U.S. patent number RE29,079 [Application Number 05/596,818] was granted by the patent office on 1976-12-14 for multiplier, divider and wattmeter using a switching circuit and a pulse-width and frequency modulator.
This patent grant is currently assigned to Motor Finance Corporation. Invention is credited to Roswell W. Gilbert.
United States Patent |
RE29,079 |
Gilbert |
December 14, 1976 |
Multiplier, divider and wattmeter using a switching circuit and a
pulse-width and frequency modulator
Abstract
In the switching circuit, the output signal from a high-gain
differential amplifier is caused to reverse polarity by means of
the switching of a pair of field-effect transistors. A resistor is
connected in series with the inverting lead of the differential
amplifier, and a signal source is connected to the resistor at an
input terminal. The field-effect transistors are used to connect
the non-inverting lead of the differential amplifier alternately
between the input terminal and ground. This causes the amplifier
output to reverse polarity, and thus provides a very accurate,
low-offset means of switching the output of the amplifier. The
switching circuit is used in a modulator which permits output
pulses to be modulated in width and/or frequency. The reversible
output signals of the switching circuit are added to the modulating
input signal and the sum is integrated. A level detector changes
the polarity of its output voltage whenever the output of the
integrator reaches either of two fixed levels. The output of the
level detector is used to drive the field-effect transistors to
control the switching of the output of the differential amplifier.
Such a pulse-width modulator is used to drive a pulse-height
modulator so as to form a multiplier. The multiplier can be used to
great advantage as a wattmeter by supplying load current and
voltage signals to the pulse-width and pulse-height modulators so
as to multiply the load current and voltage by one another. The
same device can be used as a divider by holding either one input to
the pulse-width modulator or the input to the pulse-height
modulator constant, while allowing the other of those inputs to be
variable and the input to the switching circuit to be the other
variable. .Iadd.
Inventors: |
Gilbert; Roswell W. (Palma de
Mallorca, ES) |
Assignee: |
Motor Finance Corporation
(Dunellen, NJ)
|
Family
ID: |
27485565 |
Appl.
No.: |
05/596,818 |
Filed: |
July 17, 1975 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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184026 |
Sep 27, 1971 |
3745557 |
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6075 |
Jan 27, 1970 |
3626292 |
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184026 |
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6075 |
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Reissue of: |
210456 |
Dec 21, 1971 |
03746851 |
Jul 17, 1973 |
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Current U.S.
Class: |
708/103; 327/356;
327/113; 327/172; 327/360; 702/61; 324/142 |
Current CPC
Class: |
G01R
21/127 (20130101); G01R 29/0273 (20130101); G06G
7/161 (20130101); H03F 3/72 (20130101); H03K
7/02 (20130101); H03K 7/06 (20130101); H03K
7/08 (20130101) |
Current International
Class: |
G06G
7/00 (20060101); G06G 7/161 (20060101); H03K
7/02 (20060101); H03K 7/08 (20060101); G01R
29/02 (20060101); G01R 21/00 (20060101); G01R
21/127 (20060101); H03K 7/00 (20060101); G01R
29/027 (20060101); H03F 3/72 (20060101); H03K
7/06 (20060101); G06G 007/16 (); G01R 011/32 () |
Field of
Search: |
;235/194,195,196
;307/229,230,265,235 ;328/160,161 ;324/142 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Ruggiero; Joseph F.
Attorney, Agent or Firm: Curtis, Morris & Safford
Parent Case Text
This application is a continuation-in-part of U.S. patent
applications Ser. No. 184,026 filed Sept. 27, 1971, now U.S. Pat.
No. 3,745,557, issued July 10, 1973 and Ser. No. 6,075, filed Jan.
27, 1970, now U.S. Pat. No. 3,626,292, issued Dec. 7, 1971, said
application Ser. No. 184,026 being a division of said application
Ser. No. 6,075. The disclosure of both parent applications hereby
is incorporated herein by reference. .Iaddend.
