Circuit Description; Introduction; Description - Tektronix 177 Instruction Manual

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 X 1 0 ampl ifier.
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 sim ilar 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 circu it.
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