Reference voltage source

A reference voltage source comprises a bridge circuit having two avalanche diodes in a first pair of opposite arms and two resistors in a second pair of opposite arms, as well as two operational amplifiers, An AC-DC voltage converter has its input connected to an output terminal of the source. A voltage comparator have inputs connected to the output of the AC-DC voltage converter and the output of a DC voltage setting circuit. The output of the comparator is connected to the non-inverting input of one of the operational amplifiers.

FIELD OF THE INVENTION 
The present invention relates to voltage sources and, more specifically, to 
reference voltage sources. The source of this invention is applicable to 
automatic measuring systems where it can be used as a time-stabilized 
calibration signal source. 
BACKGROUND OF THE INVENTION 
There is known a reference voltage source comprising two closed control 
loops, each including a compensation DC voltage stabilizer of opposite 
polarities, which are alternately connected to the output of the source 
(cf. U.S. Pat. No. 3,458,723). 
The source of the aforementioned patent features a low output resistance. 
On the other hand, it requires the use of time-stable resistors in the 
divider circuits to ensure a high time stability of the source's output 
voltage, keeping in mind that the error of the resistor dividers is 
proportional to the error of the source as a whole. The design of this 
square-wave voltage is such that it is hard to control the constant 
component of the output voltage or use electric signals to control the 
output voltage amplitude within a broad range. 
Another known AC voltage source includes an amplitude-controlled sinusoidal 
voltage generator, a reference square-wave AC voltage source, a 
thermoconverter to which are alternately applied the output voltage of the 
generator and the voltage of the reference voltage source, and a detector 
intended to detect the difference signal at the output of the 
thermoconverter, which controls the amplitude of the sinusoidal voltage 
generator (cf. U.S. Pat. No. 3,484,705 or F. L. Hanson, "High-Accuracy AC 
Voltage Calibration", Hewlett Packard Journal, vol. 19, No. 10, June 
1968). 
The aforementioned design accounts for a high accuracy and good stability 
of the output sinusoidal voltage, due to the stable voltage supplied by 
the reference voltage source. On the other hand, it is impossible to 
provide a square-wave voltage at the output of the source, although that 
may be necessary for certain applications. Another disadvantage of this 
source that it limits the short-time stability of the generator's output 
voltage because the noise at the input of the detector is considerably 
amplified due to the use of the deep negative feedback in the system. 
Another known reference voltage source comprises a bridge circuit which 
contains avalanche diodes in a first pair of opposite arms and resistors 
in a second pair of opposite arms, as well as two operational amplifiers 
alternately connected to the source's output terminal. 
The inverting input of the first operational amplifier is connected to the 
anode and its output is connected to the cathode of one of the avalanche 
diodes. The inverting input of the second operational amplifier is 
connected to the cathode and its output is connected to the anode of the 
second avalanche diode of the bridge circuit (cf. USSR Inventor's 
Certificate No. 445,037, Cl. G05 F, 1/56). 
The latter reference voltage source is disadvantageous in that it does not 
permit simultaneous control of the constant component of the output 
voltage and the amplitude of the square-wave AC voltage. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a reference voltage 
source which permits simultaneous control of the constant component and 
amplitude of the square-wave AC output voltage. 
The invention essentially consists of a reference voltage source built 
around a bridge circuit containing two avalanche diodes and two resistors 
in its opposite arms. The reference voltage source further includes two 
operational amplifiers, the inverting input of the first of which is 
connected to the anode and the output of which is connected to the cathode 
of one of the avalanche diodes. The inverting input of the second 
operational amplifier is connected to the cathode and its output is 
connected to the anode of the second avalanche diode of the bridge 
circuit. In accordance with the invention, the reference voltage source 
includes an AC-DC voltage converter having an input connected to the 
output terminal of the source, a DC voltage setting circuit, and a voltage 
comparator having a first input connected to the DC voltage setting 
circuit and a second input connected to an output of the voltage 
converter. The output of the comparator is connected to the non-inverting 
input of one of the operational amplifiers. 
According to the invention, the reference voltage source may include an 
integrating circuit having its first input alternately connected to the 
outputs of the operational amplifiers. A control voltage is applied to its 
second input of the integrating circuit and the output of said integrating 
circuit is connected to the non-inverting input of the other operational 
amplifier. 
The source includes an amplifier alternately connected to the outputs of 
the operational amplifiers and provided with an integrating feedback loop. 
