The final stage PA circuit provides the necessary amount of power for RF transmission through an antenna. U801 is a PA module that is capable of operating in the

800MHz band under analog or digital mode.

In analog mode linearity of the PA is not as critical as when operating in digital mode. For this reason, efficiency is a more important factor than linearity when operating

in analog mode. In digital mode, because of its IQ modulation scheme, PA linearity is a more important factor than efficiency. Efficiency and linearity is controlled by

varying the biasing of the PA(U801).

Q831 controls the supply voltage to U801. The PA supply is sourced from the B+ line. In 800MHz mode, 800KEY is held low causing Q831 to switch on. When

800KEY is pulled high, Q831 is turned off and the supply to U801 is cut off.

After the TX carrier is amplified it's sent to the duplex filter for final transmission through the antenna.

In order to maintain an accurate power level, an RF detect circuit is used to monitor the signal level being transmitted.

P8190 :