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ABSOLUTE VALUE_FUNCTIION GENERATOR
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WALTER S. DUSPIVA
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April 28, 71970
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United States Patent 0
11
3,509,369
CC
Patented Apr. 28, 1970
1
2
3,509,369
ing more particular description of preferred embodiments
of the invention, as illustrated in the accompanying draw
ABSOLUTE VALUE FUNCTION GENERATOR
William G. Crouse, Raleigh, N.C., and Walter S. Duspiva,
Eurlicott, N.Y., assignors to International Business Ma
chines Corporation, Armonk, N.Y., a corporation of
lngs.
New York
Filed July 12, 1967, Ser. No. 652,796
Int. Cl. G07g 7/12; H03k 5/153
U.S. Cl. 307—230
7 Claims
10
ABSTRACT OF THE DISCLOSURE
An electronic switch responds to the polarity of input
signals to an operational ampli?er of the differential
ampli?er type and connects the signals to one or both
of the differential ampli?er input terminals. The ampli?er
gain is alternatively plus or minus one to provide a low
cost absolute value function generator accurate at low
voltage levels in the order of plus and minus ?ve-tenths
volt.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. la-ld inclusive illustrate various operational
ampli?er characteristics to aid in a better understanding
of the improved function generator;
FIG. 2a illustrates the improved function generator
partially in schematic form and partially in diagram
matic form;
FIGS. 2b and 2c illustrate the manner in which the
embodiment of FIG. 2a operates in either of its two
states; and
FIG. 3 is a schematic diagram illustrating one preferred
form of the improved function generator.
DESCRIPTION OF THE PREFERRED
EMBODIMENTS
In the analog circuit ?eld, it is often desirable to gen
erate an output voltage which is the absolute value of
BACKGROUND OF THE INVENTION
The invention pertains to the ?eld of analog circuits,
some positive or negative input voltage. Mathematically,
this may be written as:
and speci?cally to a circuit which reproduces at its out 25 and
put signals of the same amplitude as the input signals but
of one polarity irrespective of the polarity of the input
Eout:+Eln for osElnsEmax
Eout=_Ein for _Emax$E1ns0
The drawings illustrate an improved absolute value gen
erator con?guration, and the evolution of the con?gura
value function generators. Known circuits of this type
usually utilize a pair of operational ampli?ers and diode 30 tion.
The basic circuits around which the improved circuit
current steering circuits. Known circuits are characterized
is built are a differential input operational ampli?er 1
by relatively high cost, low performance at low voltage
and a voltage threshold-switch device 2 (FIG. 3). The
levels and relatively poor response to rapid changes in
device 2 switches impedance levels for the differential
the input signal level. Examples of typical prior art cir
signal. Such circuits are frequently referred to as absolute
cuits are as follows:
(1) Analog Methods by Karplus and Soroka, published
in 1959 by McGraw-Hill, speci?cally pp. 75 and 76; and
(2) Computer Handbook by Huskey and Korn, pub
lished in 1962 by McGraw-Hill, speci?cally pp. 3-73.
It is an object of the present invention to provide a 40
ampli?er as the polarity of the input voltage changes,
thus setting the gain of the entire system to plus one for
positive inputs and minus one for negative inputs. Before
the circuit of FIG. 3 is explained in detail, a brief review
of two common feedback circuits is of value.
FIG. 1a shows the most commonly used scheme for
lower cost, improved performance absolute value func
incorporating a differential input operational ampli?er
tion generator.
as a feedback device. The gain of this circuit is approxi
mately equal to the negative ratio of the feedback resistor
to the input resistor, i.e.
SUMMARY OF THE INVENTION
An improved absolute value function generator is pro 45
vided by electronically switching the input signal to one
or both of a pair of input terminals to an operational
ampli?er of the differential ampli?er type, depending
A less common con?guration is shown in FIG. 1b. In
upon the polarity of the input signal. When the signal is
this embodiment, the polarity of the gain is positive, i.e.
applied to one of the inuts, the gain of the operational 50
ampli?er is plus one. When the input signal is applied to
both inputs, the gain of the operational ampli?er is minus
It would seem possible then that some combination
one. By providing only one operational ampli?er, the
of both con?gurations, together with a threshold switch
overall cost of the circuit in relation to accuracy is con
siderably reduced.
