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3. TECHNICAL BRIEF
3.2 Power Amplifier (SKY77328, U400)
The RF parts consist of a transmitter part, a receiver part, a frequency synthesizer part, a voltage supply
part, and a VCTCXO part.
The SKY77328 Power Amplifier Module (PAM) is designed in a low profile (1.2 mm), compact form factor
for quad-band cellular handsets comprising GSM850/900, DCS1800, and PCS1900 operation. The PAM
also supports Class 12 General Packet Radio Service (GPRS) multi-slot operation. The module consists of
separate GSM850/900 PA and DCS1800/PCS1900 PA blocks, impedance-matching circuitry for 50 Ω input
and output impedances, and a Power Amplifier Control (PAC) block with an internal current-sense 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 GSM850/900 bands and the other supports the DCS1800 and PCS1900 bands. Both PA blocks share
common power supply pins to distribute current. The GaAs die, the Silicon (Si) die, and the passive
components are mounted on a multi-layer laminate substrate. The assembly is encapsulated with plastic
overmold. RF input and output ports of the SKY77328 are internally matched to a 50 Ω load to reduce the
number of external components for a quad-band design. Extremely low leakage current (2.5 µA, typical) of
the dual PA module maximizes handset standby time. The SKY77328 also contains band-select switching
circuitry to select GSM (logic 0) or DCS/PCS (logic 1) as determined from the Band Select (BS) signal. In
Figure 1 below, the BS pin selects the PA output (DCS/PCS OUT or GSM850/900 OUT) and the Analog
Power Control (VAPC) controls the level of output power. The VBATT pin connects to an internal current-
sense resistor and interfaces to an integrated power amplifier control (iPAC™) function, which is insensitive
to variations in temperature, power supply, process, and input power. The ENABLE input allows initial turn-
on of PAM circuitry to minimize battery drain.
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