A limiting circuit is described which prevents a signal being supplied to a signal amplifier from exceeding a predetermined value. The circuit is designed to permit a signal voltage to be fed directly to a signal amplifier without passing through the operational amplifier and without being altered undesirably. When the signal level increases to the predetermined value, the summing point shifts from the input of the operational amplifier to the output of the limiting circuit

Hansen, I. G.

Peterson, V. S.

English

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https://ntrs.nasa.gov/search.jsp?R=19740013983 2020-03-17T14:42:20+00:00Z
United States Patent Office 3,404,348Patented Oct. 1, 1968
3,404,348
LOW LEVEL SIGNAL LIMTTER
Irving G. Hansen, Bellevue, and Victor S. Peterson, San-
dusky, Ohio, assignors to the United States of America
as represented by the Administrator of the National
Aeronautics and Space Administration
Filed Sept. 22, 1966, Ser. No. 582,213
10 Claims. (Cl. 330—103)
The invention described herein was made by employees
of the United States Government and may be manu-
factured and used by or for the Government of the United
States of America for governmental purposes without the
payment of any royalties thereon or therefor.
This invention relates to electronic limiting circuits and
is dircted more particularly to a low level signal limiter.
. As is well known, signal amplifying devices are designed
to accept signal input voltages within a certain range of
magnitude. If the input signal is excessive, the amplifier
may saturate or become nonlinear in operation, both of
these results being normally undesirable.
An example of an instance in which an excessive signal
may be applied to an amplifier occurs with regard to
equipment utilized in the measurement of the temperature
of extremely cold bodies, as for example, cryogenic
fluids. When the probes used to make such measurements
are subjected to normal room temperatures, the signal
voltage of such probes become so great that the amplifiers
into which the signal is being fed become overloaded. This
problem has been overcome to a large extent by the use
of limiting circuits which prevent the signal voltage from
excedeing a predetermined value.
However, none of the prior art limiting circuits are
completely satisfactory where a signal voltage is to be
limited to a low level such as on the order of 12 millivolts.
One of the reasons for the unsatisfactory performance of
prior art limiting circuits is that the signal to be limited
must pass through an operational amplifier section
included in the limiting circuit. Consequently, the signal
level is affected by the zero drift, the gain drift, and non-
linearities of the operational amplifier. Additionally, in
limiting circuits of the prior art, the output voltage
of the operational amplifier adds to the signal voltage so
that the signal voltage as supplied to a signal amplifier
includes an error component.
Accordingly, it is an object of the invention to provide
a novel limiting circuit which prevents a signal being sup-
plied to a signal amplifier from exceeding a predetermined
value.
It is another object of the invention to provide a limit-
ing circuit in which the output voltage of an operational
amplifier included therein is prevented from adding to the
signal voltage.
It is a further object of the invention to provide a limit-
ing circuit in which the zero drift, gain drift and non-
linearities of the operational amplifier included therein do
hot affect the signal level.
Still another object of the invention is to provide a'
limiting circuit in which the summing point shifts from
the input of the operational amplifier to the output of
the limiting circuit when the signal level increases to a
predetermined value.
It is yet another object of the invention to provide
circuitry of the above type in which a signal voltage is
fed directly to a signal amplifier without passing through
the operational amplifier and, consequently, is not altered
undesirably thereby.
An additional object of the invention is to provide a
limiting circuit in which an operational amplifier is con-
trolled by the voltage supplied to the signal amplifier
input and provides a correcting voltage to that point when
the signal voltage exceeds a predetermined value.
Other objects and advantages of the invention will be-
come apparent from the following description and accom-
panying drawings in which:
FIG. 1 is a schematic diagram of circuitry embodying
5 the invention; and
F.IG. 2 is a schematic diagram of circuitry embodying
the invention but modified with respect to the electrically
common connections.
Referring to FIG. 1, it will be seen that the limiting
jO circuit embodying the invention may include signal input
terminals 10 and 11 which are connected to respective
junction points 12 and 13 through respective isolating
resistors 14 and 15. These resistors provide isolation be-
tween the input terminals 10, 11 and the junction points
15 12, 13 because of their resistance values. A lead 16 con-
nects the junction point 12 to a signal output terminal 17
while a lead 18 connects the junction point 13 to a signal
output terminal 19. To amplify a signal voltage supplied
to the signal input terminals 10 and 11 from a transducer
20 21, a signal amplifier 20 may be connected between the
signal output terminals 17 and 19. The transducer 21
may be, by way of example, a temperature probe for
cryogenic liquids together with a bridge circuit or other
signal conditioning circuit.
25 With the foregoing arrangement, it will be seen that
the voltage appearing at the signal output terminals 17,
19 of the limiting circuit and across the signal amplifier
20 will be identical to that between the junction points
12 and 13 because of the direct metallic connections
30 established by the leads 16 and 18. The voltage appearing