Claims
I claim:
1. A multiplier device comprising a pulse-width modulator
controlled by a first input signal, and a pulse height modulator
controlled by a second input signal and connected to said
pulse-width modulator, said pulse-width modulator comprising source
means for producing an output signal alternating between two levels
in response to input switching signals, means for modifying at
least one of said levels in response to a modulating signal,
integrator means for integrating the output of said source means so
modified, said source means including a switching device, said
device including a high-gain differential amplifier having first
and second input terminals, an impedance connected between the
first of said input terminals and a third terminal for receiving an
input signal, and gating means for connecting the second of said
input terminals alternately between said third terminal and a
fourth terminal for receiving a signal of a magnitude lower than
the input signal applied to said third terminal, level detector
means for detecting the output of said integrator means and
producing an output signal change whenever the integrator output
reaches either of two different pre-determined levels, and means
for transmitting the output of said level detector means to said
source means as input gating signals to said source means.
2. A multiplier as in claim 1 including a smoothing circuit for the
output of said pulse-height modulator.
3. A wattmeter including a multiplier as in claim 1 with means for
receiving inputs proportional to the load voltage and the load
current of the power being measured.
4. A device as in claim 1 in which said switching means includes a
pair of field effect transistors, one connected between said second
and third terminals, and the other connected between said second
and fourth terminals.
5. A device as in claim 1 in which said differential amplifier is
an operational amplifier with a feed-back impedance connected
between its output terminal and said first input terminal.
6. A device as in claim 1 in which said first terminal is an
inverting terminal, and said second terminal is a non-inverting
terminal.
7. A device as in claim 4 in which one of said transistors is of
the P-channel type, and the other is of the N-channel type, a
common connection to the gate leads of the transistors, the output
of said level detector means being connected to said common
connection.
8. A device as in claim 4 in which said transistors are of the same
channel type, means connecting the output of said level detector
means directly to the gate lead of one of said transistors, and an
inverter connecting said output of said level detector means to the
gate lead of the other transistor.
9. A device as in claim 1 in which said impedance is a
resistor.
10. A device as in claim 1 in which said fourth terminal is common
to the input and output of said source means.
11. A mathematical circuit device comprising a pulse-width
modulator controlled by a first input signal, and a pulse height
modulator controlled by a second input signal and connected to said
pulse-width modulator, said pulse-width modulator including a
circuit device for producing an output signal alternating between
two levels in response to input switching signals, said circuit
device including a high-gain differential amplifier having two
input terminals and an output terminal, and means for reversing the
polarity of the output signal on said output terminal in response
to a change in polarity of the difference between the input signals
on said input terminals, an impedance connected at one end to a
first one of said input terminals and at the other end to a third
signal-receiving terminal, a fourth terminal for receiving a
reference signal, and switching means for connecting the second of
said input terminals alternately to said third terminal and said
fourth terminal, integrator means for integrating said output
signal, level detector means for detecting the output of said
integrator means and producing an output signal change whenever the
integrator output reaches either of two different pre-determined
levels, and means for transmitting the output of said level
detector means as input gating signals to said circuit device.
12. A device as in claim 11 including a smoothing circuit for the
output of said pulse-height modulator.
13. A wattmeter including a device as in claim 11 with means for
receiving inputs proportional to the load voltage and the load
current of the power being measured.
14. A device as in claim 11 in which said differential amplifier
includes a summing circuit, an inverter connected between one of
said input terminals and said summing circuit, said other input
terminal being connected to deliver an uninverted input signal to
said summing circuit, a signal amplifier connected to receive the
output of said summing circuit, and a feedback impedance connected
between said one input terminal and the output of said signal
amplifier.
15. A device as in claim 11 in which said differential amplifier is
an integrated operational amplifier and said feed-back impedance is
an external resistor, the first-named impedance is a resistor, said
switching means comprises a first field-effect transistor with its
source-drain path connected between said second and third
terminals, a second field-effect transistor with its source-drain
path connected between said second terminal and ground, said
transmitting means comprising means for connecting the output of
said level detector means to the gate leads of said transistors and
turning one of said transistors on while turning the other off.
