Motorola GM950E, Service Manual, Page: 189 / 228

Transmitter Power Amplifier (PA) 5-25W
Theory of Operation 6.3-21
Op-amp U0701-2 and resistors R0701 to R0703 and R0731 subtract the negative PWR DETECT
voltage from the PA PWR SET D/A output U0731 pin 2. The result is connected to Op-amp inverting
input U0701-4 pin 9 which is compared with a 4.6 volt reference VAG present at noninverting input
U0701-4 pin 10 and controls the output power of the PA via pin 8 and control line PWR CNTL. The
4.6 volt reference VAG is set by a resistive divider circuit (R0171, R0172) which is connected to
ground and 9.3 volts, and buffered by Op-amp U0105-1.
During normal transmitter operation the voltages at the Op-amp inputs U0701-4 pins 9 and 10
should be equal to 4.6 volts and the PA control voltage output at pin 8 should be between 4 and 7
volts. If power falls below the desired setting, PWR DETECT becomes less negative, causing the
output at U0701-2 pin 7 to decrease and the Op-amp output U0701-4 pin 8 to increase.
A comparator formed by U0701-4 increases the PA control voltage PA CNTL until PWR DETECT is
at the desired level. The power set D/A output voltage PA PWR SET (U0731-2) at U0701-2 pin 5
adjusts power in steps by adjusting the required value of PWR DETECT. As PA PWR SET (U0731-
2) decreases, transmitter power must increase to make PWR DETECT more negative and keep the
inverting input U0701-4 pin 9 at 4.6 volts.
Loop frequency response is controlled by Op-amp feedback components R0712 and C0711. Op-
amp U0701-3 compares the power control voltage PWR CNTL divided by resistors R0717 to R0719
with the voltage limit setting VLTG LIMIT SET from the D/A converter (U0731-13) and keeps the
control voltage constant via Q0711 if the control voltage, reduced by the resistive divider (R0717 to
R0719), approaches the voltage of VLTG LIMIT SET (U0731-13).
Rise and fall time of the output power during transmitter keying and dekeying is controlled by the
comparator formed by Op-amp U0701-3.
During normal transmitter operation the voltage at U701-3 pin 13 is higher than the voltage at pin 12
causing the output at pin 14 being low and switching off transistor Q0711. Diode D0732 reduces the
bias voltages BIAS VLTG, BIAS VLTG 2 for low control voltage levels.
The temperature of the PA area is monitored by Op-amp U0701-1 using thermistor R8641 (located
in the PA section). If the temperature increases, the resistance of the thermistor decreases,
decreasing the voltage PA TEMP. The inverting amplifier formed by U0701-1 amplifies the PA TEMP
voltage and if the voltage at Op-amp pin 1 approaches 4.6 V plus the voltage (ON) across D0721,
U701-1 simulates an increased power which in turn decreases the power control voltage until the
voltage at U0701-4 pin 9 is 4.6V again. During normal transmitter operation the output voltage of
Op-amp U701-1 pin 1 is below 4.6V. Diode D8601 located in the PA section acts as protection
against transients and wrong polarity of the supply voltage.
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