CIRCUIT DESCRIPTION

800/900 MHz TRANSMITTER DESCRIPTION

Over Current Shutdown

Current to the second and third amplifier stages in power module U600 on the PA board is monitored by sensing the voltage drop across R601. Pins 5 and 6 of U500B are effectively connected across this resistor.

As current increases, the voltage on U500B, pin 6 decreases which causes the output voltage on pin 7 to increase. The gain of each U500B input is set at about ten by R509/R504 and R502/R507.

Emitter biasing for Q501 is provided by R506 and R511. Normally, the output voltage of U500B is not high enough to turn on Q501. However, if current becomes excessive, for example because of an antenna mismatch, Q501 begins turning on. This decreases the base voltage of Q502 which turns off Q500 slightly and cuts back power output. Over-current shutdown is disabled in 15-watt models because CR500 is not used.

 

February 2001

3-32

Part No. 001-9800-001