16. A device as in claim 1 in which the frequency of the output of
said level detector means is at least 100 times the frequency of
said modulating signal.
17. A device as in claim 1 in which said levels of said level
detector means are fixed and independent from said modulating input
signal.
18. A device as in claim 1 in which said level detector means
comprises an operational amplifier having inverting and
non-inverting input terminals, the integrator output being
connected to said inverting terminal, a bias impedance connected to
said non-inverting terminal, and a positive feed-back impedance
connected between said non-inverting terminal and the output of
said operational amplifier.
19. A device as in claim 1 including a node at the output of said
source means and the input of said integrator means, and means for
conducting said modulating input signal into said node.
20. A device as in claim 1 including means for changing at least
one of said levels of said level detector means in response to
another modulating input signal, thereby changing the frequency of
the output of said level detector as a function of said other
modulating input signal.
21. A device as in claim 11 in which said level detector means
includes an operational amplifier having inverting and
non-inverting input terminals, the integrator output being
connected to said inverting terminal, a bias resistor connected to
the non-inverting terminal of said operational amplifier, and a
feed-back resistor connected between the non-inverting terminal and
the output of said operational amplifier.
22. An electrical device comprising, in combination, an integrator,
level sense means responsive to the output of said integrator for
producing a first signal of alternating polarity in which each
polarity reversal occurs when said integrator output reaches one of
two different signal levels, means for conducting said first
alternating signal to the input of said integrator, first
modulating means for changing the magnitude of said first signal in
accordance with a first input variable X, second modulating means
for changing the magnitude of said signal in accordance with a
second input variable Y and thus forming a second signal, and means
for controlling the separation between said different signal levels
in accordance with said second signal.
23. A device as in claim 22 in which at least one of said first and
second modulating means comprises a circuit device for producing an
output signal alternating between two levels in response to input
switching signals, said circuit device including a high-gain
differential amplifier having two input terminals and an output
terminal, and means for reversing the polarity of the output signal
on said output terminal in response to a change in polarity of the
difference between the input signals on said input terminals, an
impedance connected at one end to a first one of said input
terminals and at the other end to a third signal-receiving
terminal, a fourth terminal for receiving an input variable signal,
and switching means for connecting the second of said input
terminals alternately to said third terminal and said fourth
terminal.
24. A device as in claim 22 in which said level sensor means
comprises an operational amplifier having inverting and
non-inverting input terminals, the integrator output being
connected to said inverting terminal, a bias impedance connected to
said non-inverting terminal, and means for conducting said second
signal to said non-inverting terminal.
Description
This invention relates to switching devices, and particularly to
devices for providing rapid output switching of an amplifier,
particularly a differential amplifier. The invention further
relates to pulse-width modulators, pulse-height modulators, and to
multipliers, dividers and wattmeters and methods.
There are many needs for rapid switching devices, and particularly
for devices which reverse the polarity of a signal rapidly. It is
an object of the invention to produce such a device which is simple
and relatively inexpensive, and yet is highly accurate for use in
precision instrumentation.
There also is a need for very accurate, stable and reliable
multipliers, dividers and wattmeters. In particular, the lack of
wattmeters having sufficient accuracy is believed to cost the
electrical power distribution industry in the United States a very
substantial amount of money each year. Although many uses easily
could be found for electronic wattmeters, the lack of sufficient
accuracy, the high cost, and other factors have limited the
usefulness of prior electronic wattmeters.
In accordance with the foregoing, it is another object of the
present invention to provide a highly accurate and relatively
inexpensive and reliable multiplier, divider, and wattmeter, and
methods of use. It also is desired to provide unique modulators
which are capable of pulse-width, pulse-height and frequency
modulation. It is another object to provide such devices which make
advantageous use of the switching circuit of the invention.
