After the desired transmit informati on is modulated with a TX carrier frequency, enough signal power needs to be provided for RF transmission through the antenna. The

exciter stage is used to relieve amplifi cation of the final stage PA. This reduces excess heat dissipati on and overloading of the final stage PA.

In 800MHz mode, the TX signal passses through FL452 and then is a mplified by Q401. Q401 is driven on by supp ling DRIVER_B+ through Q475-A. Q475-A can be

switched on or off via the 800KEY line. In 8 00MHz TX mode, 800KEY is low. Thi s state brings the gate of Q475-A to a low level, causing Q475-A to switch on.

There are two operating modes in the 800MHz band, analog and digital . In 800MHz mode Q810 is switched on with a low level of the gate. The output of Q810 will then

be used to provide the biasi ng voltage for the final st age PA(U801).

The 800BIAS voltage level determines the operating condition of the fina l stage PA. A lower voltage level at 800BIAS denotes digital mode and a higher voltage level

denotes analog mode. In analog mode, A_D is pulled high caus ing Q811 to be switched off. This state causes a voltage drop across R811, thus, having a lower voltage

level at 800BIAS. In digital mode A_D is pulled low causing Q811 to be switched on. During this state R8 11 is bypassed an d 800BIAS will ha ve a higher voltage level.

In 1900MHz mode, the TX signal is ampli fied by Q411. Q411 is driv en on by suppling DRIVER_B+ through Q475-B. Q475-B can be switched on or off via the

1900KEY line. In 1 900MHz mode, 1900KEY is low. This state brings the gate of Q475-B to a low level, causi ng Q475-B to switch on.

1900BIAS provides the proper biasing voltage for the final stage PA(U901). 1900BIAS is supplied from TX_2.75. In 1900MHz mode, having 1900KEY low will swit ch

Q910 on.