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SECTION
IV
THEORY OF OPERATION
MODEL
150
and
155.
Figure 4-1
is
a
block
diagram
of the
Model
150
and
155
Function Generator,
A
square
wave
is
applied
to
the input
of the
integrator (operational amplifier).
This
integrator
is
composed
of a
wide band high
gain
DC
amplifier and
inte-
grating resistor
R
and
integrating capacitor C.
The
output
of
the integrator
(a
triangle
waveform)
is
fed
into
the
Hysteresis and
output switches.
The
Hysteresis and
output switch function
like
a
Schmitt
trigger with
the limit
points set
wide
apart.
As
a result
they
fire
when
the
triangle
wave
reaches
+2.
5
volts
and
-2.
5
volts.
The
firing of the
Hysteresis and
the
output switches
reverses
the
square
wave
voltage fed
into
the
integrator
and
the triangle
wave
then
reverses
direction.
The
result
is
simultaneous generation
of
a
square
wave
and
triangle
wave
of the
same
frequency
with
the
positive half cycle
of
the
square
wave
coincident with
the
negative going portion
of
the triangle
wave.
The
frequency
of
oscillation
is
determined by two
conditions.
The
first
con-
dition
is
the
values
of the
resistance and capacitance selected with
the fre-
quency
range
switch.
The second
condition
is
the
amplitude
of the
square
wave
fed
to
the input of the
integrator.
Unique
circuitry
is
provided
whereby
the
amplitude
of
the
square
wave
fed
to
the
integrator
is
controlled
by an
analog
voltage.
The
integrating resistor
R
is
also
connect
to
a
PNP
and
NPN
transis-
tor switch.
The bases
of
these transistor switches
are driven
from
the
output
square wave.
Negative square
waves
cause conduction
of the
PNP
transistor,
and
positive
square
waves
cause conduction
of
the
NPN.
These
transistors
are connected
in
an
inverted configuration
and
when
in
the
conduction
mode,
have
in
the vicinity of 10
mv
drop between
their
emitter and
collector.
The
VCG
input
and
the
binary coded
decimal
switches both drive
two opera-
tional
amplifiers
in
cascade
whose
final
output
is
proportional
to,
but inverted
from,
the
sum
of
the input
voltages.
The
first
operational amplifier drives
the
second.
As
a result, a positive
voltage
appearing on
the input of the first
am-
plifier
causes
a
negative voltage
at the
output
of the first
amplifier and a posi-
tive
voltage
at the
output
of the
second
amplifier.
A
negative
square
wave
to
the
PNP,
NPN
transistor switch thus
causes
integrating resistor
R
to
be
shorted
to
the
negative voltage output
of the first
amplifier.
A
positive
square
wave
voltage
causes
resistor
R
to
be shorted
to
the positive
voltage output
of
the
second
amplifier.
The
circuit
would
be
complete and
work
accurately except
for the
small drop
across
the
PNP,
NPN
switching transistors.
This drop
is
compensated
for
by
inserting a
matching
transistor
in
the
feedback
legs of the
operational
ampli-
fiers,
The
drive
to
the
compensating
transistors
is
designed
to
equal
the
base
drive
to
the
switching transistors.
The
switching transistors and
the
operational
4-1
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