In accordance with the present invention, the foregoing objects are
met by the provision of a switching device including a high-gain
differential amplifier with first and second input terminals. An
impedance is connected between one of the input terminals and a
third terminal for receiving the input signal to the circuit.
Switching means are provided for switching the second terminal back
and forth between the third terminal and a fourth terminal to
change the polarity of the difference between the input signals on
the input terminals to the differential amplifier, thus causing the
output of the device to switch.
The modulator of the invention includes source means (preferably a
switching device as described above) for producing an output signal
alternating between two levels in response to input switching
signals. The output of the source means is added to the modulating
input signal, and that sum is integrated by an integrator. A level
detector detects the output of the integrator and produces an
output signal change whenever the integrator output reaches either
of two different pre-determined levels. The output of the level
detector is fed back as an input signal to the source means. This
modulator provides pulse-width modulation, as well as frequency
modulation. A pulse-height modulator (preferably using another of
the switching devices) operates on the output of the pulse-width
modulator to provide a multiplier device. By making one of the
input signals to the multiplier proportional to a load current, and
the other input proportional to a load voltage, the device can be
used as a high-precision electronic wattmeter. The same device also
can be used as a divider by maintaining one of the inputs constant
while allowing the source level to vary, together with one of the
inputs. The same device can be used as another type of frequency
modulator by connecting the output of the pulse-height modulator
back to the level detector to modify the levels at which integrator
output signals are detected.
The foregoing and other objects and advantages of the invention
will be pointed out or apparent from the following description and
drawings.
In the drawings:
FIG. 1 is a schematic circuit diagram of a switching device
constructed in accordance with the present invention;
Each of FIGS. 2 and 3 is an equivalent circuit diagram of the
circuit shown in FIG. 1, each in a different switching
condition;
FIG. 4 is a schematic circuit diagram of a pulse-width-frequency
modulator, pulse-height modulator, multiplier, divider and
wattmeter device constructed in accordance with the present
invention;
FIG. 5 shows waveform diagrams for various electrical signals in
the circuit shown in FIG. 4; and
FIG. 6 shows the same waveforms as in FIG. 5, except that certain
input signal parameters have been changed.
SWITCHING CIRCUIT DEVICE
FIG. 1 shows a switching circuit device 10 constructed in
accordance with the present invention. The device 10 includes an
integrated operational differential amplifier 12, which is shown in
dashed outline. The differential operational amplifier 12 has a
non-inverting input lead 16, an inverting lead 14, and an output
lead 18. A negative feed-back resistor 28 is connected between the
output lead 18 and the inverting input lead 14 to form a high-gain
differential amplifier.
Connected to the inverting lead 14 is a resistor 26 whose other end
is connected to an input terminal 30. A gating circuit 20 is
provided. The gating circuit includes two field-effect transistors
("FETs") 22 and 24. The source-drain path of the FET 22 is
connected between the input terminal 30 and the non-inverting lead
16 of of the differential amplifier. The source-drain path of the
FET 24 is connected between the non-inverting lead 16 and the
common point in the circuit, e.g., ground. The gate leads 32 and 34
of the two FETs are connected to one or more switching sources (not
shown in FIG. 1) which will operate to turn on one of the FETs
while the other FET is turned off.
The operational differential amplifier 12 is of the conentional
integrated type. This amplifier 12 includes a summing circuit 36
which receives the input from the non-inverting lead 16. The
summing circuit 36 also receives an inverted input signal from the
inverting lead 14 through an inverter 38. The output of the summing
circuit is delivered to an amplifier 40 whose output lead is the
output lead 18 of the differential amplifier.
FIG. 2 shows the effective connection of the FIG. 1 circut in the
"inverting" mode; that is, in the mode in which FET 24 is turned on
and FET 22 is turned off. The non-inverting lead 16 is connected to
ground, and the inverting lead 14 is connected through resistor 26
to the input terminal 30. The relationship between the input
voltage E.sub.IN applied at terminal 30 and the output voltage
E.sub.OUT at terminal 18 is given by the following equation:
in which R.sub.28 is the resistance of resistor 28, and R.sub.26 is
the resistance of resistor 26.
