HP 6296A - Page 28

so
that
the
difference
between
the two
input
volt-
ages
applied
to
the
differential
amplifier
is
re-
duced
to
zero.
This
action
maintains
the
output
voltage
constant.
4-24
Stage
QlOOA
of
the
differential
amplifier
is
connected
to
a common
(+S)
potential through
im-
pedance
equalizing
resistor
Rl06.
Resistors
Rl02
and
Rl
08
are
used
to
zero
bias
the input
stage.
offsetting
minor
base
to
emitter
voltage
differ-
ences
in
QlOO.
The
base
of
QlOOB
is
connected
to
a summing
point
(A4)
at
the
junction
of
the
pro-
gramming
resistors
and the
current
pullout
resis-
tor
RSOS.
Instantaneous
changes
in the
output
(due
to
load
variations)
or
changes
due
to
the
manipulation
of
RBl
3,
result
in
an
increase
or
decrease
in
the
summing
point
potential.
01008
is
then
made
to
conduct
more
or
less,
in
accord-
ance
with
summing point
voltage
change. The
change
in
QlOOB's
conduction
also
varies
the
conduction
of
Ql
OOA
due
to
the
coupling
effects
of
the
common
emitter
resistor,
RIOS.
The
"errorn
voltage
is
taken
from
the
collector
of
QlOOA
and
ultimately
varies
the
conduction
of the
series
regulator.
4-25
Resistor
Rl04,
in
series
with the
base
of
QlOOB.
limits
the
current through the
programming
resistors
during rapid
voltage
tum-down.
Diodes
CRlOO
and
CRlOl
form
a limiting network
which
prevents
excessive
voltage
excursions
from
over
driVing
stage
QlOOB.
Capacitor
C801,
shunting
-i.2V
?O
GATING--~-
.....
---+--
...
ClllCUIT
-L2V
the
programming
resistors,
increase
the
high
frequency
gain
of
the
input
amplifier.
Resistor
R806,
shunting
the
pullout
resistor,
serve1s
as
a
trimming
adjustment
for
the
programming
current
••
Diode CR102
establishes
the
proper
collector
bia
for
QlOOA
while
R103
and
ClOO
proVide low
fre-
quency
equalization
for
the
feedback
loop.
4-26
CONSTANT
CURRENT
INPUT
CIRCUI'I' (See
Figure
4-7)
4-27 This
circuit
is
similar
in
appearance
and
operation
to
the
constant
voltage
input
circ:uit.
It
consists
of
the
coarse
and fine
current
pr01;ram-
ming
resistors
(R809
and
RS!
O),
and
a
difforential
amplifier
stage
(Q200
and
associated
components).
Like
transistor
QlOO
in
the
voltage
input
ciircuit,
Q200
consists
of two
transistors,
having
matched
characteristics,
that
are
housed
in
a
singl1~
pack-
age.
4-28
The
constant
current
input
circuit
continu-
ously
compares
a fixed
reference
voltage
with
the
voltage
drop
across
the
current
sampling
re:sistor.
If
a
difference
exists,
the
differential
amplifier
produces
an
"error"
voltage
which
is
propo1rtional
to
this
difference.
The remaining
components
in
the
feedback
loop (amplifiers
and
series
re·;ulator)
function
to
maintain
the
drop
across
the
cu:rrent
sampling
resistor,
and
consequently
the
output
current,
at
a
constant
value.
R827
-r..zv
1801
P11U.OVT
RESISTOR
CIOS
RIOOA
•
ltl08
Figure
4-7.
Constant
Current
Input
Circuit,
Simplified
Schematic
•
4-6