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HP 654A - 4-19. Amplitude Control and Automatic Leveling Control; 4-25. Amplitude Control Integrator

HP 654A
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Model
654A
Section
IV
4-19.
AMPLITUDE
CONTROL
AND
AUTOMATIC
LEVELING
CONTROL.
4-20.
The
amplitude
of
the
balanced
sine
wave
signal
from
the
Balanced
Amplifier
is
independent
of
the
Attenuator
settings
but can
be
varied
over
a 2
dB
range
by
the
front
panel
AMPLITUDE
control (
Schematic
2).
The
Automatic
Leveling
Circuit (
ALC)
consists
of
the
Average
Detector,
the
Amplitude
Control
Integrator,
and
the
Amplitude
Current
Reference (
varied
by
the
AMPLITUDE
control).
The
Average
Detector
monitors
the
output
of
the
Balanced
Amplifier
and
produces
a
dc
current
proportional
to
the
amplitude
of
the
Balanced
Amplifier
signal.
The
Amplitude
Control
Integrator
compares
this
dc
current
with
a
current
of
opposite
polarity
from
the
Amplitude
Current
Reference;
any
difference
in
magnitude
between
the
current
from
the
Average
Detector
and
the
reference
current
is
used
to
apply
negative
feedback
to the
photosensitive
resistor
at
the
input
of
the
Buffer
Amplifier
until
the
output
of
the
Balanced
Amplifier
is
at
the
level
where
the
two
currents
are equal.
The
output
of
the
Balanced
Amplifier
is
at
the
required
level
when
the current
from
the
Average
Detector
is
equal
to
the
reference
current.
4-21.
AVERAGE
DETECTOR. (
Schematic
2)
4-22.
The
detector
monitors
the
output
of
the
Balanced
Amplifier.
A2Q24
ad
A2Q25
form
a
high
gain
amplifier
which
is
a
current
source
for
the
detector (
A2CR21
and
A2CR22).
A2CR21
supplies
the
metering
circuits
with
a
positive
dc
current
and
A2CR22
supplies
the
ALC
circuits
with
a
negative
dc
current.
These
currents
are
equal
in
amplitude
to
each
other
and
proportional
to
the
amplitude
of
the
Balanced
Amplifier
output
signal.
A2Q24
and
A2Q25 (
together
with
the
components
connected
to
the
base
of
A2Q25)
form
essentially
one
transistor
with
high
gain,
high
output
impedance
and
very
low
output
capacitance;
these
factors
together
with
the '
bootstrap'
capacitor
A2C42
account
for
the
amplifier's
effectiveness
as
a
current
source over
a
wide
frequency
and
temperature
range.
A2C43 (
Freq.
Response)
is
adjusted
for
flat
frequency
response
of
the
detector
circuit.
4-23.
AMPLITUDE
CURRENT
REFERENCE.
(Schematic
2)
4-24.
Zener
Diode
AlCR8
maintains
a
constant
voltage
across
R3 (
front-
panel
AMPLITUDE
control)
and
A2R91
in
series,
the
Amplitude
Control
Integrator
maintains
essentially
0
Vdc
at
the
output
of
the current
reference (
at
the
base
of
A3Q6):
thus, for
any
given
setting
of
R3,
there
is
a
fixed
voltage
drop
across
A3R19
and
A3R20
and
a
fixed
amount
of
current
flows
from
the
Amplitude
Current
Reference.
When
the
setting
of
the
AMPLITUDE
control
(R3)
is
changed,
the
voltage
drop
across
A3R19
and
A3R20
is
changed;
this
sets
a
new
fixed
value
of
current
flowing
towards
the
base
of
A3Q6.
el
4-25.
AMPLITUDE
CONTROL
INTEGRATOR.
(Schematic
2)
4-26.
The
circuit
consists
of
A3Q6
through
A3Q9
and
associated
circuitry,
including
the
lamp
of
the
photo-
sensitive
resistor
A2DSV1 (
Schematic
1).
A3Q6
and
A3Q7
form
a
differential
amplifier;
any
change
in
output
from
the
collector
of
A3Q7
is
amplified
by
A3Q8
and
applied
to
the
base
of
A3Q9
to
change
the
current
through
the
lamp
of
the photosensitive
resistor (
A2DSV1).
Changes
in
lamp
current
change
the
impedance
of
the
voltage
divider
at
the
input
of
the
Buffer
Amplifier (
Paragraph
4-12),
thus
changing
the
sine
wave
signal
level
through
the
Buffer
Amplifier
and
the
Balanced
Amplifier.
The
dc
current
from
the
Average
Detector
is
compared,
at
the
base
of
A3Q6,
with
the
current
from
the
Amplitude
Current
Reference.
In
the
differential
pair (
A3Q6,
A3Q7)
the
base
of
A3Q7
is
connected
directly
to
ground;
therefore,
as
long
as
the
base
of
A3Q6
is
held
at
0
Vdc,
there
will
be
no
change
in
output
from
the
collector
of
A3Q7.
The
amplitude
of
the
positive
current
flowing
from
the
Amplitude
Current
Reference
is
fixed;
the
amplitude
of
the
negative
current
flowing
from
the
Average
Detector
depends
on
the
level
of
signal at
the
output
of
the
Balanced
Amplifier.
These
currents
are
summed
at
the
base
of
A3Q6.
The
difference
current
flows
into
the
base
of
A3Q6.
The
amplitude
of
the
base
current
is
set
by
the
AMPLITUDE
CAL
adjustment
and
the
AMPLI-
TUDE
control.
As
long
as
this
current
level
is
not
changed
by
a
variation
in
the
Average
Detector
output,
there
will
be
no
change
in
current
through
the
lamp
of
the
photo-
sensitive
resistor;
thus
the
ac
signal
level
at
the
output
of
the
Balanced
Amplifier
will
be
constant.
In
this
condition,
the
ALC
loop
is
in
the "quiescent"
state
and
the
output
of
the
654A
is
at
the
required
level.
4-27.
Suppose
that
the
output
level
of
the
Balanced
Amplifier
now
changes
for
some
reason (
e. g.
the
frequency
of
the
Wien
Bridge
Oscillator
is
changed);
then
the
Amplitude
Control
Integrator
will
act
to return
the
signal
back
to
its
original
level
in
the
following
manner:
a.
The
negative
current
from
the
Average
Detector
(A2CR22)
will
change
proportionally
with
the
change
in
ac
signal
level.
This
current
flowing
into
the
summing
node
at
the
base
of
A3Q6
diminishes
the
positive
current
amplitude
thus
decreasing the
base
current
to
A3Q6.
This,
in
turn,
begins
to
shut
off
the
transistor.
b.
The
output
of
the
Amplitude
Control
Integrator
changes
in
response
to
the
new
input
so
as to
change
the
ac
signal
level
into
the
Buffer
Ampli-
fier.
This
will
be
in
such
a
direction
as to
return
the
Balanced
Amplifier
ac
output
back
to
the
level
where
the
negative
dc
current
from
the
Average
Detector
is
again
equal
to
its
quiescent
value.
In
this
way
the
654A
output
signal
is
maintained
at
a
constant
level.
4-28.
To
manually
control
the
654A
output
level
over
the
2
dBm
range,
the
AMPLITUDE
control
setting
is
changed;
this
changes
the
current
from
the
Amplitude
Current
Reference
thus
changing
the
base
current
to
A3Q6.
The
Amplitude
Control
Integrator
now
acts
as
before
to
change
the
Balanced
Amplfier
ac
output
level
until
the
current
4-3

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