tions)
and a
shunt
capacitor
selected
by
the
HF
rolloff single-ended
for each
half
of
the
circuit.
But
differential
switch.
The
LF rolloff
filter
is composed
of
the series
signals
cause
the
emitter-coupled
pair,
Q114A
and
Q1148,
capacitor
selected by
the
LF rolloff
switch
and the
1.59
to
provide
equally
and
oppositely
changing
signal
currents
megohm
shunt
resistor at
the input
to
A2.
The
transfer
to
their
collector
circuits,
and
common-mode
signals
tend
curves
for these
filters are
given
in
Section
Ill.
to
cause
equal
changes
in the
collector
currents
of
Q114A
1
and
Q1148.
Differential
signals
cause
no
change
in
the
emitter
currents
of
Q114A
and
Q1148
but
common
mode
4-‘IE
QVERI-OAD
FAST-RECQVERY
CIRCUIT
signals
do.
Therefore
the
common-mode
feedback
signal
is
taken
from the
emitter
resistor
8146.
The
overload
recovery
button,
when
pressed,
turns
on
FE-i-,5
Qiop
and
Qiop
eenneeted
pet‘/Veen
eeen
input
pi
Q116
shifts
the
Q114A
collector
voltage
level
for
driving
,
A1 (Pp
terminals
5 &
6) end
stewed, and iuriie en
e FET
0117.
0117
inverts
the
Q114A
signal,
and
helps
Q1148
switch
connected
between
the
input
of A2_and
ground.
drive
the
Compiementary
Output
pair
Q118
and
Q119_
Q105
and
Q106
discharge
the
input capacitors,
C1 and
O
Osin
the Q114A
nd Q114-B
out
uts
this
Wa
CanCe|S
C2,
and
the FET
switch
between
the
input
of
A2
and
pp
9
.
a
F.)
.
.
V
ground discharges
the
LP and
HP
filter
capacitors, °9‘“".‘°“'"?°?"e
S'gna|s
that
may
remam
m
tms
part
of
the
thereby
providing
fast
Overload
recovery
circuit.
This
is
because
the
common-mode
feedback
loop,
in
keeping
the
voltage
across
8146
constant,
cancels
the
4-1F
POWER
SUPPLY
common-mode
signal
component
that
would
otherwise
appear
in
the
collector
currents
of
Q114A
and
Q1148.
AC
iine
p0Wer
is
Stepped
d0Wn
bV
e
transformer!
then
C8108
temperature-compensates
the
emitter
junction
of
rectified.
The
+
& —
rectifier
outputs are
shunted
with
12
V
Q113_
QR112
and
CR113
are
forward
biased
diodes
whose
nickel-cadmium
batteries.
Batteries
across
the
dc
supply
Constant
()_6
V
iunotion
dr()p5
keep
Q118
and
Q119
outputs
provide
sufficient
regulation
so
that other
regu|a-
forward
biased.
The
primary
gain-determining
feedback
is
tien
i5
net
needed
When
ee
line
p0Wer
is
u$eCl-
it
the
taken
from
the
output
through
8131,
and
will
be
explained
batteries
are
in
a state
of discharge,
the
transformer-rectifier
below,
circuit will automatically
charge them
while
operating
from
ac.
For
battery
operation,
the unit
is simply
not
plugged
Q111A
and
Q1118
compare
the
common-mode
feedback
into
the
ac
receptacle,
so
that the
batteries
alone
provide
voltage
at
the
base
of
Q1118
to
a reference
voltage
at
the
power
to the
dc lines.
For convenience
in
checking
the
base
of
Q111A,
and
provide
corresponding
equal
and
batteries,
a test
circuit
is
provided.
opposite
signal
currents
to
their
collector
circuits.
Q113
inverts
the
Q111A
collector
signal
to aid
Q1118
in
driving
4.2
CIRCUIT
DESCRIPTIONS
Q107
and
Q108.
Q107
and
Q108
are
complementary
transistors
that
drive
the
junction
of
8123
and
8126,
4.2A
DIFFERENTIAL
PREAMPLIFIER
(A1)
providing
balanced
feedback
to
the
sources
of
the
input
FET's.
Refer to the
schematic on
page
Vll-3. This
circuit
differen-
tially
amplifies
the
input signals
at
the
gates of
Q100
A
&
8.
These
field-effect transistors
are
involved
in
two
over-
The
teedbeek
eetien 0t
en Operetienei
ernpiitier
i5 Sueh
85
lapping
modes
of
operational
amplifier
feedbaok
that
to
keep
the
summing
point
voltage
equal
to the
reference
determine
gain
and
prevent amplification
of
common-mode
peint
\/0ite9e-
Sinee
teedbeek
i5 u$ueiiV
taken
ir0rn
the
signals.
output,
this
condition,
being
the
only
stable
condition,
is
taken
advantage
of in
determining
gain.