It can be seen from the foregoing equation that when R.sub.26
equals R.sub.28, the circuit 10 merely is a unity-gain
inverter.
FIG. 3 shows the circuit 10 in the other of its two switching
modes, the non-inverting mode. In this mode, the FET 22 is turned
on and FET 24 is turned off. Thus, the non-inverting terminal 16
now is connected to the input signal terminal 30 instead of to
ground. The full input voltage is applied to the non-inverting
terminal 16, while the voltage drop across the resistor 26 is
applied (together with a negative feed-back signal through resistor
28) to the inverting terminal 14 of the differential amplifier. The
result of this connection is that initially a differential voltage
appears between the terminals 14 and 16 due to the operation of the
gating circuit 20. This differential lasts only during the very
brief switching time of the amplifier output. In the ensuing
steady-state condition, the signal on terminal 16 equals that on
terminal 14 when the output 18 is equal to input 30 and no current
flows through resistors 26 and 28. Therefore, the output of FIG. 1
is non-inverted; that is, if the input signal is positive, the
output signal is positive and equal, and vice versa. Thus, the
polarity of the output signals has reversed.
The circuit shown in FIG. 3 is merely a follower type of circuit.
Thus, the relationship between the output and input voltages is
independent of resistors 26 and 28, and is expressed by the
following equation:
in the FIG. 3 circuit, the values of resistors 26 and 28 can vary
widely without changing the output signal in any significant degree
because the amplifier inputs 14 and 16 have a relatively high
impedance, and no current flows through the resistors.
The switching circuit shown in FIG. 1 has a number of advantages,
particularly in electrical instrumentation. The device 10 provides
an output signal which switches as fast as the operational
amplifier 12 will permit, and this can be in the order of a few
nanoseconds. The switching also can be accurate because of the
circuitry does little if anything to degrade the inherently good
accuracy of the operational amplifier. For example, the series "on"
resistances of the FETs 22 and 24 do not adversely affect the
accuracy because the "off" resistance is millions of times greater
than the "on" resistance.
The circuit device 10 has a substantial cost advantage over
circuits previously used in instrumentation for similar purposes.
For example, a typical prior art device is shown in the article
entitled "An Electronic Multiplier for Accurate Power
Measurements," by Tomota, Sugiyama and Yamaguchi, in "IEEE
Transactions on Instrumentation and Measurement," Volume IM-17, No.
4, Dec., 1968. In such a prior art device, two separate precision
voltage sources are used, with the outputs of each being sampled
alternately by series FETs. An output operational differential
amplifier is used as a buffer. As compared with applicant's
circuit, such a prior circuit requires one additional precise
voltage source that is not required by applicant's device. Thus,
applicant's device is less expensive. Furthermore, the series "on"
resistances of the FETs and the extra voltage source are sources of
potential error which are not present in the present invention.
Other prior circuits have used a single precise voltage source,
series FETs for switching, an inverting operational amplifier, and
an output operational amplifier as a buffer. Such circuits also are
inherently more costly and error-prone than applicant's circuit
because they require one additional precise operational amplifier.
Furthermore, the series FETs and extra operational amplifier are
extra sources of error.
MODULATORS
FIG. 4 simultaneously shows a number of different circuits using
the switching device 10. The first of these circuits is a modulator
84 which is shown in the left hand portion of FIG. 4. The modulator
84 provides an alternating output pulse signal whose width and
frequency are moduated by an input signal applied at an input
terminal Z or, at a terminal X corresponding to the terminal 30
shown in FIGS. 1, 2 and 3.
The modulator 84 includes the switching device 10 which delivers
its output through a resistor 42 to a node (a). An input signal is
applied at terminal Z to the node (a) through a resistor 44, and is
added to the output of the switching circuit. The sum of the two
signals is supplied over a lead 50 to an integrating circuit 46.
