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neously by
the
feedback
from
the
output
and
the
feedback
from
the
common-mode
circuit.
The
latter
has already been
discussed regarding
0111
A and
0111
B
as
the
operational
amplifier
inputs
for
that
loop,
but
it
is
now
apparent
that
the
common-mode
loop involves
all
three
operational
amplifier inputs. Both feedbacks
work
together
in
satisfying
the
two
FET
operational
amplifier
input
requirements, and
the
output
loop
depends on
the
common-mode
feedback
in
determining
gain.
R131,
R126,
R123,
and
R128
(and
Rl06
and
R120
in
xl00
gain
mode
of
A
1)
comprise
the
feedback
voltage
divider
that
determines
gain. Even though
connected
to
the
junctions
of
R126-R131 and
R123-R128,
the
sources
of
0100
A & B
do
not
affect
the
divider ratio because
the
common
mode
feedback
to
the
junction
of
R123
and R126
supplies
the
source
currents.
Therefore
gain
is
equal
to
the
ratio
of
the
sum
of
R131 and
R128
to
the
sum
of
R123
and
R126,
as can be seen by stating
the
equilibrium
source-source voltage (across R
123
and
R126)
and deter-
mining
the
output
voltage required
to
force the required
current
through
the
divider
network
to
establish this
voltage.
To
see
how
the
common-mode
feedback
is
neces-
sary for
the
gain-determining
feedback's
proper
operation,
aside from providing
the
source
currents
to
avoid loading of
the
divider
network,
set
as
an
example
the
gate voltage of
0100A
at
0 V and the gate voltage
of
0100B
at
+1
V.
In
this
example
the
source voltage
of
0100B
must
be
+1
V
and
the
source voltage
of
0100A
0 V
(not
counting
small
source-gate
drops,
which are ignored here).
Without
the
common-mode
feedback
it
would
be
quite
impossible
for
any
value
of
output
voltage
to
force
current
through
the
divider
network
that
can satisfy
the
loop. However,
the
common-mode
feedback
to
the
junction
of
R
123
and
R
126,
in forcing
current
through
these resistors, allows
the
gain
loop
to
be satisfied with
-10
V
at
the
output.
4.2B OUTPUT AMPLIFIER (A2)
Refer
to
the
schematic
on page VII-4.
CR200
through
CR203
are
input
protective diodes, biased
through
R203
and
R204.
Notice
that
CR201 and
CR203
are forward
biased, and their forward bias voltages
(about
0.6
V)
reverse
bias
CR200
and
CR202.
This scheme fixes
the
maximum
input
voltage
at
two
diode
drops
(about
1 V).
0201
is
a field-effect
transistor
used as a switch for
discharging
the
filter
capacitors (see block diagram)
when
the
OVL
recovery
button
is
pressed. During normal
operation
this switch
is
off
and
does
not
load
the
input
signal.
The
gates
of
field-effect transistors
0205
A & B are
the
reference and
summing
inputs
of the
A2
operational
amplifier.
The
first stage
of
A2, comprising
0202,
0203A,
0203B,
0205
A &
B,
and
0206,
is
a differential stage
with
common-mode
rejection.
0206
provides
constant
current
to
the
common
source
point
of
0205
A &
B,
and
is
primarily responsible for
the
common
mode
rejection
of
the first stage.
0202
provides
constant
current
to
R213,
thereby
fixing
the
voltage across
R213
at
about
3
V.
0203A
and
0203B
are forward biased, so th at
--
because
the
emitter
junction
voltage
of
a forward biased silicon
IV-3
transistor
is
constant
at
about
0.6
V
--
the
source-drain
voltages of
0205
A & B
is
held
constant
at
approximately
the
voltage across R213.
The
differential·signal drain
currents
of
0205
A & B pass
to
the
collectors
of
0203A
and
0203B,
where
they
are developed
as
signal voltages for
application
to
0207
A and
0207B.
0207A
and
0207B
are
emitter
followers which provide
drive
for
0209
and
0210.
0209
and
0210
are an
emitter-coupled pair, which provide equally and
oppositely
changing signal
currents
to
their
collector
circuits.
0211
shifts
the
0209
collector
voltage level for driving
0212.
0212
inverts
the
0209
signal for helping
0210
drive
the
complementary
output
pair
0213
and
0214.
CR204
temperature
compensates
the
emitter
junction
of
0212.
CR205
and
CR206
are forward biased
diodes
whose
constant
0.6 V
junction
drops
keep
0213
and
0214
forward biased.
The
operational amplifier adjusts its
output
voltage
to
whatever
value
is
required
to
keep
the
summing
point
voltage equal
to
the reference poi
nt
voltage;
therefore
the
gain
of
A2
is
determined
by feedback dividers
R216-R217
and
R9-R10
(see block diagram discussion,
Subsection
4.1).
4.2C
POWER
SUPPL
Y
CIRCUIT
Refer
to
the
schematic
on page VII-5.
Ac
line
power
is
stepped
down
by
T400,
and
the
low-voltage
output
of
T400
is
rectified by
CR400
and
CR401.
S400
selects
the
transformer's
ac line voltage.
The
+ & - rectifier
outputs
are
shunted
with
12
V nickel-cadmium batteries. Batteries
across
the
dc supply
outputs
provide sufficient regulation
so
that
other
regulation
is
not
needed.
If
the
batteries
are in
a
state
of discharge,
the
transformer·rectifier
circuit
will
automatically
charge
them
while
operating
from ac. A
Zener-referenced divider circuit, comprising
CR300,
0300,
and
associated resistors, provides several biasing voltages
to
the
amplifier circuits.
4.20
BATTERY TEST
CIRCUIT
0301
and
0302
are a Zener-referenced switch,
to
wh ich
the
battery
test
switch
connects
one
battery
at
a
time,
applying
the
voltages
in
the
same polarity
for
both
batteries.
If
the
batteries are charged sufficiently,
0302
conducts
and lights
the
panel lamp.
If
a
battery
is
low,
the
lamp remains
out
when
the
switch
is
pressed
in
the
direction
of
the
corresponding
battery.
Note
that
the
power
switch
must
be
on
for
th
is
circuit
to
operate.
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