Tektronix CURVE-TRACER 576 Instruction Manual page 65

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amplifier. The output amplifier has two modes of opera-
tion, one producing current steps and the other producing
voltage steps_
The output of the Step Generator, which may be from
one to ten current steps of 350 IJ-A per step plus from one
to ten steps of offset, is applied to the base of 0105A (see
the Step Amplifier schematic). 0105A together with
01058 form a differential amplifier. As the base current of
0105A is decreased, the collector current of 01058 in-
creases, raising the voltage at the base of 0110. Each cur-
rent step at the base of 0105A, therefore, causes a positive
voltage step at the base of 0110 which is amp I ified and
inverted by 0110. Part of the output of 0110 is trans-
mitted through R113, R112 and C112 creating negative
feedback at the base of 01 05A. R 113 adjusts the feedback
gain of current to voltage amplifier 0105 and 0110foran
output at the collector of 0110 of negative-going steps with
amplitudes of 1/2 volt/step.
0117 and 0122 have been added to the current to volt-
age amplifier circuit to slow down the voltage transition
from the level of the last step generated to the zero step
level, in cases where this transition may cause damage to
the device under test. When the preset number of steps has
been produced at the 0110 output, a rapid transition
occurs as the step returns to its starting point. This transi-
tion, when applied to the base of a transistor, rapidly turns
it off. If a transistor is turned off in this manner when its
collector is at a high level, a high inductive voltage kick will
be produced in the collector supply transformer. Such an
inductive voltage kick rnay be large enough to damage the
transistor.
This circuit operates either when the 2X RATE button is
pressed or when the 300 JlS and 80 JlS PULSED STEPS
buttons are pressed together. In this case the emitter circuit
of 0122 is opened, turning the transistor off. The source of
FET 0117 is held at -11.3 volts by divider
R116-D115-R108. When 0122 turns off, divider
R 119-R 120-R 121 sets the voltage at the gate of 0117 at
-10.3 volts, turning the FET on. With 0117 on, its drain is
held at about -11.3 volts, providing a constant voltage on
the side of C114 connected to 0117. By holding one side
of C114 at constant voltage and transmitting the output of
0110 across the other side, C114 becomes an integrator.
The voltage transition of the 0110 output from the level of
its last step to the starting level is, therefore, slowed down
by integrator C114. When 0122 is turned on (normal or 0.5
times rate or DC mode), 0117 is held off by having about
-34 volts at its gate. In this case, the current through R 117
controls the voltage on 0117 side of C 114, which moves up
and down with changes in the output of 0110. C 114, there-
fore, has little effect on the output of 0110 and causes no
slowing of the voltage transition.
When relay K101A is in the- position, the output of
0110 is transmitted through inverter circuit 0130A and B
and 0133 and inverted before it is applied to the output
©
Circuit Description-Type 576
amplifier The inverter is identical in operation to the cur-
rent to voltage amplifier described previously. Since the
input resistance (R125) and the feedback resistance (R137)
are equal, the gain of the inverter is 1. INVERT ZERO
adjustment R 127 sets the voltage at the base of 0 130A so
that the initial level is the same for the non-inverted steps
and the inverted steps.
The position of relay K101A is controlled by the COL-
LECTOR SUPPLY POLARITY switch, the STEP-OFFSET
POLARITY INVERT button and the Terminal Selector
switch in conjunction with the step generator polarity logic
(see the Step Amplifier schematic). U33C and D, U72A, 8
and C form a coincidence gate. See Table 3-2 for a truth
table of this gate. The output at pin 6 of U72B causes 0101
to turn on and off, thus switching relay K101A between+
and -. If a high appears at the output of U72B, K101A
switches to the - position and if a low appears, it remains
in the + state. The inputs to U33C and D and to U72A and
Care controlled by the voltage levels on connectors T and S
as shown in Table 3-2. Setting the Terminal Selector switch
to EMITTER TERM STEP GEN has the same effect on the
voltage level of connector T as pressing the POLARITY
INVERT button. If the POLARITY INVERT button is
pressed, however, the Terminal Selector switch has no
effect on the voltage level at connector T and vice versa.
TABLE 3-2
Step Generator Polarity Logic
COLLECTOR Connectors
SUPPLY POLARITY Pin 6
POLARITY INVERT T
s
U72B
AC Pressed H L H
AC
Not Pressed H H L
+(NPN) Pressed H L H
+(NPN) Not Pressed H H L
-(PNP) Pressed L L L
-(PNP)
Not Pressed L
H H
Output Amplifier. The step output amplifier transforms
the output steps of the current to voltage amplifier (or
inverter) into current or voltage steps of various amplitudes
as determined by the AMPLITUDE switch. It is basically a
differential amplifier with separate feedback to each input.
The negative input side of the amp I ifier controls the ampli-
tude of the output steps. The positive input side of the
amplifier provides either current regulation or a constant
operating level. To obtain current steps (see Fig. 3-7A), the
gain of the negative side of the differential amplifier is set
for an output of 1 volt per step. This output is then trans-
mitted through a variable resistance in series, the current
setting resistors. With the constant voltage per step relation-
ship across the current setting resistors, the current per step
output can be varied by changing this resistance in series.
To obtain voltage steps, the input resistance to the nega-
3-11