Motorola GM1200E Service Manual page 59

Transmitter Power Amplifier (PA) 5-25W
The ALC loop regulates power by adjusting the PA control line PWR CNTL to keep the forward
power voltage PWR DETECT at a constant level.
Opamp 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 opamp 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 (R0251, R0252) which is connected to
ground and 9.3 volts and buffered by opamp U0251-1.
During normal transmitter operation the voltages at the opamp 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 opamp 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 PWR SET (U0731-2) at U0701-2 pin 5 adjusts
power in steps by adjusting the required value of PWR DETECT. As PWR SET (U0731-2)
decreases, transmitter power must increase to make PWR DETECT becomes more negative and
keep the inverting input U0701-4 pin 9 at 4.6 volts.
Loop frequency response is controlled by opamp feedback components R0712 and C0711. Opamp
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 opamp 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 voltage BIAS VLTG for low control voltage levels.
The temperature of the PA area is monitored by opamp U0701-1 using thermistor R3611 (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 opamp 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. Resistor R0724, R0722, R0723 set the factor of the decrease in output
power per temperature increase while R0721 through R0723 set the threshold were the temperature
starts reducing the output power. During normal transmitter operation the output voltage of opamp
U701-1 pin 1 is below 4.6V. Diode D3601 located in the PA section acts as protection against
transients and wrong polarity of the supply voltage.
Introduction/Theory of Operation 3.1-29