Tektronix CURVE-TRACER 576 Instruction Manual page 66

Circuit Description-Type 576
(B)
1/2 volt/step
Voltage
Setting
Resistor
Rt
R243
OUTPUT
Z
Reverse
Voltage
Limit
1 volt/step
Rt
Current
Setting
Resistors
Device
Under
Test
Device
Under
Test
Fig. 3-7. Block diagram of Step Output Amplifier: (A) Current Mode; (B) Voltage Mode.
tive input, the voltage setting resistors, is changed, thus
varying the feedback gain of that side of the differential
amplifier In this manner voltage steps of various ampli-
tudes are obtained.
Current Mode. Input to the negative side of the differ·
entia I comparator, at the base of 0 150A, is always through
VOLTAGE SETTING RESISTORS R141 through R145. In
the current mode, this input resistance is set at 3.01 kD
( R 141) for all current positions of the AMPLITUDE
switch. When 1/2 volt steps are applied to the base of
0150A through R141, they are inverted, applied to the
base of 0164 and inverted again. The steps are then trans-
mitted through emitter follower 0169 to the bases of 0172
and 0176. Depending on the position of relay contacts
K102B and K102C, either 0172 and 0180 or 0176 and
0184 are turned on. If, for example, K102B and K102C are
in the
+
positions, signifying positive-going steps out, 0176
and 0184 are on the 0172 and 0180 are off. In this case
the input to 0176 is negative-going steps. They are inverted
by 0176 and the resulting positive-going steps are trans-
mitted through emitter follower 0184 to the negative side
of the floating 50-volt supply. Each time a positive step
occurs at the negative side of the 50-volt supply, the supply
3-12
is pushed up by the amount of the step. The positive side of
the 50-volt supply is connected to both the feedback resis-
tors and the input to the current setting resistors, so that
each time the 50-volt supply is raised by a step, the voltage
at this connecting point is also raised by the amount of the
step. Due to the presence of the 50-volt supply, the voltage
at the input to the current setting resistors is offset by 50
volts. To compensate for this offset, 50 volts of opposing
offset is added to the input of the current setting resistors
through relay K 1 02A. If K 1 02B and K 1 02C are in their -
positions, 0172 and 0180 are on and 0176 and 0184 are
off. In this case negative-going steps are applied to the
positive side of the 50-volt supply and negative-going steps
appear at the input to the current setting resistors.
The output of tho negative side of the differential ampli-
fier at either K 1 02B or K 1 02C is fed back to the base of
0 150A through feedback resistor R 194. Since R 194 is
6.04 kD and the input resistance, R 141, is 3.01 kD the
feedback gain of this circuit is 2. For a half volt per step in·
put, the resulting output of the negative side of the differen·
tial amplifier (as seen by the input to CURRENT SETTING
RESISTORS R197 through R216) is steps of one volt per
step, the zero level being at ground. (If offset has been
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