The output of the integrator 46 is delivered to a comparator or
level-detector circuit 56. The output of circuit 56 is delivered
through a lead 68 to the gate leads 32 and 34 of FETs 22 and 24 of
the switching device 10.
The operation of the modulator 84 now will be explained with the
assistance of FIGS. 5 and 6. FIG. 5 shows the waveforms of the
signals appearing at points (a), (b), and (c) of FIG. 4, for the
condition in which the signal applied to terminal Z is zero, and
the signal applied to terminal X is a steady DC signal. The
rectangular wave (a) shown in FIG. 5 is integrated by the
integrator 46, which produces an output whose waveform is
substantially triangular, as is shown at (b) in FIG. 5. The reason
for this is that as the output voltage from the integrator reaches
a first level e.sub.1, the comparator causes its output voltage (c)
to reverse polarity abruptly. This causes the switching circuit 10
to operate to reverse the polarity of its output and start the
output of integrator 46 declining in a linear fashion. When the
integrator output reaches a second level e.sub.2, which is equal
but opposite in polarity to the level e.sub.1, the comparator again
causes its output voltage to reverse polarity, again causing the
switching circuit 10 to operate, thus starting the cycle once
again.
FIG. 6 illustrates how the above-described waveforms are changed
when a negative DC signal is applied to terminal Z. First, the
whole waveform (a) is shifted downwardly, meaning that the positive
portions are of lower magnitude and the negative portions are of
greater magnitude. Thus, the slope of the positive-going portions
of the waveform (b) is lower. However, since the negative portions
of the waveform (a) are of greater magnitude than before, the slope
of the negative-going portions of the waveform (b) is steeper.
Since the separation e between comparator levels e.sub.1 and
e.sub.2 is constant, the time duration t.sub.1 of the
positive-going portions of the integrator output is greater than
the time duration t.sub.2 of the negative-going portions of the
wave. This is in contrast to the waveform (b) shown in FIG. 5, in
which t.sub.1 is equal to t.sub.2.
The waveform (c) in FIG. 6 shows the result of this change on the
output of the modulator. The positive pulses have the time duration
t.sub.1, and the negative pulses have the duration t.sub.2. Thus,
the widths of the pulses have been modulated by the signal applied
to terminal Z. In addition, as will be more readily apparent from
the equations set forth below, the frequency of the waveform (b)
also has been changed by the application of the input signal. Thus,
the modulator 84 is both a pulse-width modulator and a frequency
modulator.
The following equations describe the operation of the
modulator:
in which: t.sub.1, t.sub.2 and e are defined above and in FIGS. 5
and 6.
C is the integrating capacitance, the capacitance of capacitor
52.
I.sub.z is the current input to terminal Z.
I.sub.x is the current input to terminal X.
R.sub.26 and R.sub.28 are the resistances to resistors 26 and 28,
respectively.
f is the frequency of the output of the modulator; i.e., the
frequency of the waveform (b) in FIGS. 5 and 6.
The modulator 84 also can be considered to be a free-running
oscillator whose output frequency f varies inversely with the
magnitude of the Z signal, and directly with the X signal. Equation
(8) shows this variation.
The foregoing equations also demonstrate the effect of the input
signal on terminal X in the width modulation of the output signal.
Thus, both the X and Z inputs are capable of modulating the pulse
width and frequency of the output.
Integrator 46 is a conventional high-precision integrator circuit.
It includes an operational differential amplifier 48 and a negative
feedback capacitor 52. The input signal is applied to the inverting
terminal, and the non-inverting terminal is grounded. The value of
the capacitance C of the capacitor 52 is set so that, in accordance
with equation (8) above, the frequency will be within the desired
range. For steady DC inputs, the frequency f is not particularly
critical. However, for AC or fluctuating DC inputs, the frequency f
should be high enough to ensure that a very small sample of the
input wave is taken during each cycle of modulator operation. For
precision measurements, f should be over 100 times the frequency of
the input wave, and preferably is at least 200 times the latter
frequency.
