EFJohnson 989X SERIES 用户手册

3-32

February 2001

Part No. 001-9800-001

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.