Tektronix 1S1, Instruction Manual

Page: 52 / 159

Circuit Description – Type 1S1
3-15
connecting plug. This voltage is produced by dropping the
+75 volts across the series resistance of R607, R606, and
the filaments of V263, V183 and V44. The voltage reference
for this supply is the -19-volt supply, therefore the –19-
volt supply must be set correctly for the +19-volt supply
to be correct.
The shunt regulator consists of comparator Q614 and
shunt transistors Q624 and Q627. Transistor Q614 compares
a voltage near ground (through D616) to the voltage at
the center of precision divider R623-R625-R626. Temperature
compensation for Q614 is provided by diode D616.
Assume that the output voltage tends to go slightly nega-
tive due to an increased load on the supply. The precision
divider will then decrease the bias current to Q614, causing
the voltage at the base of Q624 to become more positive.
Current will therefore decrease through Q624 and Q627,
allowing the supply to return to +19 volts.
- 138-Volt and - 139-Volt Supplies
Transistor Q667 is an emitter-follower circuit providing
a low-impedance voltage source for the -139-volt supply
that is able to provide the large current pulses required for
sampling. The action of this regulator circuit presents a
constant current load on the decoupled -150-volt supply
in the oscilloscope and eliminates voltage fluctuations that
would be coupled to other circuits.
The -138-volt supply is connected to fixed current loads
in the Type 1S1 which draw a total of about 25 ma of
current and set the operating level of zener diode D662.
The constant current through D662 holds the base of Q667
approximately 6.2 volts more positive than the -144 volts
received from the dc coupled -150-volt supply. Voltage
at the emitter of Q667 is therefore approximately –139
volts, due to the emitter follower action.
Transistor Q667, connected in parallel with the current
load on the -139-volt supply, conducts the current that is
not needed by the sampling circuits. A sudden increase in
current load in the instrument makes the voltage across R662
tend to increase and the voltage at the emitter of Q667
tend to go positive. The resulting current decrease through
Q667 returns the voltage to -139 volts and holds the
current constant through R662.