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TDMA M3090/M3097:
A_D SWITCH
SW_QMOD_KEY
Description
The final stage PA circuit provides the necessary amount of power for RF transmission through an antenna. U701 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 contro lled by varying the biasing of the PA (U701).
Q721 controls the supply voltage to U701. The PA supply is sourced from the B+ line. In 800MHz mode, 800_ON the voltage is decreased causing Q721 to switch on.
When 800_ON is pulled high, Q721 is turned off and the supply to U701 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 a accurate power level, an RF detect circuit is used to monitor the signal level being transmitted. A single RF detect circuit is
used for both transmission paths, 800MHz and 1900MHz.
The switching circuit defines the proper PA loads when in 800MHz analog and 800MHz digital mode. Q270 is a FET switching circuit which allows SW_QMOD_KEY
to be sourced from the TX_2.75 supply. During non-transmit states, Q270 is switched off by pulling QMOD_KEY high. This state will not allow SW_QMOD_KEY to
be sourced from TX_2.75, thus redu cing unnecessary current consumption in numerous T X circuits that use the SW_QMOD_KEY supply.
When transmitting in 800MHz analog mode, LOAD A is used and LOAD B is bypassed. LOAD B is bypassed by allowing CR730 to be forward biased. Forward biasing
CR730 will provide a ground state at the anode side, thus allowing on ly LOAD A to be present.
CR730 is forward biased by having a supply voltage present at the AD_SW line. When the A_D line is pulled down, Q261 is switched on, forcing the base of Q260 low.
PNP transistor Q260 will t hen be switched on, allowi ng the output(AD_SW) to be pulled high from SW_QMOD_KEY.
PA_A_D line controls the PA bia sing in 800MHz digital and anal og mode.
There are two operating modes in the 800MHz band, analog and digital. The 800BIAS voltage level determines the operating condition of the final stage PA. A lower
voltage level at 800BIAS denotes digital mode and a higher voltage level denotes analog mode. In analog mode, PA_A_D is pulled high causing Q723 to be switched on.
This state causes R723 to be bypassed, thus, having a high voltage level at 800BIAS. In digital mode PA_A_D is pulled low causing Q723 to be switched off. During this
state R723 has a voltage drop across it and 800BIAS will have a lower voltage level.
PA Circuit (800MHz)
800_ON
B+
800BIAS
U701
TX_800
CR723
PA_A_D
A_D
Q711-A
Q723
Q721
R723
TL921
TL920
Load
Load
B
A
CR730
Q270
Q261
Q260
PA_A_D
800BIAS
TX_800
DET_800_1900
QMOD_KEY
SW_QMOD_KEY
TX_2.75
A_D SWITCH
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