The output of the amplifier serves as the output of the source. 
The comparator preferably includes an operational amplifier having two 
resistors placed in parallel with its inverting input. The resistors serve 
as the input resistors of the comparator. The output of the operational 
amplifier is coupled to its inverting input via a capacitor. 
The integrating circuit and the integrating feedback loop are preferably 
built around operational amplifiers. 
According to the invention, the reference voltage source includes a circuit 
with a time-varying transfer voltage ratio. The input of the circuit with 
a time-varying transfer voltage ratio is alternately connected to the 
outputs of the operational amplifiers of the bridge circuit, whereas the 
output of this circuit serves as the output of the source. 
The circuit with a time-varying transfer voltage ratio is preferably built 
around a summing operational amplifier with a resistor and at least two 
switching circuits having in parallel with its inverting input, each of 
the switching circuits having a controlled switch and a resistor connected 
in series. 
The point of connection between the controlled switch and resistor of each 
of the aforedescribed circuits may be connected via a respective switch to 
the non-inverting input of the summing operational amplifier. 
A low pass filter is preferably connected to the output of the circuit with 
a time-varying transfer voltage ratio.

DETAILED DESCRIPTION OF THE INVENTION 
The reference voltage source of the invention is based on a bridge circuit 
having avalanche diodes 1 and 2 (FIG. 1) in a first pair of opposite arms 
and resistors 3 and 4 in a second pair of opposite arms. The source 
includes two operational amplifiers 5 and 6. The output of the first 
operational amplifier 5 is connected to the cathode of the avalanche diode 
1 and the output of the second operational amplifier 6 is connected to the 
anode of the avalanche diode 2. The inverting input 7 of the amplifier 5 
is connected to the anode of the avalanche diode 1. The inverting input 8 
of the amplifier 6 is connected to the cathode of the avalanche diode 2. 
The outputs of the amplifiers 5 and 6 are connected to inputs 9 and 10 of 
first and second switches 11 and 12, respectively. Control inputs 13 and 
14 of the switches 11 and 12, respectively, are connected to outputs of a 
clock pulse generator 15. The outputs of the switches 11 and 12 are 
combined and connected to an output terminal 16 of the source. 
According to the invention, the reference voltage source further includes 
an AC-DC voltage converter or full-wave rectifier 17 having an input 
connected to the output terminal 16, and a DC voltage setting circuit 18. 
The outputs of the converter 17 and of the circuit 18 are connected to the 
inputs of a voltage comparator 19, whose output is connected to a 
non-inverting input 20 of the amplifier 5. A non-inverting input 21 of the 
operational amplifier 6 is connected to an input terminal 22, whereto 
control voltage is applied. 
In order to provide for a specific bias of the output voltage with respect 
to the DC voltage, the source of the invention includes an integrating 
circuit 23 (FIG. 2) having an input 24 combined with the input terminal 22 
of the source. An input 25 of the integrating circuit 23 is connected to 
the combined outputs of the switches 11 and 12. The output of the 
integrating circuit 23 is connected to the non-inverting input 21 of the 
amplifier 6. 
The integrating circuit 23 is preferably built around an operational 
amplifier 26 with an input resistor 27 and a capacitor 28 connected in its 
feedback loop. The non-inverting input of this amplifier is the input 24 
of the circuit 23. A switch 29 is connected to the non-inverting input of 
the amplifier 26. The control input 30 of the switch 29 is connected to 
the generator 15. 
In order to provide for high voltages with a low-resistance output, it is 
necessary that an amplifier 31 (FIG. 3) be connected at the output of the 
source and provided with an integrating feedback loop 32. An input 33 of 
the amplifier 31 is connected to the combined outputs of the switches 11 
and 12. The amplifier 31 is built around an operational amplifier 34 with 
resistors 35 and 36 connected to its input and in the feedback loop, 
respectively. 
Also connected in the integrating feedback loop 32 is an integrating 
operational amplifier 37 with a resistor 38 connected to its inverting 
input and a capacitor 39 connected in the feedback loop. A switch 40 is 
connected to the inverting input and has a control input 41 connected to 
the output of the clock pulse generator 15. 
FIG. 4 presents an alternative embodiment of the converter 17 and the 
comparator 19. The AC-DC voltage converter 17 includes, as shown in FIG. 
