Linear Integrator - HP 202A Operating And Service Manual

Sect. III Page 3
reference. When V3B conducts the Bi-Stable Unit
is flipped back to its original state, completing one
cycle of operation.
Voltage regulator tubes V5 and V6 a r e connected
by a voltage divider from which the switching refer-
ence voltages a r e taken. They also provide the
limiting voltages applied to tubes V7 and V8 which
a r e seen to be a push-pull clamping system. In-
asmuch a s the integrator output
is
directly related
to the input, it is seen that the magnitude of square-
wave applied must be carefully controlled. Al-
though only the squarewave appearing a t the plate
of Vl is needed to drive the integrator, the clamp
is made push-pull to prevent excessive current
variations in the regulator tubes. The action of
V7B and V8B
is
such that if the applied waveform
h a s peak excursions in excess of the potentials on
the remaining cathode and plate, these being deter-
mined
by
regulator tubes V5 and V6, a current will
flow through R20 which drops the voltage to very
nearly the potential of the regulated element of the
conducting section of the diode. The action of the
other diodes
is
the s a m e , but 180" out of phase,
inasmuch a s they a r e coupled to the plate of V2.
In this way, waveforms appearing on the clamped
sides of R21 and R20 a r e a s s u r e d to be of equal
magnitude a s well a s 180" out of phase, and further
the average of dc level of the squarewave is ac-
curately controlled.
3-3 LINEAR INTEGRATOR
Consider the block diagram of the linear of feed-
back integrator a s shown in Figure 3-3. Starting
with the output voltage E,,
it is
seen that
if
the gain
of the amplifier is high, then the signal appearing
a t the junction of R and C (the amplifier input) must
be
small. For a fixed output E o a s the gain is in-
creased the resultant signal at the input of the am-
plifier becomes arbitrarily small. Since the voltage
a t the junction a t R and C is arbitrarily small, a
squarewave applied to the input will cause a constant
current in R. Because the current charging and dis-
charging C is constant, except f o r direction, the
voltage a c r o s s C will be triangular. Since there
is
virtually no signal at the junction of R and C the
output voltage must also be triangular.
In this case the frequency of the applied signal
is
so
low that the amplifier used must be direct coupled.
There is a net voltage rise between input level and
output level in a dc amplifier. In this particular
application the average output level
is
determined
a s the average of the "plus reference" and "minus
reference" levels, since the output excursion
is
limited to these levels. B this level does not coincide
with the average level of the applied squarewave,
then the positive and negative excursions of the
squarewave will not be equal, resulting in unequal
rise and fall rates of the output triangle. Because
the squarewave input is generated from the triangular
output by the bi-stable circuit, the net result is that
under such conditions the squarewave is really a
rectangular wave. The resulting rectangular wave
has an average value just equal to that demanded
of the amplifier input by virtue of the pre-set output
level. The average levels of the input and output
a r e stabilized by the use of a differential amplifier
that has high gain to the difference between the volt-
age applied to its inputs but little o r no gain to any
voltage change common to both inputs.
Figure 3-4 shows how this
is
done. The right hand
grid of the differential amplifier V15, is the signal
input and is driven through R by the rectangular
wave appearing on the FREQUENCY control. The
average voltage of this rectangular wave is depen-
dent on the clamping levels and the ratio of "on" to
"off" time. When the system is adjusted for equal
on-off times (squarewave) the average is just the
average of the clamping levels. The left hand grid
has no signal because the voltage divider which in-
cludes the balance control is connected to the no-
signal sides of the clamping tubes. However, any
change in the clamping level changes the average
level appearing on both input g r i d s in the same
amount. Due to the large common cathode resistors
of V15 and V16 a common mode change has very
little effect. The input to the left hand grid has
another function. If the balance control R60,
is
varied slightly, the output of the amplifier will show
a considerable change in average level; and therefore
Figure 3-3. Generalized Miller o r Feedback Integrator
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