Tektronix CURVE-TRACER 576 Instruction Manual page 69

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current through R 165, R 166 and R 167. As the voltage
steps increase through 0176 and 0184 or through 0172
and 0180, the current increases through the current lim-
iting resistors. This current increase causes the voltage drop
across the resistors to increase. If positive-going steps are
being produced, this increase in voltage drop is transmitted
through 0176 and 0169 to the junction of R 166 and R 167.
As the voltage drop increases, the voltage at this junction
point goes down. When the voltage reaches about -2.3
volts, D165 forward biases, clamping the voltage at the base
of 0169. This prevents generation of further steps. When
negative-going steps are being produced, the drop across the
current limiting resistors is transmitted through three base-
emitter junctions, 0180, 0172 and 0169, to the junction
of R 166 and R 167. As voltage drop increases, the voltage at
the collector of 0164 goes up. When this voltage reaches
+12.5 volts, 0164 is saturated, and again no further steps
can be generated. The CURRENT LIMIT switch determines
the number of resistors to be included in the current lim-
iting resistance, therefore determining the amount of cur-
rent necessary to either turn on D 165 or saturate
Q
169.
Standard Test
Fixture Schematic
Pins
Circuit Description-Type 576
VERTICAL AND HORIZONTAL DISPLAY
Signal Sensing and Display Sensitivity
Once the Collector Supply and the Step Generator Out-
put have been applied to the device under test, measure-
ments of the voltages and currents seen at the terminals of
the device under test may be displayed on the vertical and
horizontal axes of the CRT. These measurements are made
by first sensing the current or voltage through current
sensing resistors or voltage dividers, then amplifying the
resulting voltage with the display amplifiers and applying
them to the deflection plates of the CRT. The positions of
the HORIZONTAL, the MODE and the Terminal Selector
switches determine which measurements are made.
Collector Current Sensing.
If the MODE switch is set to
either NORM or DC, collector current (lc) is measured on
the vertical axis of the CRT. Collector current is measured
by placing a resistor (Rsl between ground and the current
return to the collector supply and measuring the voltage
developed across this resistor (see Fig. 3-8 and Fig. 3-9). By
15, 16, 32
/
1!,
AD
\. - +-
Pin 17
Collector
Supply
Output
Device
Under
EMITTER
Test
GROUNDED
Step
~
Generator
I
BASE
I
GROUNDED
/
'Terminal
EMITTER
Selector ... -f>'2_0UND_ED
-;:I"'
~~~
GROUNDED
~'V
Pins ' \ /
~425
9, 10,26
·y,
R~
r-
A /
AA
·v;v
"R'4'22
Rc
A~
~
To Horizontal
Display Switching
"""""
.,
R449
Pins
Current
Return
Pin 25
-,
..
J..
11, 12, 27"
Pins
~ 13,28,29
/
vry
~
A's
To Vertical
Display Switching
Display Sensitivity
Switching Schematic
Rs
R405 through R409
Rc R414 through R422
R' B R442 through R445
RD & RE R427 through R430
Note:
Pin numbers refer to
connectors J360
Fig. 3-9. Simplified schematic of Display Sensitivity Switching and Standard Test Fixture schematics for measurement of collector current
Oc) and base-emitter voltage <VBE) or emitter-base voltage <VEB>·
©I 3-15