55 ing device, might result in a circuit that has a gain of
either plus or minus one, depending upon the polarity of
The input switch includes a differential ampli?er to
the input signal.
sense input signal polarity with additional amplifying
I If the con?gurations of FIGS. 1a and 1b are combined
stages for high gain. The last stage performs the switch
as illustrated in FIG. 10, the resultant circuit is one that
ing function. This permits accurate operation at voltage
includes two inputs. The equation for the output voltage
levels much lower than a volt. The differential portion
of the switch includes a pair of transistors preferably hav
ing matched base-emitter voltage-current characteristics
for good temperature stability. In addition, the impedance
presented by the ?nal ampli?er stage is very low com
in FIG. 10 is as follows:
Rf
Rf
To make
Eout?Einl
the circuit_'Fi)+Einz
even more specialized, allow the
pared to that provided by oppositely poled diodes of prior 65
second input, Elm, to be proportional to the ?rst input,
art devices. This provides signi?cantly better accuracy
Em, as shown in FIG. 1d, giving rise to the circuit
equation:
The improved generator exhibits better performance
in an environment of rapid changes in the input signal
level.
70
The foregoing and other objects, features and advan
It can be seen from an analysis of the equation, that
tages of the invention will be apparent from the follow
the gain of the circuit in FIG. 1d may be either positive
at extremely low voltages close to zero volts.
3,509,369
3
or negative, depending upon the value of the four
resistors external to the operational ampli?er, i.e. R1,
R2, Ri and Rf. If these resistance values were to be
switched properly by a zero input threshold device, the
gain of the system could be set to either plus or minus
one, thus effecting the desired absolute value function.
One method of properly controlling the impedances is
to break the input resistor Ri into two series input
4
The junction between the resistors Rx and Ry is con
nected to ground potential by way of a common emitter
transistor ampli?er 10‘ and a low-valued resistor 11. The
emitter electrode of the ampli?er 10 is connected to a neg
ative supply terminal 12 by way of a resistor 13 and an
adjustable potentiometer 14.
The voltage divider comprising resistors 11 and 13 and
potentiometer 14 ?xes the emitter potential of transistor
10 at a low neagtive voltage equal in value to the Vce
resistors Rx and Ry, as shown in FIG. 2a. The threshold
device controls a switch to connect the junction of the 10 drop across the transistor 10 when it is operated at satu
resistors Rx and Ry to ground potential. The switch is
ration. This will set the collector voltage of the transistor
closed for input voltages which are zero or positive and
10 at ground potential when it is saturated.
is open for input voltages which are negative. Then,
The input. terminal 3 is connected to one input of a
differential ampli?er switch 15 comprising a pair of tran
sistors 16 and 17. More speci?cally, the input terminal 3
is connected to the base electrode of the transistor ampli
?er 16 by way of a resistor 18. The base electrode of the
depending upon the position of the switch, the ampli?er
will be utilized in one of the two con?gurations of FIG.
2b or FIG. 2c. FIG. 2b’ illustrates the ampli?er connec
tions of the embodiment of FIG. 2a when the switch is
closed incident to detection of a positive polarity at the
transistor ampli?er 16 is clamped to ground potential by
way of oppositely poled diodes 19 and 20*. The function
input. FIG. 20 shows the ampli?er connections when the
switch is open due to a negative potential at the input.
20 of these diodes is to limit the amplitude of signals applied
to the base electrode of the transistor by way of re
The component values which will achieve the desired
sistor 18.
results in FIG. 2a can be determined by use of the for
The base electrode of the transistor ampli?er 17 is
mulas set forth below. It will be recalled that a gain of
connected to ground potential by way of a resistor 21 and
plus one is desired for the con?guration of FIG. 2b’ and
a gain of minus one for FIG. 2c. Arbitrarily assign the 25 to the wiper of a potentiometer 22 by way of a resistor
23. The potentiometer 22 is connected to positive and
following values:
negative supply terminals 24 and 25 and the wiper is
Ohms
adjusted so that it is close to ground potential.