between these junction points is slightly less than the
signal voltage applied to the signal input terminals 10 and
11 due to the presence of the resistors 14 and 15.
In order to limit the signal voltage supplied to the
35 signal amplifier, the potential between the junction points
12 and 13 must be limited to a predetermined value which
is within the range of voltages which may be supplied to
the signal amplifier 20 without causing it to operate in a
nonlinear region or without otherwise adversely affecting
40 its operation. To this end, there is provided an operational
amplifier 22 having input terminals 23 and 24 and out-
put terminals 25 and 26. Input driving voltage is supplied
to the operational amplifier 22 by means of a lead 27
connected between the junction point 13 and input
45 terminal 24 and, through a dropping resistor 28 connected
between the junction point 12 and the input terminal 23.
•When the voltage appearing between the junction points
12 and 13 is less than the value to which it is to be lim-
ited, no limiting action is required and, therefore, the volt-
50 age applied to the input terminals 23 and 24 of the op-
erational amplifier 22 should toe maintained at substan-
tially zero magnitude. This is accomplished by providing
a first negative feedback network which may comprise
a resistor 29 connected between the output terminal 25
55 and the input terminal 23 of the operational amplifier 22.
It will be understood by those skilled in the art that a
capacitor and/or an inductor may be substituted for the
resistor 29 or connected in series or parallel therewith
depending on the feedback characteristic desired. The out-
60 put terminal 26 of the operational amplifier 22 is con-
nected to the lead 18 at a junction point 30 to provide
an electrically common connection to the lead 18.
As explained previously, it is desired to prevent the
voltage appearing between the junction points 12 and 13
65 from exceeding a predetermined magnitude. To this end,
there is provided a nonlinear feedback network compris-
ing a unidirectional current conducting device such as a
diode 31 having its anode electrode connected to the out-
put terminal 25 of the operational amplifier 22 and hav-
70 ing its cathode electrode connected to the junction point
12 by means of a lead 32. Since the junction point 12 is
connected to the limiting amplifier output terminal 17 by
3,404,348
the lead 16, the cathode electrode of the diode 31 is di-
rectly connected to the terminal 17. Accordingly, it will
be seen that the output voltage of the operational ampli-
fier 22 as seen at the terminals 25 and 26 is applied to
the signal amplifier 20 at terminals 17 and 19 through the 5
diode 31.
The diode 31 has a prescribed threshold voltage and
will not conduct until the voltage impressed thereon ex-
ceeds this threshold value. Consequently, .no voltage will
be fed tack from the output terminal 25 to the June-
 1Q
tion point 12 until the output voltage of the operational
amplifier exceeds the prescribed threshold voltage of the
diode 31. The diode 31 serves as a voltage dropping means
having a prescribed threshold voltage.'
Operation of the foregoing circuitry will now be de-
 15
scribed.
Taking 12 millivolts, as an exemplary predetermined
value to which the input voltage of the signal amplifier 20
must be limited, the voltage .between the junction points
12 and 13 must be limited to this same value because of
 20
the direct metallic connections to the signal output termi-
nals 17 and 19 as previously described. When the signal
voltage being applied to the signal input terminals 10
and 11 is substantially less than 12 millivolts, the voltages
between the junction points 12 and 13 will be slightly less
 25
than the signal voltage at the input terminals 10 and 11
due to the resistors 14 and 15. Similarly, the driving volt-
age at the input terminals 23 and 24 of the operational
amplifier 22 should be only slightly less than the voltage
between the terminals 12 and 13 because of the dropping 30
resistor 28. However, because of the negative feedback
provided by resistor 29 from the operational amplifier the
voltage between the input terminals 23 and 24 is held to
substantially zero voltage. The gain of the operational
amplifier is chosen so-as to maintain the voltage between 35
the terminals at substantially zero, the gain being deter-
mined by the ratio of resistor 29 to resistor 28 as is well
known to those skilled in the art.
Under the foregoing operating conditions, the diode 31
is nonconductive. Assuming now that the voltage supplied 40
by the transducer 21 increases, the output voltage of
the operational amplifier 22 as seen between terminals 25
and 26 likewise increases. When the voltage between the
terminals 25 and 26 becomes greater than the threshold
voltage of the diode 31, current will flow from the ter- 45
minal 25 to the junction point 12 via the diode 31 and
the.lead 32. This conduction of the diode 31 causes the
full output voltage of the operational amplifier 22 minus
the voltage drop of the diode to be fed back to the junc-
tion point 12. The summing point is thus transferred from 50
terminal 23 to the junction 12 when the diode 31 con-
ducts. Since the voltage at the output terminals 25 and 26
of the operational amplifier 22 is 180° out of phase with
the voltage at the junction points 12 and 13, the increase
of voltage above 12 millivolts which would otherwise ap- 55
pear between the junctipns 12 and 13 is effectively can-
celed.
Assuming that the voltage applied to the terminals 10
and 11 continues to increase, the current flow through the
diode 31 also increases. As current through the diode 31 60
increases, the resistance of the'diode decreases. Conse-
quently, the magnitude of voltage fed back to the junction
points 12 and 13 increases thereby preventing the voltage
