Section
3-171
Service
CIRCUIT
DESCRIPTION
Introduction
This section of the manual contains a description of the
circuitry
in
the 177 Standard Test Fixture. Complete
schematic diagrams, with
component
numbers and parts
values, are shown on
the
pullout pages at
the
rear
of
this
manual.
Description
The
Standard
Test
Fixture contains
the
vertical pream-
plifier (variable gain)
that
measures voltage across a
current-sensing resistor.
The
entire vertical preamplifier, including its power
supply, floats with respect
to
ground. See Fig. 3-1.
The
vertical preamplifier consists
of
two FET source
followers,
0604A
and
B;
a non-inverting feedback ampli-
fier, U616, and an inverting feedback amplifier, U624-
0626-0628,
which provides
the
1-2-5 current per division
switching.
The
inverting amplifier
output
connects
to
the
577 vertical amplifier.
Current sensing resistors, R
630
through R636, are
connected between
the
collector supply and
the
device
under test.
0604A
and B (source followers) are connected across
the
current-sensing resistors, and
the
source-follower out-
puts are fed differentially
to
U
616,
the
X10 amplifier.
R612
is
the
input
resistor for U616; R616 and R615
(Gain Adj) make up
the
feedback resistor.
The
gain
is
set
for slightly more than 10,
to
compensate for
the
FET
source-follower losses.
C632, LOOPING COMPENSATION (front-panel con-
troll.
compensates for stray capacitance
at
0604B
gate and
the
test
adapter, as well as some
of
the
device-under-test
capacitance.
C630 and R660 through R665 (ganged with
the
current-sensing resistors, R630 through R636), compensate
REV.
B,
MAR.
1975
for
the
stray capacitance across
the
current sensing resis-
tors, keeping
the
time
constant
similar
to
the
time
constant
of
the
stray capacitance and R630 through R636.
U616
output
is
connected
to
a 10:1 divider,
R618-R619. This divider
output
is
connected
to
the
horizontal
attenuator
in
the
low-current ranges
of
the
VERTICAL CURRENT/DIV switch
(2
nA/DIV
to
2 mA/DIV). This voltage
to
the
horizontal
attenuator
is
the
same
as
the
voltage on
the
collector of
the
device under
test. This method permits monitoring
the
collector voltage
without
drawing
current
through
the
current-sensing
resistor.
On the high-current ranges
(5
mA/DIV
to
2 A/DIV)
the
horizontal
attenuator
is
connected directly
to
the
collector
sense terminal of
the
device under
test
to
permit Kelvin
sensing. If Kelvin sensing
is
not
used,
the
22.Q
resistor
between C and C SENSE terminals connects
the
horizontal
attenuator
to
the
C
term
ina
Is.
U616
output
is
connected
to
the
inverting amplifier,
U624-0626-0628.
The
input
resistance
is
R621 and
the
feedback resistance
is
composed of three resistors, R627
(gain of
5L
R627 and R628
in
parallel (gain of
2.5L
and
R627
in
parallel with R629 (gain
of
1).
The inverting amplifier,
U624-0626-0628,
is
connected
to
pin
21
of
J110,
to
the
vertical amplifier via pin 4
of
P129.
0626-0628
provide low-output impedance
to
prevent signal load
ing
by
the
chopper
circuit.
All
power
is
supplied by
the
floating 0+15 and
0-15-volt
supplies. 0 (test fixture common)
is
connected
to
the
junction
of
the
collector supply and
the
current
sensing
resistors.
When using
the
177
Test
Fixture
in
the
577,
the
highest
ranges of
the
MAX
PEAK VOLTS switch
(100V,
400
V,
and 1600
V)
are
not
enabled unless
the
protective cover
is
placed over
the
test
adapter
or
the
interlock
defeat
button
is
depressed. When
the
cover
is
closed,
S626
is
closed and
permits these three ranges
to
be
enabled.
3-1