R1 ___________________________________ __
10,000
The emitters of the transistors 16 and 17 are connected
R2 ___________________________________ __
1,000
Rf ____________________________________ __ 100,000
The values of Rx and Ry must now be determined, utiliz
ing the following equations:
/
30 to a negative supply terminal 26 by way of a common
resistor 27. A resistor 28 connects the collector electrode
of the transistor 16 to the collector electrode of the tran
sistor 17. The collector electrode of the transistor 17 is
connected to ground potential by way of a resistor 30
FOR POSITIVE INPUTS
35 and to a positive supply terminal 31 by way of a re
sistor 32.
The collector electrode of the transistor 16 is con
nected directly to the base electrode of a common emitter
transistor ampli?er 35. The collector electrode of the
ampli?er 35 is connected to a positive supply terminal
36 by way of a resistor 37. The collector electrode of
the ampli?er 35 is also connected to the base electrode of
the transistor 10 by way of a resistor 38. The base elec
trode of the transistor 10 is connected to a negative sup
(2)
45 ply terminal 39 by way of a bias resistor 40.
The potentiometer 22 is set so that the transistors 16
By substituting the values of R1, R2 and R)‘ in Equation
and 17 conduct equally when the input signal Em is at
1 above, we compute the value of Ry to be ten thousand
ground potential. The gain of the system will therefore
ohms.
Substituting the values of R1, R2, R7‘ and Ry into
be unde?ned when the input is at ground potential; how
Equation 2 above, we ?nd the value of Ex to equal 50 ever, no problem arises since Em equals zero and the out
put Eon, will therefore equal zero. When Em goes slightly
seventy-three thousand, three hundred ohms.
positive, the transistor 16 becomes saturated and the tran
It will be appreciated that this is not the only set of
sistor 17 turns 011‘. When Em goes slightly negative, the
component values which will produce the desired results.
transistor 17 saturates and transistor 16 turns off.
However, it has been found that these particular values
When the transistor ampli?er 35 is in its off state, the
give results which are quite good when practical factors 55
positive potential from the supply terminal 36 will bias
(such as the linearity of the transistors used in the
the transistor switch 10 on to saturation, thereby substan
threshold circuit, etc.) are taken into consideration.
tially connecting ground potential to the junction between
In a practical circuit, the switch illustrated in FIG. 2a
the resistors Rx and Ry. This occurs only when the tran
is replaced by some type of transistor switching arrange
ment which detects the polarity and level of the input sig 60 sistor 16 is in its highly conductive condition as a result
of the input signal Em being at a positive potential with
nal and clamps the junction of Rx and Ry to ground
potential when the polarity of the input signal is positive.
The transistor switching circuit 2 of FIG. 3 serves this
purpose very well.
.
The preferred embodiment of FIG. 3 will now be
described in detail. In FIG. 3, the ampli?er 1 has one
respect to ground.
When the input signal Em is at some negative poten
tial, the transistor 16 is in its low conductivity state,
whereby the transistor 35 is turned on to apply ground
potential to the base circuit of the transistor switch 10.
With the transistor 35- in its conductive state, the swich 10
is turned off, presenting an open circuit to the junction
R2. The same input is connected to the input terminal 3
between the resistors Rx and Ry.
by way of the resistor R1. The other input of the ampli
The use of extremly well-matched transistors 16 and 17
?er 1 is connected to the terminal 3 by way of the resistors 70
will obviate the need for the adjustment potentiometer 22.
Ry and Rx. The resistor Rx is shown in the form of a
The use of a transistor 10 having an unusually low
fixed resistor and an adjustable potentiometer for accurate
impedance at saturation will obviate the need for the
sizing. The feedback resistor R)‘ is also in the form of a
adjustment potentiometer 14 and resistors 13 and 11.
?xed resistor and an adjustable potentiometer for accurate
dimensioning.
75 In addition, a ?xed value resistor can be used for Rx.
input connected to ground potential by way of the resistor
3,509,369
These resistors and potentiometers permit the use of a
lower performance, low-cost transistor switch without
the gain of the ampli?er to plus one when the input‘
signals are of a selected polarity.
