LG U890, Service Manual

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The single Gallium Arsenide (GaAs) Microwave Monolithic Integrated Circuit (MMIC) contains all
active circuitry in the module. The MMIC contains on-board bias circuitry, as well as input and
interstage matching circuits. The output match is realized off-chip within the module package to
optimize efficiency and power performance into a 50 Ω load. This device is manufactured with
Skyworks’ GaAs Heterojunction Bipolar Transistor (HBT) process that provides for all positive voltage
DC supply operation while maintaining high efficiency and good linearity.
Primary bias to the SKY77410 is supplied directly from a three-cell Ni-Cd, a singlecell Li-Ion, or other
suitable battery with an output in the 3.2 to 4.2 volt range. Power down is accomplished by setting the
voltage on the low current reference pin to zero volts. No external supply side switch is needed as
typical “off” leakage is a few microamperes with full primary voltage supplied from the battery.
An isolator between the Power Amplifier and the duplexer is highly recommended to provide constant
load and source impedances (respectively) to those devices. This improves PA stability, ACPR
performance, and harmonic suppression, as well as duplexer isolation, insertion loss, and ripple.
Transmitter performance is always improved when an isolator is included and is well worth the added
cost.
3.5.4 Thermistor (RT100)
This thermistor senses temperature variations around UMTS PA to adjust PA gain deviation for assure
compliance with the applicable transmit power control standards. Negative temperature compensation
thermistor is used in the U890.
3. TECHNICAL BRIEF
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Figure 3.5.3-1 UMTS PA functional block diagram