In this
circuit,
0102
provides
constant
current
to
8108,
thereby
fixing
the
however,
we
really have
twe
dependent
operational
ameli-
voltage across
8108.
8105
and
8119
provide
an
average
of
tier i00p$ that
determine
Qein
end
pr0\/ide
e0rnrn0n'nn0de
the
source voltages
of
Q100
A &
8 to
the
base
of
Q103.
reieeti0n-
Because
Q103
and
both
sections
of
Q101
are
forward
biased (so
that
their
emitter
junctions
have
constant 0.6
V
The
tir5t
ieedpeek
eempenent
We
Wiii
e0n$ider
i5 the
across
them,
approximately),
the source-drain
voltagesof
eerninen'rn0de
teedheek
trern
Q107
end
Qip8-
C0rre-
Q100
A & 8
are
held
constant
at approximately
the
voltage
$p0ndin9
Currents
threugh
R123
end~
R126
drive
the
across
8108
(5
V).
C8101 and
C8107 are
input
protective
Spurees
pi
Q100
A 81
B te
epuntereet
the e0mrnen'rn0de
diodes
reverse
biased by
the
collector
voltage
of
Q104. If
eernpenent
Oi the
input
$i9nei-
in
this
i00p
the
be$e
0t
either
input
voltage becomes
larger
than
about
-3
\/,
the
Q111A
is the
reference
point
and
the
base
of
Q1118
is
the
corresponding
diode
will conduct.
The
current
path
is
summing
D0int-
The hi9h
Qein
Oi
the
i00p
Causes
the
through
the Q104
collectorjunction
and
C8104
to
ground.
feedback
te
the
Spureee
t0
e0rnpieteiV
eduntereet
the
The gate
junctions
of
the
input
FET's
can
handle
positive
e0rniTi0n'n10de
edrnpenent
Qt
input $i9nei-
in d0in9
5° it
input
over-voltages
without
being
damaged;
they
simply
aide
the
teedpeek
irern
the
Output
in
$eti5tVin9 the
beoome
forward
biased
and
oondnot_
requirements
of
the
gain-determining
loop,
as
will
be
seen
from
the
following discussion.
The
drain
currents
of
Q100A
&
8 pass
to
the
collectors
of
Q101A
& Q1018,
and
are
developed
as
voltages
for
The gates
of the
two
input
FET's
can
be
considered
as
the
\-.
application
to the
bases
of
Q109A
and
Q1098.
Q109A
and
reference points of
two
separate
operational
amplifiers,
and
Q1098
are
emitter
followers
which
provide
drive
for
the
sources
the
corresponding
summing
points.
Both
of
Q114A
and
Q1148.
Up
to
now
amplification
has been
these
operational amplifiers
must
be
satisfied simulta-
IV-2
tions)
and
a
shunt
capacitor
selected by
the
HF rolloff
switch.
The
LF
rolloff filter
is
composed
of
the
series
capacitor
selected
by
the
LF
rolloff switch and
the
1.59
megohm
shunt
resistor
at
the
input
to
A2.
The
transfer
curves
for
these
filters are given
in
Section
III.
4.1E
OVERLOAD
FAST·RECOVERY
CIRCUIT
The overload recovery button, when pressed, turns on
FET's
0105
and
0106
connected between each input
of
A 1
(PC
terminals
5 &
6)
and ground, and turtls on a FET
switch
connected
between the
input
of
A2
and ground.
0105
and
0106
discharge the
input
capacitors,
Cl
and
C2,
and
the
FET
switch
between the input of
A2
and
ground
discharges the
LP
and H P
filter
capacitors,
thereby providing fast overload recovery.
4.1
F POWER
SUPPL
Y
Ac line
power
is
stepped
down
by a
transformer,
then
rectified.
The
+ & - rectifier
outputs
are
shunted
with
12
V
nickel-cadmium batteries. Batteries across
the
dc
supply
outputs
provide
sufficient
regulation so
that
other
regula-
tion
is
not
needed
when
ac line
power
is
used.
If
the
batteries
are
in
a
state
of
discharge,
the
transformer-rectifier
circuit
will
automatically
charge
them
while
operating
from
ac. F or
battery
operation,
the
un
it
is
simply
not
plugged
into
the
ac receptacle, so
that
the
batteries
alone provide
power
to
the
dc
lines.
For
convenience
in
checking
the
batteries, a
test
circuit
is
provided.
4.2
CIRCUIT
DESCRIPTIONS
4.2A DI
FFERENTIAL
PREAMPLIFIER (A
1)
Refer
to
the
schematic
on page VII-3. This circuit differen-
tially amplifies
the
input
signals
at
the
gates
of
0100
A &
B.
