The final s tage PA circuit provides the n ecessary amount of power for RF trasmission through an antenna . U801 is a PA module that is capable of operating in the

800MHz band under anal og or digital mode.

In analog mode line arity of the PA is not as critical as when operating in digital m ode. For this reason, effeiciency is a more important factor than linearit y when operat-

ing in analog mode. In digital mode, because of its IQ modulation scheme, PA lin earity is a more important factor than efficie ncy.

Efficiency and lineari ty is controlled by varying the biasing of the PA(U801). In 1900MHz mode only one bias ing method is needed since 1900M Hz mode only operates

in digital mode .

Q1807 controls the supply voltage to U801. The PA supply is sourced from the B+ line. In 800MHz mode, 800KEY is held low ca using Q1807 to switc h on. When

800KEY is pulled high, Q1807 is turned off and the supply to U801 is cut off.

Q901 controls the supply voltage to U901. The PA supply is sourced from the B+ line. In 800MHz mode, 1900KEY is held low causing Q901 to switch on. When

1900KEY is pulled high, Q901 is turned off and the supply to U901 is cut off.

After the TX carrier is ampli fied it's sent to the duplex filter for final tr ansmission through the antenn a.