SEA 222 Operator's Manual page 60

7 .1.2
The +10 Volt Rx and Tx busses are generated through computer control
of inverted transistor switches, QlO and Qll, while the +13.6 Volt Tx
buss is generated on the PA/Filter
boar~
through compu_ter contra 1 of
switching transistor Q2.
The
+B
Volt and +5 Volt logic buss voltages are generated from the
switched +13 .6 Volt buss through the use of standard three terminal
regulators, U29 and U30.
+10 VOLT REGULATOR AND THE Tx/Rx SWITCHES
The +10 Volt regulator in the SEA 222 makes use of the PNP power
transistor as a series pass device. This transistor, Q7, is connected
as an inverted power stage with its base drive derived from a negative
feedback amplifier consisting of QB and Q9.
In operation, the base of emitter follower Q9 is provided with a sample
of the +10 Volt regulator output through resistor network R193, R194.
This sample is supplied to the emitter of feedback amplifier QB.
The base of QB is provided with a stable reference voltage from zener
diode CR30. This device has a nearly "flat" temperature vs voltage
characteristic and is further stabilized by obtaining its voltage drive
from the +10 Volt buss.
Since the base voltage of QB is stabilized by CR30, the collector
current in QB and thus the base current of the series pass transistor
Q7 is inversely proportional to the voltage at the base of Q9. That
is, a RISE in the voltage at the base of Q9 causes a REDUCTION in base
drive to Q7. This in turn causes the output voltage to be reduced.
Similarly, a DROP in the voltage at the base of Q9 will result in an
INCREASE in base drive to Q7 which causes the.output voltage to be
increased.
Rl89 is shunted across Q7 to provide "start up" voltage for the regula-
tor system. Potentiometer R193 is the +10 Volt adjustment and allows
the regulator to be "trimmed" to exactly +10 Volts by setting the base
voltage on Q9. Resistor R190 is provided to limit the base drive to
Q7. This provides a degree of short circuit immunity for the regu-
1 ator, since the base drive for Q7 tends to "starve off" at about 1 Amp
of output current.
The use of the inverted PNP transistor, 07, as the series pass device
has the advantage that the regulator will remain operational under low
line voltage conditions. The low line limits are primarily a function
of the E-C saturation voltage in Q7, which is normally less than
n.5
44
I