The comparator circuit 56 consists of another operational
differential amplifier 58 with a resistor network connected to the
non-inverting input lead 60 of the amplifier 58. The inverting
input lead receives the output from the integrator circuit 46. The
resistor network consists of resistors 62 and 64 and is connected
with the non-inverting input 60 and the output of the amplifier 58
in order to provide positive feedback.
In operation, the comparator produces a large (e.g., 10 volts) DC
output signal of one polarity when the signals applied to the input
terminals are different from one another in one direction, and a DC
signal of equal magnitude but the opposite polarity when the input
voltage difference is in the opposite direction. The ratio of the
resistance of resistor 64 to the resistance of resistor 62 sets the
value of the voltages or "break points" e.sub.1 and e.sub.2 (FIGS.
5 and 6) at which the signals on the input leads of the amplifier
58 equal one another. In essence, the circuit 56 can be thought of
as a level detector, since it produces a change in the output
signal whenever the input reaches a predetermined, fixed level. In
this context, the circuit 56 also can be thought of as a
level-sensitive flip-flop circuit.
In applying the output of the comparator 56 to the FETs 22 and 24,
an inverter 92 is connected between the leads 32 and 34. This is
because the FETs 22 and 24 are of the same channel type; that is,
both FETs are of either the N-channel or P-channel type. The
inverter 92 ensures that the gate of one FET always will receive a
signal of a polarity opposite to that received by the other gate.
If FETs of opposite channel type are used, that is, if one FET is
of the N-channel type and the other of the P-channel type, the
inverter can be omitted and replaced by an ordinary conductor.
PULSE-HEIGHT MODULATOR
The middle portion 86 of the circuit shown in FIG. 4 is a
pulse-height modulator. Actually, the modulator is nothing more
than a switching device 10 as is shown in FIGS. 1, 2 and 3, with an
input terminal Y corresponding to the terminal 30 to receive the
modulating input. Pulses from the comparator 56 are applied to the
input gate leads 32 and 34 in the manner described above. The curve
(d) in each of FIGS. 5 and 6 shows how the height of the pulses has
been increased by the signal Y. It is readily apparent from
equations (1) and (2) how this pulse-height modulation is
accomplished.
MULTIPLE-INPUT FREQUENCY MODULATOR
A different frequency modulator can be produced by simply
re-arranging the circuitry of the modulators 84 and 86 already
described. In this embodiment of the invention, the connection 66
coupling the resistor 64 to the output of amplifier 58 is removed.
Also, the output lead 70 of the pulse-height modulator 86 is
connected by means of a lead 90 (shown as a dashed line) to the
right end of resistor 64. Further, there is no input on the Z
terminal.
With this arrangement, the output of the pulse-height modulator 86
is used to vary the separation e of the breakpoints e.sub.1 and
e.sub.2 of the comparator circuit 56. This provides a further means
of changing the frequency f of the output signal at point (c) in
FIG. 4. Equation (8) above shows the effect of such a change of the
quantity e on the frequency f of the output wave. The following
equation describes the simple relationship between the inputs at
terminals X and Y in the circuit here being described:
In which I.sub.y is the current input at terminal Y, and I.sub.x is
defined above.
It is readily evident that this circuit has valuable utility as a
voltage-to-frequency convertor, and specifically as a dividing
circuit with an output in terms of frequency. This can be very
valuable in analog-to-digital conversion.
MULTIPLIER, WATTMETER AND DIVIDER CIRCUIT
The entire circuit shown in FIG. 4 forms a high-precision
multiplier, wattmeter and divider circuit. The circuit includes the
modulators 84 and 86 described above, together with a smoothing
circuit 88 which smooths the output from the pulse-height modulator
86 to produce a DC signal on the output lead 82 which is
proportional to the product or division of two input signals.
