OPERATING INSTRUCTIONS
GENERAL DESCRIPTION OF
INSTRUMENT OPERATION
The semiconductor curve tracer is essentially a
signal generator that generates precision test signals
for application to a semiconductor, the results of
which are displayed on an oscilloscope. See Figure 2.
Figure 2. Basic Curve Tracer Operation
The curve tracer generates two signals that are
applied to the semiconductor device under test. One
signal is a variable amplitude 120-Hz sweep voltage
(full-wave rectified DC) which is normally applied
to the collector. The other signal consists of constant
current steps which are normally applied to the
base. For FET's, constant-voltage steps rather than
constant-current steps are generated and applied to
the gate.
The 120-Hz sweep voltage is continuously adjust
able from O to 100 volts peak with the SWEEP
VOLTAGE control. This pulsating de voltage is of
positive polarity when the POLARITY switch is set
to NPN (N CHAN) and of negative polarity when
the switch is set to PNP (P CHAN).
The step generator signal consists of five constant
current steps plus a zero-current step. The STEP
SELECTOR switch offers 11 settings from 1 µ.A to
2 mA per step to match normal operating conditions
for a full range of transistor types. For example,
with the STEP SELECTOR switch set at 50 /tA per
step, the current steps are 0, SO JtA, 100 JtA, 150 /.LA,
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200 µ.A and 250 µA, then repeated. When the PO
LARITY switch is in the NPN position, the steps are
of positive polarity, in the PNP position, negative
polarity. In the VOLTS PER STEP positions of the
STEP SELECTOR switch, constant-voltage steps are
generated for testing FET's. Five selections from
.05 to 1 volt per step are offered. The polarity of the
voltage steps is inverted in relation to the current
steps. That is, the N CHAN position produces nega
tive voltage steps, and the P CHAN position pro
duces positive voltage steps (reverse bias steps are
required for testing FET's).
The sweep voltage that is applied to the collector
of the transistor is also applied to the horizontal
input of the oscilloscope. As the voltage increases
from zero to maximum and returns to zero, a hori
zontal sweep is produced. A precision resistor in
series with the sweep voltage source develops a
voltage that is proportional to the resultant collector
current. The voltage developed across this resistor
is applied to the vertical input of the oscilloscope;
therefore, vertical deflection represents collector cur
rent. Actually, four different precision resistors are
included and selected by the VERTICAL SENSITIV
ITY switch on the curve tracer. This allows greater
versatility for displaying a wide range of collector
currents.
The steps of the step generator are synchronized
with the pulses from the sweep generator. The base
current remains at a fixed value while the sweep
voltage increases from zero to maximum and returns
to zero. The base current then steps to the next high
er fixed value while the sweep voltage completes
another cycle. The result is a family of six curves,
one for each step, with each base current step in
crease producing a higher collector current. The
display is a dynamic collector current (le) vs. col
lector voltage (Ve) graph.
OSCILLOSCOPE
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Figure 3. Basic Block Diagram
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