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4-2
Section IV
feedback
signals
is
amplified
by the differential amplifier
(A2Q2
and
A2Q3) and
applied,
through
emitter
follower
A2Q4, to the complementary
symmetry pair A2QS and
A2 Q6.
The
very
low
output
impedance
of
the
complementary pair
acts
as
a
constant
voltage source
for
the Buffer Amplifier and
also allows feedback
to be applied
to the bridge without loading the output
of
the
oscillator.
A2QS
and A2Q6 are forward biased through
A2CR7
and
A2CRS
so as
to
conduct slightly when
no
signal
is applied;
this eliminates
crossover
distortion
of
the
output
signal.
4-10
.
A2Q7
acts
as
a
peak detector. Part
of
the
oscillator
signal
is
superimposed on a negative
bias
at
the base
of
A2Q7; A2Q7
conducts only
when the positive peaks
of
the
signal overcome
the negative bias. The negative dc
output
of
A2Q7
is filtered by A2C2, A2C3
and
A2C4
and
used to
bias the diodes A2CRI and A2CR2
so as
to vary the
impedance of the negative feedback
side of
the bridge to
control the
amplitude of
oscillations. A2RS is
switched
into
the
circuit on
the XI 00 through XI M ranges
and
A2CS
on
the XIO through XI K ranges for extra filtering.
A2CR3
limits
any
reverse
voltage
transients
across
the polarized
capacitors A2C3
,
A2C4
and
A2CS. A2R9* is
selected
to
draw
current
from the bridge
so
as to balance the current
through A2CR I
and
A2CR2 and improve distortion in
the
output signal.
4·11. BUFFER AMPLIFIER. (Schematic
1)
4·12
.
The BtJffer Amplifier (A2QS
through
A2Q I0)
provides isolation
between
the
oscillator and
the Balanced
Amplifier
and is also used
as
the point
at
which the
6S4A
signal is leveled. A2QS
and A2Q9
form a differential
amplifier
whose
output,
from the collector
of
A2Q9, is
amplified
by
A2QlO
and fed to the Balanced Amplifier.
Part of the signal
output
from A2Q lOis
applied as negative
feedback through A2R39 to the base of A2QS to stabilize
the
amplifier
gain
over its
10Hz to 10 MHz
frequency
range. The signal from the
oscillator
is divided down
at the
input to the
Buffer
Amplifier by
a
resistive
divider
consisting of
A2R33
and the
resistive
element
of
the
photosensitive resistor
(A2DSV
I). The resistance
of
the
resistive element
is determined by the
current through
the
lamp,
which
is
controlled
by the Automatic Leveling
Circuits (Paragraph
4-19 and
following)
.
The signal level
at
the input of the Buffer Amplifier
varies
with
changes
in the
resistance
of
the photosensitive resistor to
control the
level
of
the
6S4A output
signal.
4·13. COUNTER EMITTER FOLLOWER. (Schematic
1)
4-14.
The Counter Emitter Follower (A3Ql)
serves as
isolation between the Oscillator
circuit,
and the rear panel
COUNTER
OUTPUT (12). The output
frequency
is the
same as
the Variable Test Oscillator frequency
.
Signal
amplitude
at the
COUNTER OUTPUT is
approximately
0.1 V rms into a
50
ohm load
.
4-15. BALANCEO AMPLIFIER. (Schematic 2, Figure 7-3)
Model6S4A
4-16. The
single-ended sinusoidal
output
from
the Buffer
Amplifier is amplified
and converted
into
a
balanced
signal\\
in the Balanced Amplifier. The Amplifier
consists
of
three'll
cascaded
differen
tial
amplifiers (A2Q II through A2Q 16)
and
two
complementary symmetry
pairs
(A2Q
IS, A2QI9
and A2Q20, A2Q21). The balanced
output
signal from the
complementary
symmetry
pairs
is developed
across
summing
resistors
A2R74 and
A2R7S and then
applied
through A2R76
and
C9,
and
through A2R77 and
CIO
to
the
attenuators (S4). The
input sinusoidal
signal at
the base
of
A2QII is
converted
into two
signals
of
equal amplitude,
but
ISOo
out
of
phase, taken from the
collectors of
A2QII
and
A2Q 12.
The
two
signals
then follow parallel paths,
while maintaining the
ISOo
phase relationship, through the
Balanced
Amplifier.
One
signal
path is through A2QII,
A2Q 13
and
A2Q
15
to the
complementary symmetry
pair
A2QIS
and
A2Q19
;
the other signal
path
is
through
A2Q12, A2QI4
and
A2QI6 to
complementary symmetry
pair A2Q20
and
A2Q21. The
complementary
pairs are
biased through diodes A2CRIS,
A2CRI6
and A2CRI7
,
A2CRIS
so
that the transistors
conduct
slightly
when
no
signal is applied;
this eliminates
crossover
distortion of the
signal
at the output
of
the Balanced Amplifier.
4-17.
The
gain of
the Balanced Amplifier
is stabilized
by
means
of
negative
feedback.
A2RSI
couples
the
output
signal from
the top of
A2
R74 back to the base
of
A2Q II
and A2RS3 couples
the
opposite
signal from the bottom
end
of A2R7S back to the base
of A2Q12.
Note that the
~r
application
of feedback
together
with
the high
gain of
th
e
~.
Amplifier
and
the
very low signal source
impedance
(from
'
the Buffer
Amplifier) causes
the Balanced Amplifier to
function
as
an
operational amplifier
with differential input
and
output;
as a result
,
the
overall
gain
of
the Balanced
Amplifier is determined by the ratio
of
A2RSI to A2R44
and
A2RS3 to A2RS4.
Capacitors A2C1S and A2C1S
improve the high frequency response of the feedback paths.
4-IS. To maintain proper balance
of
the
output signal,
the
junction
of summing resistors
A2R74 and A2 R7S is held at
virtual
ground
by
means of
negative
feedback
to the third
differential
amplifier
(A2QIS, A2QI6). The differential
pair A2Q22
and
A2Q23
compares
the
voltage at
the
junction
of
A2R74
and
A2R7S with ground
(the
base of
A2Q23 is
connected
directly to
ground).
Any
signal
unbalance
or common
mode
signal
across the two resistors
moves
their junction
away
from
ground;
this
voltage at
the
junction is
amplified
by A2Q22 and A2Q23 and
applied
to
the base of A2Q 17 to change its
collec
tor
current.
A2Q 17
is the
source of current
for A2Q 15 and
A2Q
16 so that the
change
in
current through
them will be
such as
to restore
the balance between the two
sinusoidal signals and
return
the
junction of
A2R74
and
A2R7S back to
ground.
A2C32
is
adjusted
for
equal
signal voltage
across
A2R74
and
A2R7S. A2R47,
A2C16,
A2C23,
A2C26
,
A2C27,
A2C2S
and
A2C29
all serve
as frequency
shaping elements
to
improve the frequency
response and
to insure stability
of
the Balanced Amplifier
over
its frequency range
of 10Hz
to
10 MHz.
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