4, a high-voltage resistor 42 and two resistors 43 and 44. The resistor 42 
is connected to a common point 45 of connection of the resistors 43 and 
44. An operational amplifier 46 is connected to a common point in the 
connection between the resistor 43 and a capacitor 47. 
The capacitor 47 is connected to the input of the amplifier 46. A capacitor 
48 is connected to the output of the amplifier 46. Diodes 49 and 50 are 
connected in the feedback loop and are also connected to the resistors 43 
and 44. 
The comparator 19 is built around an integrating operational amplifier 51 
with a capacitor 52 connected in its feedback loop and with input 
resistors 53 and 54. A switch 55 is connected to the input of the 
amplifier 51. The control input 56 of the switch 55 is connected to the 
output of the clock pulse generator 15. The resistor 53 is coupled 
directly to the inverting input of the integrating operational amplifier 
51 via the switch 55. The resistor 54 and the capacitor 47 are coupled to 
the inverting input of the integrating operational amplifier 51 via the 
switch 55. 
Non-inverting inputs 57, 58 and 59 of the amplifiers 37, 46 and 51, 
respectively, are grounded. 
In the alternative embodiment of FIG. 5, the reference voltage source of 
the invention includes a circuit 60 with a time-varying transfer voltage 
ratio. The input 61 of the circuit 60 is connected to the combined outputs 
of the switches 11 and 12 an its output serves as the output of the 
source. 
The circuit 60 is built around a summing operational amplifier 62 (FIG. 6) 
with a resistor 63 connected in its feedback loop. An inverting input 64 
of the amplifier 62 is connected to a resistor 65 and "n" parallel 
switching circuits, each comprising a resistor and a switch. FIG. 6 is a 
simplified circuit comprising three circuits containing resistors 66, 67 
and 68 and switches 69, 70 and 71, respectively. Respective switches 75, 
76 and 77 are connected to points 72, 73 and 74 of connection of the 
resistors 66, 67 and 68 and switches 69, 70 and 71, respectively. The 
switches 75, 76 and 77 are all connected to a non-inverting input 78 of 
the amplifier 62. 
Control inputs 79, 80 and 81 of the switches 69, 70 and 71, respectively, 
are connected to the outputs of the clock pulse generator 15. 
A low pass filter 85 is connected to the output of the circuit 60. 
The reference voltage source of the invention (FIGS. 1 to 4) operates as 
follows. 
The operational amplifiers 5 and 6 are provided with a bipolar supply so 
that their output voltages are variable within broad limits. 
When operating under steady-state conditions, the voltage produced at the 
output of the operational amplifier 5 is positively biased with respect to 
the voltage across its non-inverting input 20 by a value corresponding to 
the voltage drop across the avalanche diode. The voltage produced at the 
output of the operational amplifier 6 is negatively biased with respect to 
the voltage at its non-inverting input 21 by a value corresponding to the 
voltage drop across the avalanche diode 2. The output voltages of the 
amplifiers 5 and 6 are applied by the switches 11 and 12 to the output 
terminal 16, either directly or via the amplifier 31. As a result, a 
square-wave AC output voltage is produced at the output of the source. 
The AC to DC voltage converter 17 smoothes the output voltage of the source 
and converts it to a DC voltage by full-wave rectification. The voltage 
comparator 19 then compares the rectified voltage with the preset DC 
voltage, whereupon the rectified voltage is applied to the non-inverting 
input 20 of the amplifier 5. The phase is selected so that if the output 
voltage is in excess of the set voltage, a negative signal is produced at 
the output of the comparator 19, whereby the output voltage of the 
amplifier 5 is reduced until the signals at the inputs of the comparator 
19 are equalized. 
The comparator 19 operates as follows. A positive full-wave detected 
voltage is applied to its input, that is, to the inverting input of the 
amplifier 51, from the converter 17 via the resistor 54. A negative DC 
voltage is applied to its input from the voltage setting circuit 18 via 
the resistor 53. The difference between the currents flowing through the 
resistors 53 and 54 is integrated by the operational amplifier 51 and 
applied to the input 20 of the amplifier 5. If the output voltage of the 
source is in excess of the preset voltage across the circuit 18, the 
current flowing through the resistor 54 is in excess of the compensation 
current flowing through the resistor 53, which accounts for a negative 
change in the voltage across the output of the amplifier 51. As a result, 
the output voltage of the amplifier 5 and the voltage across the input and 
output of the amplifier 31 are reduced until the voltages at the inputs of 
the comparator 19 are equalized. This means that the currents flowing 
through the resistors 53 and 54 are of equal magnitudes, but opposite 
polarities. 