between those terminals from exceeding the predeter-
mined 12 millivolt limit. 65
The characteristics of the diode 31 are such that the
resistance thereof decreases faster than the current there-
through increases, and in a nonlinear relationship after
the threshold voltage has been exceeded. As a result of
these characteristics, after the voltage between the junc: 70
tion points 12 and 13 reaches 12 millivolts, the voltage
feedback to those junction points will increase faster than
the voltage being supplied to them from the transducer 21
increases. Thus the canceling feedback voltage limits the
voltage between the junction points 12 and 13 to the de- 75
sired predetermined value of 12 millivolts. The diode 31
and the lead 32 comprise what may be termed a non-
linear negative feedback network having a prescribed
threshold voltage.
Of course, when the transducer 21 voltage drops to a
magnitude at which the voltage between the junction
points 12 and 13 would be less thatn 12 millivolts without
the presence of the operational amplifier 22 and its asso-
ciated networks, the voltage applied to the terminals 23
and 24 becomes sufficiently low as to allow the voltage
between the terminals 25 and 26 to drop below the thresh-
old voltage of the diode 31. When this occurs, the diode
31 becomes nonconductive and feedback to the junction
point 12 ceases. As a result, the summing point shifts
from junction point 12 to terminal 23 where it remains
as long as the voltage between the junction points 12 and
13 is below 12 millivolts.
The circuit of FIG. 2 is similar to that shown in FIG. 1
and like parts are identified by like numerals. In the cir-
cuit of FIG. 2, terminals 11, 24 and 26 are connected
to common grounds 33. Also, the side of the signal am-
plifier 20 originally connected to the terminal 19 is con-
nected to a ground 33. The circuit of FIG. 2 is thus a
single ended input as compared to the differential input of
FIG. 1.
Frm the foregoing it will be seen that, in the limiting
circuit of the present invention, a signal voltage to be
limited is transferred directly from a transducer to a sig-
nal amplifier without passing through any other amplifier
stages. To limit the signal voltage, an operational am-
plifier is driven through a dropping resistor by the signal
voltage applied to the signal amplifier. The output voltage
of the operational amplifier is fed through a nonlinear
impedance network to the signal amplifier when the volt-
age supplied to the signal amplifier tends to exceed a pre-
determined magnitude thereby canceling the voltage in
excess of the predetermined magnitude.
It will be understood that changes and modifications
may be made to the foregoing circuitry without departing
from the spirit and scope of the invention as set forth in
the claims appended hereto.
What is claimed is:
1. In a signal limiting circuit including signal input
means adapted to receive a signal from a transducer and
signal output means adapted for connection to a signal
amplifier, the combination of: isolating means connecting
said signal input means to said signal output means; 'ah
operational amplifier having an input and an output; volt-
age dropping means connected between said operational
amplifier input and said signal output means to isolate the
voltage at the input of said operational amplifier from
the voltage at said signal output means; a linear feedback
network connected between said output and said input of
said operational amplifier to reduce the voltage at said
input of said operational amplifier to substantially zero
when the voltage between said signal output terminals
is below a predetermined magnitude; and, a nonlinear
feedback network connected between said output of said
operational amplifier and said signal output means where-
by, as the output voltage of said operational amplifier in-
creases, the feedback to said signal output means increases
faster than the feedback to said input of said operational
amplifier to limit the voltage at said signal output means
to a predetermined value.
2. The circuit as set forth in claim 1 in which said non-
linear feedback network includes voltage dropping means
having a prescribed threshold voltage. ' '
3. The circuit as set forth in claim 1 in which said non-
linear feedback network comprises a unidirectional cur-
rent conducting device having a prescribed threshold volt-
age. .
4. The circuit as set forth in claim 1 in .which said non-
linear feedback network comprises a semiconductor diode.'
5. The circuit as set forth in claim 4 in which said linear
feedback network comprises a resistor.
Oct. 1, 1968 I. G. HANSEN ETAL
LOW LEVEL SIGNAL LIMITER
Filed Sept. 22, 1966
3,404,348
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INVENTORS
I R V I N G G. HANSEN
VICTOR S. PETERSON
ATTORNEYS
3,404,348
6. The circuit as set forth in claim 1 in which said iso-
lating means is a resistor and, said voltage dropping means
connected between said operational amplifier input and
said signal output means is a resistor.
7. The circuit as set forth in claim 6 in which said non-
linear feedback network includes voltage dropping means
having a. prescribed threshold voltage.
8. The circuit as set forth in claim 6 in which said non-
linear feedback network comprises a unidirectional cur-
rent conducting device having a prescribed threshold volt-,
age.
9. The circuit as set forth in claim 6 in which said
nonlinear feedback network comprises a semiconductor
diode.
10. The circuit as set forth in claim 9 in which said
linear feedback network comprises a resistor.
10
3,041,535
3,248,569
References Cited
UNITED STATES PATENTS
6/1962 Cohran 330—110 X
4/1966 Weekes 330—103 X
ROY LAKE, Primary Examiner.
JAMES B. MULLINS, Assistant Examiner.



Low level signal limiter

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