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3. TECHNICAL BRIEF
C. Power Amplifier (PA)
The PA functionality of the SKY74400 consists of separate GSM850/EGSM900 and DCS1800/PCS1900
blocks, impedancematching circuitry for 50 Ω output impedances, and a PAC block with an internal
currentsense resistor. The custom BiCMOS integrated circuit provides the internal PAC function and
interface circuitry. Fabricated onto a single Gallium Arsenide (GaAs) die, one Heterojunction Bipolar
Transistor (HBT) PA block supports the GSM850 and EGSM900 bands, and the other supports the
DCS1800 and PCS1900 bands. Both PA blocks share common power supply pins to distribute current. The
RF output ports of the SKY74400 are internally matched to a 50 Ω load to reduce the number of external
components for a quad-band design. The PA also contains band-select switching circuitry to select GSM
(logic 0) or DCS/PCS (logic 1) as determined from the BAND_SELECT signal. The VBAT pin connects to an
internal current-sense resistor and interfaces to an iPAC function, which is insensitive to variations in
temperature, power supply, and process.
The ENABLE input allows initial power-on of PA circuitry to minimize battery drain.
(4) Low Drop-Out (LDO) Linear Voltage Regulators
The SKY74400 includes integrated LDO linear voltage regulators to eliminate the need for a separate
power management integrated circuit or discrete voltage regulators. Each functional block in the
SKY74400 includes a separate, internal LDO voltage regulator.
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