The smoothing circuit 88 includes an input resistor 72, a
differential operational amplifier 78, and a negative feed-back
network consisting of parallel-connected resistor 74 and capacitor
76. The input signal is received on the inverting terminal 80. In
essence, the smoothing circuit 88 in a low-pass filter.
MULTIPLICATION, POWER MEASUREMENT AND DIVIDING METHODS
The mode of operation of the circuit shown in FIG. 4 depends upon
the method by means of which it is used. The general equation of
the circuit is set forth below:
In which I.sub.OUT is the output current from terminal 82, and the
other quantities have been defined above.
The circuit can be used as a multiplier by holding the signal input
at terminal X constant, and allowing the signals applied to
terminals Y and Z to be the input variables. When this is done, it
can be seen from equation (10) that the output is simply the
product of the inputs at terminals Z and Y.
When the device is used as a wattmeter, it is used as a multiplier;
one of the input signals at Y or Z is made equal to the load
current of the power being measured, and the other input is made
equal to the load voltage. The output then is the product of the
load current and voltage; i.e., the load power.
In use as a dividing circuit, either of the output signals on
terminals Z and Y is held constant while the signals on terminals X
and the other one of the terminals Z and Y is allowed to vary. It
can be seen from equation (10) above that the output is
proportional to the result of dividing either the Z or the Y signal
by the X signal.
The circuits described above have a number of advantages,
particularly in precision instrumentation. It is believed that the
invention provides a multiplier, divider and wattmeter which, in
practical use, is significantly more accurate than devices
previously available. Furthermore, this greater accuracy is
obtained with a substantial reduction in the number of components,
and particularly in the number of expensive precision operational
amplifiers required in previous circuits. Also, the device can be
fast-operating (depending upon the cost one is willing to pay for
the operational amplifiers), and is very reliable in operation.
The invention is particularly useful and unexpectedly accurate when
operating with time-varying input signals whose waveform is
relatively steep at low voltages and relatively flat at higher
voltages. Typical of such a waveform is a sine wave, which has the
greatest slope at zero volts and the lowest slope at its peaks. As
noted above, typically, the frequency f at which the circuit
operates is at least 100, and preferably 200 times the frequency of
the Z input signal. The reason for the desirability of such
frequent sampling is that it ensures that each portion of the input
wave most closely approximates a DC signal. Since accuracy is
reduced if this approximation is not carefully maintained, it is
desired to have a relatively high operating frequency in areas in
which the waveform has a steep slope and tends to degrade the
approximation, whereas, a lower operating frequency will be
sufficient where the waveform is relatively level and is close to
being DC anyway. In the present invention, the frequency f of the
operation of the circuit automatically is highest with the lowest
input signals. In fact, when the input signal Z is zero, the
highest frequency f occurs, and when the highest input signal
occurs (Z must be lower than X), the lowest frequency results.
Thus, it is an unexpected advantage of the circuit that is
automatically operates with such waveforms in the most precise
manner.
Following is a list of components which have been used in a circuit
as shown in FIG. 4 which has been built and successfully operated.
It should be noted that the components for the switching circuit 10
are included in this list.
______________________________________ Component Operational
Amplifiers 12, 48, 53 and 78 and Inverter 92 The Model 44
Operational Amplifier sold by Analog Devices Corp.; a fast-setting
operational differential amplifier. Switching time: 100
nanoseconds. Amplitude precision: 2 parts per million. FETs 22 and
24 3N153 (N-channel depletion type) Resistors 26 and 28 2,000 ohms,
(matched precisely) Resistor 42 1,000 ohms Resistors 62, 64 and 72
10,000 ohms Resistor 44 2,000 ohms Resistor 74 5,000 ohms Capacitor
52 0.02 microfarads (precise for voltage/frequency converter)
Capacitor 76 2 microfarads
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The above description of the invention is intended to be
illustrative and not limiting. Various changes or modifications in
the embodiments described may occur to those skilled in the art and
these can be made without departing from the spirit or scope of the
invention.
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