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Section IV
4 - 1 2 .
INTERMEDIATE
4-13.
The output of Q2 is f e d to the intermediate sec‑
of
attenuator
t i o n
the
range
voltage divider that works w i t h the
compensated
l i m i n a r y
attenuator
to p r o v i d e the conventional 1, 3, 10
correct meter
sequence f o r
4 - 1 4 . FIXED
AMPLIFIER.
G A I N
4-15.
Refer to the schematic
the back of this manual.
4-16. Transistors Q3
through
which
gain a m p l i fi e r
i s
used
current
scale meter deflection
f o r f u l l
meter
the
c i r c u i t w i t h
a
linear
a l l current
o p e r a t i o n a t
4-17.
The
of the
output
difference
Q3 and
a m p l i fi e r
the meter circuit.
feedback
s i g n a l f r o m
is fed to Q5. Q5
ference
signal
tional
g r o u n d e d
e m i t t e r d i r e c t
The direct
feature of the amplifier, neces‑
coupled
s a r y
because
of the extremely
403A,
the Model
response of
bias stabilization. R34 through R39 make up the dc
which tends
feedback loop,
due to ambient temperature change. The
f o r de d r i f t
bias level is set w i t h R36.
4-18.
The
output of Q6 is
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
circuit.
The meter source
even more by the use of negative
the output o f the meter r e c t i fi e r
f r o m
of Q4.
R53
to R55AB, C27-28,
base
the feedback
the
phase of
403A uses
positive voltage feedback to
further
the source
to
increase
4-19.
The necessity of high meter source
can be explained by
r e f e r r i n g to figures
4-20.
In order to have correct voltmeter action, it
is necessary that the change in
change i n a m p l i fi e r input voltage. The
p o r t i o n a l t o a
resistance, then,
load
should r e m a i n
4-3,
however,
f r o m fi g u r e
Eg) decreases,
the diode voltage
Rg (and therefore the load resistance)
meter
l i n e a r i t y. Note i n fi g u r e 4-4 that R g
fecting
series w i t h Ro, the source
appears in
on output current, of
The effect
sistance with voltage, can be minimized by feeding
circuit
meter
f r o m a constant
the
source.
impedance
In
this
sistance w i l l have a negligible effect on the t o t a l
p a s s i n g through
them and hence through the
The effect of diode resistance
4 - 2 1 ,
ther minimized by Q7 current through
0.3
fixed
v o l t across both
i m p r e s s e sa
biasing them close t o t h e i r
4-22.
C21-22
C17-18,
and C25-26 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
ATTENUATOR.
,;
a
whichis
s i m p l y
frequency
p r e ‑
operation.
(figure 5-15) in
d i a g r a m
Q7
up
make
the fixed
develop the necessary
t o
to
and
p r o v i d e
source
high impedance
f o r
levels.
attenuator is fed to
r a n g e
Q4,
with a
and compared
This
d i f ‑
Q6
a r e conven‑
and
coupled
a m p l i fi e r s .
cycle)
low frequency
(1
feedback f o r
r e q u i r e s dc
minimize any tendency
to
Q7,
fed
base
to
a grounded
meter
impedance is increased
current feedback
the
b r i d g e t o
L3, correct
and
at high frequencies. The
the base of Q3
impedance.
impedance
4-4.
4-3 and
meter current be
p r o ‑
constant. Note
that when Eg (and therefore
the diode resistance
increases,
a f ‑
impedance.
in
changes
diode r e ‑
current or
h i g h
w a y, changes of diode r e ‑
current
meter.
change is f u r ‑
R41,
which
CR4,
CR3 and
contact
p o t e n t i a l .
0.3
fe)
VOLTAGE
DIODE VOLTAGE (Eq)
F i g u r e 4-3. Diode Current Vs Diode Voltage
< ‐ ‐ P O S I T I V E VOLTAGE
FEEDBACK (MINOR)
Fixed
F i g u r e 4 - 4 ,
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
bridge-type configuration, w i t h a c r y s t a l diode and a
in each branchand a dc microammeter
capacitor
nected across i t s midpoints. The
meter
the
i s p r o p o r t i o n a l t o
waveform. Since calibration of the meter in
voltage
r m s volts is basedon the r a t i o that exists between the
and effective values of a sine-wave voltage,
average
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
the l i m i t s
cation o f
o f p o s s i b l e
ence of harmonics in the measured voltage waveform.
4-25.
meter r e c t i fi e r c i r c u i t o p e r a ‑
The Model 403A
t i o n can best be explained by analyzing the c i r c u i t in
F i g u r e 4-5
a s i m p l i fi e d f o r m .
‐‐‐‐‐‐>
INCREASING
RO
R30
R31
arranged i n a
c o n ‑
current
through
the
value
the
average
o f
input
an
gives
i n d i ‑
e r r o r due
t o the p r e s ‑
shows
avoltage source
00125-3
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