Keithley 237 Service Manual

SECTION 4
Principles
of
Operation
Part 2:
Assume the instrument is still programmed to
source +5mA with a compliance of 27V. However, for
one
reason or another, the bad resistance changes
to
6k2.
This bad change causes the
source to
go
into compliance
making voltage the controffing function.
With the 4k2 bad, the +1 CLAMP is on making current
the controlling function
(as
explained
in
the Part
1).
When
the bad resistance is increased
to
6k2, the voltage
drop across the bad would
be
30V
(5mA
x
6k2 =
30V).
However,
voltage is limited
at
27V and the source
goes
into compliance.
In
voltage compliance, the 27V is scaled down
to 3V
and
inverted, This
3V
is
fed back
(via
V
FEEDBACK)
to
U15A. The non-inverting
(+)
input ofUl5A
will go to 0V
(3V +
3V =
0V)
forwardbiasingCR12. As
a
result, the
+V
CLAMP will turn
on.
Current from ±10V REF
starts flowing into the
+V
CLAMP.
With less current flowing into the
+1
CLAMP,
the voltage
at
the input
of U14 decreases causing the out
put current of the source
to decrease. The resultant feed
back voltage
to
U17
wffl cause its
(+)
input
to
increase
above
0V
causing CR10
to become reversedbiased. As
a
result, the +1
CLAMP will hirn off.
With the
+V CLAMP on, the instrument is voltage con
trolled.
The output from the
V DAC
and
the feedback
configuration
wffl apply
a
voltage
to
the error amplifier
(U14)
that wffl effect
a constant output voltage of +27V.
Bootstrap
amplifier gain is
controlled at the
output
of the
error
amplifier(U14)by the FET switch
Q24
and
relay
K4.
On the 110V
and 1100V ranges, the
switches are
open and
xllO
gain is provided. On
the lower voltage ranges the
switches
are closed providing xli gain.
Device Ull
is
a xl
buffer
between the V/I control circuit and the output
module.
Measurement is accomplished
by
routing
properly con
ditioned
voltage feedback levels (1 FEEDBACK and
V
FEEDBACK)
through
a multiplexer to the analog4o
digital (A/D) converter.
Op amp U9A is configured as a
unity gain amplifier
to function
as
buffer for 1
FEED
BACK.
Op
amp U9B is
configured
as
a
non4nverting am
plifier for
V
FEEDBACK. On the 1,1V range, the FET
switch
Q33
is closed providing xlO gain. On the higher
voltagc ranges, the FET switch
is
open configuring U9B
as a umty
garn amplifier The
four signals
(+10V
refer
ence,
V
Measure,
1
Measure
and
floating
common)
are
multiplexed
(Ui) and routed to
the A/D converter via
buffer U21.
422
Output
Stage
The output
stage
of the bootstrap
amplifier
basically con
sists
of
a
dass AB circuit
whose bias is
controlled by the
output of the error
amplifier.
Model 236
The simplified
schematic diagram
of the output stage
for
the Model 236
is shown in Figure
4-3. The
detailed sche
matic of
the
output
stage
is
found on drawings
236-166
and 236-126
(page
2).
At the heart of the output stage
are power
transistors Q21
and Q19. The collector of
Q21 is effectively
connected
to
the +150V,
l2OmA power
supply, while the collector of
Q19 is effectively
connected
to
the —150V,
l2OmA power
supply.
When programmed for
a
zero
output, both
Q21 and Q19
are zero-biased
(ofß,
The
±150V
supplies are dropped
across
the high impedance of
these two
transistors. As a
result, zero voltage is present at
the
output (floating com
mon to output
common).
When the source
is pro-
grammed for
a
positive output,
Q21 becomes
forward-bi
ased
(on).
The voltage drop
across
Q21 becomes
less
than
+150V. As
a
result,
the
voltage
present at
the
output is the
difference between the supply
voltage
(+150V)
and the
voltage drop
across Q21. For a
positive
output, Q19 is re
verse-biased
(ofO
keeping
the
negative supply
isolated
from the output.
Circuitry for a negative
output
functions
in
a
similar
manner. When the source
is
programmed
for
a
negative
output, Q19 becomes
forward-biased,
resulting with a
negative voltage present on
the output.
Transistor Q21 is
reverse-biased isolating
the output
from the positive
supply.
Bias for the
two
power
transistors
are provided
by
the
±3OVF supplies
and
two
15V
zener diodes (VR1O
and
VR9).
This bias
circuit is
referenced
to
floating common
in
order
to
maintain the
bases
of the transistors at a
volt-
age
that is higher
than the
output
so
that
they can be
for
ward-biased. Bias
control is
performed by
the output of
4-4

Keithley 237 Specifications

General

Brand	Keithley
Model	237
Category	Measuring Instruments
Language	English