HP 6296A - Page 31

•
•
•
also
selects
one
of
two
meter
ranges
on
each
scale.
The metering
circuit
consists
basically
of
a
selection
circuit
(switch
S2
and
associated
voltage
dividers),
a
stable
differential
amplifier
staqe
(QBSlA
and
Q8518),
two
meter
amplifiers
(Q852
and
Q853),
and
the
meter
movement.
4-48
The
selection
circuit
determines
which
voltage
divider
is
connected
to
the
differentia 1
amplifier
inpu~.
When
S2
ls
in
one
of
the
voltage
positions,
the
voltage
across
divider
RBSO,
RBSl.
and
R852
(connected
across
the
output
of
the
sup-
ply)
is
the
input
to
the
differential
amplifier.
When 52
is
in
one
of
the
current
positions,
the
voltage
across
divider
R853, R854,
and
RSSS
(con-
nected
across
the
sampling
resistor)
is
the input
to
the
differential
amplifier.
With
S2
in the higher
voltage
range
(position
2)
the
voltage
drop
across
R852
is
applied
to
stage
Q851A
while
stage
Q851B
is
grounded
tothe
+s
terminal.
For low
out-
put
voltages,
S2
can
be
set
to
position
(1)
result-
ing in
the
application
of
a
larger
percentage
of the
output
voltage
(drop
across
RSS
l
and
RBS
2)
to
stage
Q851A.
With
52
in
the
higher
current
posi-
tion
(3)
the
voltaqe
drop
across
R853
is
applied
4-9
to
stage
Q851B
while
stage
Q851A
is
grounded
to
the
+S
terminal.
In
the
low
current
range,
the
voltage
drop
across
R853
and
R854
is
applied
to
Q851B.
4-49
Differential
amplifier
stage
Q851
is
a
stable
device
having a
fixed
gain
of
ten.
To
minimize
temperature
effects,
the
two
stages
are
housed in a
single
package
that
is
similar
to
those
used
in
the
constant
voltage
and
current
in-
put
circuits.
The
outputs
of
the
differential
am-
plifier
drive
meter
amplifiers
Q852
and
Q853
which,
in
turn,
deflect
the meter.
Transistor
Q850
provides
a
constant
bias
current
to
the
emit-
ters
of
Q852
and
Q853.
Potentiometer
R870
per-
mits
electrical
zeroing
of
the
meter.
4-50
The
meter
circuit
contains
an
inherent
cur-
rent
limiting
feature
which
~rotects
the meter
movement
against
overloads.
For
example,
lf
ME1'~R
switch
52
is
placed
in
the
low
current
range
when
the
power
supply
is
actually
deliver-
ing a
higher
ampere
output,
the
differential
am-
plifiers
are
quickly
driven
into
saturation,
limit-
ing
the
current
through
the
meter
to
a
safe
value
.