Tektronix CURVE-TRACER 576 Instruction Manual page 67

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\.,.
\.,.
added in the step generator circuit, the zero step
level
may
range from 0 to 10 volts.)
The output end of the current setting resistors is con-
nected through the device under test to ground. When
volt-
age steps of 1
volt
per step are applied between the input
end of the current setting resistors and ground, current
steps of variable amplitude flow through the device under
test. The current amplitude of the steps is determined by
AMPLITUDE switch SW195 (see Step Generator Switching
schematic), which chooses various combinations of resistors
R197 through R216.
In order to obtain calibrated current steps, the
voltage
across the current setting resistors must be held at 1 volt
per step. The voltage at the output, however, may vary by
the amount of the turn-on voltage of the device under test
thus altering the current per step output of the step gener-
ator. To compensate for this turn-on voltage, any variation
from ground of voltage at the input to the device under test
is transmitted through the +1 amplifier to the positive side
of the differential amplifier. This starts a regulating process
which causes the voltage at the input to the current setting
resistors to move in the same direction as the turn-on volt-
age at the output, thus nullifying its effect.
The +1 amplifier is made up of paraphase amplifier
0229A and B, constant current sources 0233 and 0226,
and emitter followers 0235 and 0241. In the current mode,
any voltage at the input of the
device
under test is trans-
mitted through R220 to the high impedance gate input to
0229B. If, for example, this
variation
is a rise in
voltage
at
the gate input, it will be accompanied by a rise in voltage at
the drain of 0229A, due to the paraphase operation of
0229A and B. Raising the voltage at the 0229A drain raises
the base of emitter follower 0235, and thus the base of
emitter follower 0241. As the emitter of 0241 follows its
base up, it pulls the voltage at the gate of 0229A up so that
it is equal to the
voltage
at the gate of 02298. This rise in
voltage
at the gate of 0229A is then transmitted to the base
of 0150B (positive side of the differential amplifier)
through feedback resistors R243 and R244. The +1 ampli-
fier, therefore, transmits any voltage variation from the in-
put to the
device
under test to the input to the base of
0150B with no change in amplitude or polarity. In per-
forming this task, the +1 amplifier provides the voltage vari-
ation with a high impedance input and a low impedance
output. When the rise in voltage at the base of 0 150B has
been transmitted to the input to the current setting resis-
tors, it compensates for voltage variations at the input to
the device under test holding the voltage across the current
setting resistors at 1 volt per step. AMP BAL adjustment
R224 adjusts the DC balance of paraphase amplifier 0229,
and also compensates for unbalance in 0150. OUTPUT Z
adjustment R243 adjusts the output impedance of the step
amplifier.
Relay K 101 B and 0248 or 0250 are used to limit the
voltage which may be applied to a
device
under test in the
reverse direction using opposing offset. If, for example,
Circuit Description-Type 576
positive going steps are to be applied to the device under
test, K101 B is in the+ position. If negative offset is applied
to the device under test by pushing the OPPOSE button
and turning the OFFSET MUL T control clockwise, the step
generator wi II attempt to conduct negative current at the
input to the device under test. In doing this, the
voltage
at
the input to the device under test and thus the voltage at
the 02298 gate input is driven down. When the
voltage
goes approximately 2 volts below ground, 0248 turns on.
With 0248 on, the negative-going voltage steps at the base
of 0150A are limited, thus limiting the output of the out-
put amplifier (the input to the
device
under test) to about 2
volts. This amount of voltage should not damage a
device
under test.
Voltage Mode. Voltage steps are obtained from the out-
put amplifier in a manner similar to that used to obtain
current steps. For
voltage
steps, however, the VOLT AGE
SETTING RESISTORS are changed to obtain the various
voltage amplitudes, rather than the CURRENT SETTING
RESISTORS (which are held constant in the
voltage
mode).
Also since it is not desirable to regulate the
voltage
at the
input to the CURRENT SETTING RESISTORS in the
volt-
age mode, the feedback to the positive side of the differen-
tial amplifier through the +1 amplifier is disconnected and
the input to the +1 amplifier is connected to ground. The
base of 0 150B is, therefore, held at essentially ground.
Since the output of the +1 amplifier is at ground, reverse
voltage limiting transistors 0248 and 0250 are disabled in
the
voltage
mode.
In the voltage mode when steps of 1/2 volt per step are
applied to the step output amplifier, they are transmitted
through VOLTAGE SETTING RESISTORS R141 through
R 145, the input resistance. By varying this input re-
sistance with respect to constant feedback resistor R 194,
the feedback gain of the negative side of the differ-
ential amplifier is changed, thus varying the amplitude of
the voltage steps. After being conducted through the volt-
age setting resistors, the steps are amplified and transmitted
through the
negative
side of the differential amplifier in the
same manner as described in the current mode section.
When the voltage steps reach the CURRENT SETTING
RESISTORS, they are transmitted through a nominal resis-
tance (R215 and R216) of 5 il, for all
voltage positions of
the AMPLITUDE switch, before being applied to the device
under test. Voltage steps of varying amplitudes, as deter-
mined by the AMPLITUDE switch, are then applied across
the input impedance of the device under test. Feedback to
the input to the differential amplifier occurs at the output
of the current setting resistors, therefore, minimizing the
effect of R215and R216.
When using
voltage
steps, the current conducted at the
step generator input to the device under test may increase
quite rapidly and possibly damage the device under test
(especially when testing transistors). As a means of limiting
this current in .othe voltage mode, current limiting resistors
R 185, R 186 and R 187 are added to the output amplifier
circuit by the CURRENT LIMIT switch. These resistors
limit current at the Step Generator Output
by
limiting
3-13