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Section VIII
THEORY
OF OPERATION
Model
3455A
8-54.
Squaring
Amplifier.
The
squaring amplifier
is
a log
amplifier
circuit
which
takes the log
of
the input
voltage,
or
in
this
case since there
are
two
transistors
(Q9
A
and
QUA)
in
the
feedback
loop, takes twice the log
of
the input
voltage.
Therefore, the
output of
the squaring amplifier
is
equal to
2
log
I
V
in
I
or
log
1
V
in
I
*
.
8-55.
Square
Root
and Averaging
Amplifier.
The
square
root amplifier reverses the action
of
squaring
amplifier.
The
input to the amplifier
is
through
logging
transistors
QllB
and
Q9B.
Output of
the square
r
oot ampli
fier
is
equivalent to
1/2
log
IV
in I*
or
log
V
V
In
I
^
.
The
operations of the square root
amplifier
and
the averaging
amplifier
are
simultaneous
and
inter-dependent.
The com-
bined output
of
the
two
circuits
is
a
dc
level
proportional
to the
rms
value
of
the input
signal.
B-S6.
AVERAGE
RESPONDING
AC
CONVERTER
(Option 001).
8-57. General.
8-58.
The
average ac converter
is
an
average
responding
cir-
cuit calibrated
to the
rms
value
of
a sinusoidal input. Full
scale
output
of
the converter
is
6.6667
V
dc
for
all
ranges.
Figure 8-20
shows
a
simplified
schematic
of
the
converter.
8-59. Circuit Description.
8-60.
AC
Input Attenuator.
The
ac
input attenuator
is
an
RC
circuit
which
provides
a fixed
attenuation
of
100-to-l
on
the
100
V
ac
and 1000
V
ac ranges.
Attenuator
switch-
ing
is
done by
reed relays
which
are
controlled
by
the
inguard
controller.
Input
resistance
of
the
AC
Converter
is
approximately 2
megohms.
8-61.
Converter
Amplifier.
The
converter
amplifier uses
a
dual
FET
input
stage
to
maintain
a
high input
impedance.
An
operational amplifier provides the necessary gain
to
drive
the
output
stage
of
the converter
amplifier.
The
out-
put
stage
of
the amplifier
is
a
current
driver
circuit.
Two
ac
feedback paths
provide
fixed gains
of
1
or
0.1.
An
integrat-
ing amplifier
(U4)
is
used
to
maintain
a
dc
level
of
0
V
dc
at
the
output of
the
Converter
Amplifier.
The
integrating
amplifier
also
determines the
low
frequency
cut-off point
of
the
Converter
Amplifier.
(The
cut-off
frequency
is
ap-
proximately
300
llz
on
the
FAST
ACV
mode
and 30
Hz
on
the
ACV
mode.)
A
diode
protection
circuit
is
used
to
limit
the
output
of
the
Converter Amplifier
to
approximately
±
6
V
peak
to prevent saturation
of
the
amplifier.
8-62.
AC
Ranging.
AC
ranging
is
accomplished
by
attenu-
ating
the input
signal
and changing
the gain
of
the converter
amplifier.
The
input attenuator provides
a fixed
attenuation
of 100
to
1.
The
Converter Amplifier has
fixed gains
of
1
or
0.1.
Table
8-1
shows
the various
combinations of
amp-
lifier
gain
and
input attenuation necessary for the input
voltage ranging. Full
scale
output of
the
Converter
Amp-
lifier
is
approximately
1
volt
rms
for
all
ranges.
8-63.
Rectifier
and
Filter
Amplifier.
The
output of
the
Converter Amplifier
is
applied
to a
rectifier circuit
which
produces both
a positive
going
and
a negative
going
half-
wave
rectified signal
output
(sec
Figure 8-20).
The
rectified
signals
are
summed
to
provide ac feedback
for
the
Con-
verter
Amplifier.
The
Filter
Amplifier has
a
fixed gain
of
approximately
6.6.
The
feedback
circuitry
of
the
Filter
Amplifier provides
one
pole
of
filtering.
The
output of
the
Filter
Amplifier
is
applied to
a
one
pole
RC
filter
network
for
FAST
ACV
operation
and
a
two
pole
RC
filter
network
for
ACV
operation.
Figure 8-20. Simplified
Average Responding
AC
Converter.
8-12
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