These
field-effect transistors are involved
in
two
over-
lapping
modes
of
operational
amplifier feedback
that
determine
gain and
prevent
amplification
of
common-mode
signals.
0102
provides
constant
current
to
Rl0S,
thereby
fixing
the
voltage across R lOS. R 1
05
and R
119
provide an ave rage
of
the
source
voltages of
0100
A & B
to
the
base of
0103.
Because
0103
and
both
sections
of
0101
are forward
biased (so
that
their
emitter
junctions
have
constant
0.6
V
across
them,
approximately),
the
source-drain voltages
of
0100
A & B are held
constant
at
approximately
the
voltage
across
RlOS
(5 V).
CR10l
and
CR107
are
input
protective
diodes
reverse biased by
the
collector
voltage of
0104.
If
either
input
voltage
becomes
larger
than
about
-3
V,
the
corresponding
diode
will
conduct.
The
current
path
is
through
the
Ql04
collector
junction
and
CR104
to
ground.
The
gate
junctions
of
the
input
FET's
can handle positive
input
over-voltages
without
being damaged;
they
simply
become
forward biased and
conduct.
The
drain
currents
of
0100A
& B pass
to
the
collectors
of
0101A
&
0101B,
and are developed as voltages
for
application
to
the
bases
of
0109A
and
01098.
0109A
and
0109B
are
emitter
followers which provide drive for
0114A
and
0114B.
Up
to
now
amplification has been
IV-2
single-endeq for each half
of
the circuit.
But
differential
signals cause
the
emitter-coupled
pair,
0114A
and
0114B,
to
provide equally and
oppositely
changing signal
currents
to
their
collector circuits, and
common-mode
signals
tend
"")
:
to
cause equal changes
in
the
collector
currents
of
0114A
~
and
0114B.
Differential signals cause no change
in
the
emitter
currents
of
0114A
and
0114B
but
common
mode
signals
do.
Therefore
the
common-mode
feedback
signal
is
taken
from
the
emitter
resistor R146.
0116
shifts
the
0114A
collector voltage level
for
driving
0117.0117
inverts
the
0114A
signal, and helps
0114B
drive
the
complementary
output
pair
011S
and
0119.
Opposing
the
0114A
and
0114B
outputs
this
way
cancels
common-mode
signals
that
may remain in this
part
of
the
circuit. This
is
because
the
common-mode
feedback
loop, in
keeping
the
voltage across
R146
constant,
cancels
the
common-mode
signal
component
that
would
otherwise
appear
in
the
collector
currents
of
0114A
and
0114B.
CR 1
OS
temperature-compensates
the
emitter
junction
of
0113.
CR112
and
CR113
are forward biased
diodes
whose
constant
0.6
V
junction
drops
keep
011S
and
0119
forward biased.
The
primary
gain-determining feedback
is
taken
from
the
output
through
R131,
and will be explained
below.
0111
A and
0111
B
compare
the
common-mode
feedback
voltage at
the
base of
0111
B
to
a reference voltage
at
the
base of
0111A,
and provide
corresponding
equal and
opposite signal
currents
to
their
collector
circuits.
0113
inverts
the
0111
A
collector
signal
to
aid
0111
B
in
driving
0107
and 01OS.
0107
and
010S
are
complementary
transistors
that
drive
the
junction
of
R 123 and R 126,
...)/
providing balanced feedback
to
the
sources
of
the
input
FET's.
The
feedback action
of
an
operational
amplifier
is
such
as
to
keep
the
summing
point
voltage equal
to
the
reference
point
voltage. Since
feedback
is
usually
taken
from
the
output,
th
is
condition,
being
the
only stable
condition,
is
taken
advantage
of
in
determining
gain.
In
this circuit,
however, we really have
two
dependent
operational
ampli-
fier loops
that
determine
gain and provide
common-mode
rejection.
The
first feedback
component
we
will consider
is
the
common-mode
feedback
from
0107
and
0 lOS. Corre-
sponding
currents
through
R 123 and. R
126
drive
the
sources
of
0100
A & B
to
counteract
the
common-mode
component
of
the
input
signal.
In
this loop
the
base
of
0111
A
is
the
reference
point
and
the
base
of
0111
B
is
the
summing point.
The
high gain
of
the
loop causes
the
feedback
to
the
sources
to
completely
counteract
the
common-mode
component
of
input
signal.
In
doing
so
it
aids
the
feedback from
the
output
in
satisfying
the
requirements
of
the
gain-determining loop,
as
will
be
seen
from
the
following discussion.
The
gates
of
the
two
input
FET's
can
be
considered as
the
J',
reference
points
of
two
separate
operational
amplifiers, and .
the
sources
the
corresponding
summing
points.
Both
of
these
operational
amplifiers
must
be satisfied simulta-
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