Section IV
4 - 1 2 .
INTERMEDIATE
ATTENUATOR.
,;
4-13.
The
output
of Q2 is f e d to the intermediate sec
‑
t i o n
of
the
range
attenuator
whichis
s i m p l y
a
frequency
compensated
voltage
divider that works w i t h the
p r e
‑
l i m i n a r y
attenuator
to
p r o v i d e
the
conventional 1, 3, 10
sequence f o r
correct
meter
operation.
4 - 1 4 . FIXED
G A I N
AMPLIFIER.
4-15.
Refer
to
the schematic
d i a g r a m
(figure 5-15)
in
the
back
of this manual.
4-16.
Transistors Q3
through
Q7
make
up
the
fixed
gain a m p l i fi e r
which
i s
used
t o
develop
the necessary
current
f o r
f u l l
scale meter deflection
and
to
p r o v i d e
the
meter
c i r c u i t
w i t h
a
high
impedance
source f o r
linear
o p e r a t i o n a t
a l l
current
levels.
4-17.
The
output
of
the
r a n g e
attenuator is
fed
to
difference
a m p l i fi e r
Q3 and
Q4,
and
compared
with a
feedback
s i g n a l
f r o m
the
meter
circuit.
This
d i f
‑
ference
signal
is
fed
to
Q5. Q5
and
Q6
a r e
conven
‑
tional
g r o u n d e d
e m i t t e r d i r e c t coupled a m p l i fi e r s .
The
direct
coupled
feature
of the amplifier, neces
‑
s a r y
because
of
the
extremely
low frequency
(1
cycle)
response of
the
Model
403A,
r e q u i r e s dc
feedback f o r
bias
stabilization.
R34
through R39 make
up
the
dc
feedback
loop,
which tends to minimize any tendency
f o r
de d r i f t
due to ambient temperature change. The
bias level is
set
w i t h
R36.
4-18.
The
output of Q6 is
fed
to
Q7,
a grounded
base
a m p l i fi e r, which
not
only
a m p l i fi e s
the signal, but
also provides a
high
output impedance f o r the
meter
circuit.
The
meter
source impedance is increased
even
more
by
the use
of negative
current
feedback
f r o m
the output o f the
meter
r e c t i fi e r
b r i d g e
t o
the
base
of Q4.
R53
to R55AB, C27-28,
and
L3, correct
the phase of
the
feedback at high frequencies.
The
403A
uses positive voltage feedback to the base of Q3
to
further
increase
the
source
impedance.
4-19.
The necessity of high
meter
source impedance
can be explained by r e f e r r i n g to figures
4-3
and
4-4.
4-20.
In order to have correct voltmeter
action, it
is necessary that
the
change
in
meter
current
be p r o
‑
p o r t i o n a l t o a
change i n a m p l i fi e r
input
voltage. The
load
resistance, then,
should
r e m a i n
constant.
Note
f r o m
fi g u r e
4-3, however,
that when
Eg
(and therefore
the diode
voltage
Eg) decreases,
the diode resistance
Rg
(and therefore the load resistance)
increases,
a f
‑
fecting
meter
l i n e a r i t y.
Note i n fi g u r e
4-4 that R g
appears in
series w i t h
Ro,
the
source
impedance.
The
effect
on output current, of
changes
in
diode
r e
‑
sistance with
voltage,
can be minimized by feeding
the
meter
circuit
f r o m
a constant
current
or
h i g h
impedance
source.
In
this way,
changes
of
diode
r e
‑
sistance
w i l l have
a negligible effect on
the
t o t a l
current
p a s s i n g
through
them and
hence
through
the
meter.
4 - 2 1 ,
The effect of diode resistance
change
is
f u r
‑
ther minimized by Q7
current
through
R41,
which
i m p r e s s e sa
fixed
0.3
v o l t
across both
CR3 and
CR4,
biasing them
close
t o t h e i r
contact
p o t e n t i a l .
4-22.
C17-18,
C21-22 andC25-26 a r e switched in on
the
1CPS - 1MC function to provide the long t i m e
c o n
‑
stants
necessary
f o r
1
cps response.
4-2
fe)
0.3
VOLTAGE
INCREASING
‐‐‐‐‐‐
>
DIODE VOLTAGE
(Eq)
RO
F i g u r e
4-3. Diode Current Vs Diode Voltage
R30
R31
<
‐ ‐
P O S I T I V E
VOLTAGE
FEEDBACK
(MINOR)
F i g u r e
4 - 4 ,
Fixed
A m p l i fi e r
B l o c k
D i a g r a m
4 - 2 3 .
METER
RECTIFIER C I R C U I T.
4-24.
The
meter
r e c t i fi e r c i r c u i t i s
arranged
i n a
bridge-type configuration,
w i t h a
c r y s t a l
diode
and a
capacitor
in each branchand a dc microammeter
c o n
‑
nected
across i t s midpoints. The
current
through
the
meter
i s p r o p o r t i o n a l t o
the
average
value
o f
the
input
voltage
waveform.
Since calibration of the meter in
r m s volts
is basedon the
r a t i o
that exists
between the
average
and
effective
values of a sine-wave voltage,
deviation f r o m a true
sine
wave m a y cause
meter
measurements t o b e i n
e r r o r. Table 3 - 1
gives
an
i n d i
‑
cation o f
the
l i m i t s
o f p o s s i b l e
e r r o r due
t o
the p r e s
‑
ence
of harmonics in the measuredvoltagewaveform.
4-25.
The Model
403A
meter
r e c t i fi e r c i r c u i t o p e r a
‑
t i o n
can best
be explained by
analyzing the
c i r c u i t in
a s i m p l i fi e d f o r m .
F i g u r e
4-5
shows
avoltage
source
00125-3
