Mhz Band; 35-Watt Transmitter; Figure 3-31. 35-Watt Rf Power Amplifier (Rfpa) Gain Stages (700-800 Mhz) - Motorola ASTRO XTL 1500 Detailed Service Manual

Theory of Operation: Transmitter
Protection Mechanisms
Final-stage temperature is sensed in the RFPA resulting in VTEMP, which is proportional to
temperature. VTEMP is compared against a reference voltage TEMP_1 (U0959, pin 9) via U0957-1.
When VTEMP exceeds TEMP_1, the U0957-1, pin 1, voltage increases and forward biases one of
the D0951 diodes, which cuts back power. Power continues to cut back with rising temperature until
the voltage level at the junction of R0978 and R0983 is high enough to forward bias D0952, thus
clamping the cut back so that the radio meets its duty cycle specification while providing protection
against high-temperature conditions. The clamping level is set via TEMP_2 (U0959, pin 10) and
U0957-2. U0957-3 is used to sense if a high A+ battery voltage condition exists and, if it does, the
Q0955 gate is biased on, which increases the clamp voltage allowing for additional power cutback
for a high A+, high temperature condition.
Final-stage current is also monitored via VCURRENT, which is proportional to current. VCURRENT
is compared against a reference CURR_LIM_SET (U0959, pin 1) which is tuned after power
characterization. If VCURRENT exceeds CURR_LIM_SET, then U0957-4, pin 14, voltage rises and
forward biases one of the D0951 diodes, which limits power.
Finally, control voltage is limited by U0956-4 and D0950. RFPA_CNTRL can rise to the control
voltage limit set by R0942-4. U0965 provides protection against supply voltage transients. When
transients on the A+ voltage exceed 24 volts U0965 pin 1 is pulled high thus turning on Q0954 via
TX_DISABLE, R0932, U0958 disabling the control voltage to the PA first stage and momentarily
turning off the transmitter. In addition, Q0900 is turned on, forward biasing one of the D0950 diodes
and thus reducing the control voltage. When the voltage transient drops back below 20 volts, U0965
pin 1 goes low thus enabling the control loop and turning the transmitter on again.
3.12.4 700–800 MHz Band

3.12.4.1 35-Watt Transmitter

The following text discusses the 35-W transmitter.
3.12.4.1.1 RF Power Amplifier (RFPA)
The RFPA consists of three gain stages, which are shown in
2mW
TX_INJ
From
FGU
RFPA_CNTRL
Figure 3-31. 35-Watt RF Power Amplifier (RFPA) Gain Stages (700–800 MHz)
6815854H01-A
FIRST DRIVER
STAGE STAGE
200mW
U6500
Q6520
C65
1517
K9.1V
VGBIAS3
Figure 3-31.
FINAL
STAGE
Q6540
9045M
A+
VGBIAS1
4W
A+
Q6541
9045M
A+
VGBIAS2
3-41
42W
RFPA_OUT
To Antenna
Switch
MAEPF-27887-O
June 15, 2005