2. An absolute value function generator comprising
an operational ampli?er of the differential ampli?er
type having a pair of input terminals and an output
adversely alfecting accuracy.
Chart 1 below sets forth the test results with respect
to two circuits constructed in accordance with the details
of FIG. 3, utilizing the following component values. The
terminal,
circuits for which the test results are set forth in Chart 1,
utilized the same resistive components and operational
ampli?er, but different transistors were utilized to verify
input resistor means adapted to receive bipolar input
signals and feedback resistor means normally bias
ing the ampli?er for operation with a gain of minus
satisfactory operation of the circuit independent of the
one,
transistors utilized. The component values for one suit
able implementation of the embodiment of FIG. 3 are as
control means modifying the input resistor means to
follows:
the input signals are of a selected polarity,
a source of ground potential and a single input signal
change the gain of the ampli?er to plus one when
Ohms
R1
___
R2
___
_
Rx __..
Ry
_____
Rf ..___
__._
terminal,
9,988
..__.._
9,971
said feedback resistor means connected between the
72,380
ampli?er output terminal and one of the ampli?er
input terminals, and
9,939
___ 99,590
11
13
3.3 20
10,000
14
and
____ 11,660
18
.
2,400
21
22
23
28
1,100
50,000 25
5,100
24,000
30
560
32 _
____
30
CHART N0. 1
Input (volts)
Circuit N o. 2 output
(volts)
(volts)
. 010
. 100
. 010
. 100
1. 000
3. 000
5. 000
8. 001
10. 002
1. 000
3. 000
4. 998
7. 998
9. 997
. 019
. 108
. 019
. 108
l. 008
3. 006
5. 003
8. 000
9. 998
1. 008
3. 007
5. 006
8. 004
10. 001
third and fourth series resistors connecting the signal
input terminal to ground potential,
the junction between the third and fourth resistors
being connected to the other ampli?er input terminal.
3. The function generator of claim 2 wherein the
control means includes
560
Circuit No. 1 output
said input resistor means including
?rst and second series resistors connecting the signal
input terminal to said one ampli?er input terminal,
a switch for selectively connecting the junction between
the ?rst and second resistors to ground potential
to change the gain of the ampli?er from minus one
to plus one.
4. The function generator of claim 3 wherein the con
trol means further includes
?rst transistor ampli?er means connected to the input
35
terminal and responsive to the polarity of the input
signals to render the switch alternatively effective
or ineffective.
5. The function generator of claim 4 wherein the tran
40 sistor ampli?er means render the switch effective to
connect the junction between the ?rst and second resistors
to ground potential when the polarity of the input signals
is positive.
6. The function generator of claim 5 wherein the
While the invention has been particularly shown and 45 switch comprises
a common emitter transistor ampli?er having its col
described with reference to preferred embodiments there
lector electrode coupled to the junction between
of, it will be understood by those skilled in the art that
the ?rst and second transistors and having its emitter
the foregoing and other changes in form and details may
electrode coupled to ground potential.
be made therein without departing from the spirit and
50
7. The function generator of claim 5 wherein said ?rst
scope of the invention.
What we claim is:
transistor ampli?er means comprises a high gain differen
tial ampli?er.
1. An absolute value function generator comprising
an operational ampli?er of the differential ampli?er
References Cited
type having a pair of input terminals and an output
UNITED STATES PATENTS
terminal,
55
means each serially connected to a respective one
3,281,723
3,292,098
10/1966
12/1966
Mercer ________ __ 307—235 X
Bensing ___________ __ 330~—69
input terminal and adapted to receive bipolar input
signals and including negative feedback resistor
3,305,729
3,413,492
2/1967
11/1968
Stein ____________ __ 307—235
Schneider ________ __ 307-—235
an input circuit including at least two input resistor
means connected to one of the input terminals nor 60
mally biasing the ampli?er for operation with a gain
of minus one,
electronic switch means responsive to the input signals
for modifying the input resistor means to change
JOHN S. HEYMAN, Primary Examiner
US. Cl. X.R.
307-235, 236; 330—30, 69