The SKY77518-21 is compatible with logic levels from 1.2 V to VCC for BS and TX_EN pads,
depending on the level applied to the VLOGIC pad. This feature provides additional flexibility for
the designer in the selection of FEM interface control logic.
Figure.3-2-1 SKY77518 FUNCTIONAL BLOCK DIAGRAM
Figure 3-2-1
below. The band select pad (BS)
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