Apart from rough stabilization of the output voltages of the amplifiers 5 
and 6 due to the use of the bridge circuit, the reference voltage source 
circuitry of the invention provides for a finer stabilization of these 
voltages by eliminating minor deviations of the output voltage from preset 
values. The reference voltage source of the invention also features a high 
noise immunity and good time stability. 
The integrating circuit 23 serves to adjust the voltage at the 
non-inverting input 21 of the amplifier 6 so that the constant component 
of the signal at the output of the switches 11 and 12 is equal to the 
control voltage across the input terminal 22. 
If the constant component of the signal at the output of the switches 11 
and 12, this is at the input 25 of the integrating circuit 23, differs 
from the control voltage at the input terminal 22, current is passed 
through the resistor 27 to charge the capacitor 28 until the output 
voltage of the integrating circuit 23 and voltages at the non-inverting 
input 21 of the amplifier 6, the output of the amplifier 6 and the input 
25 are changed to equalize the constant component of the signal at the 
input 25 and the control voltage at the input terminal 22. 
The source of FIGS. 5 and 6 operates as follows. The output voltages of the 
amplifiers 5 and 6 are alternately applied via the switches 11 and 12 to 
the input of the circuit 60 with a time-varying transfer voltage ratio, 
and then, via the low pass filter 85, to the output of the source. The 
sinusoidal output voltage thus produced is converted to a smoothed DC 
voltage by the converter 17, whereupon the comparator 19 compares it with 
the DC voltage. The difference of these voltages is applied to the 
non-inverting input 20 of the amplifier 5, and the phase is selected so 
that if the output voltage is in excess of a desired value, the output 
signal of the comparator 19 is negative, whereby the output voltage of the 
amplifier 5 is reduced. According to the preferred embodiment of FIG. 6, 
the circuit 60 with a time-varying transfer voltage ratio comprises the 
summing operational amplifier 62, whose inverting input 64 is connected to 
the outputs of the switches 11 and 12. 
As shown in FIG. 7a, voltage is continuously applied to the input 64 of the 
amplifier 62 via the resistor 65, whereas voltage is applied via the 
resistor 66 and the switch 69 to the input 64 only during periods from 
t.sub.1 to t.sub.7 and from t.sub.9 to 1.sub.15 (FIG. 7b). Accordingly, 
voltage is applied via the resistor 67 and the switch 70 during periods 
from t.sub.2 to t.sub.6 and from t.sub.10 to t.sub.14 (FIG. 7c). Voltage 
is applied via the resistor 68 and the switch 71 during periods from 
t.sub.3 to t.sub.5 and from t.sub.11 to t.sub.13 (FIG. 7d). 
Current pulses passing through the resistors 65, 66, 67 and 68, and 
represented in FIGS. 7a, 7b, 7c and 7d, respectively, are added together 
so that a voltage is produced at the output of the operational amplifier 
which approximates a sinusoid (FIG. 7e). 
The approximation can be made highly accurate by appropriately selecting 
the resistances of the resistors 65, 66, 67 and 68 and the switches 69, 70 
and 71, as well as the number of parallel circuits. 
The signal thus produced is applied to the low pass filter 85 which 
suppresses the higher harmonic components. For example, the filter 85 may 
be a three-pole filter, in which case the overall distortion of the 
sinusoidal signal at its output is less than 0.005 percent. 
When the switches 69, 70 and 71 are in the off position, the switches 75, 
76 and 77 serve to direct the current flowing through the resistors 66, 67 
and 68, respectively, to a common bus. As a result, there are no voltage 
surges at the output of the switches 11 and 12, and the output signal of 
the operational amplifier is free from voltage peaks at the times that the 
switches 69, 70 and 71 are switched. The remainder of the source of FIGS. 
5 and 6 operates as hereinbefore described. 
The comparator 19, the integrating circuit 23 and the integrating feedback 
loop 32 are provided with the switches 55, 29 and 40 in order to maintain 
the original DC voltage across the non-inverting inputs of the operational 
amplifiers 5, 6 and 34 when DC voltage is applied to the output terminal 
16. The switches 55, 29 and 40 are brought to the "on" position, and the 
DC voltages at the outputs of the amplifiers 51, 26 and 37 remain 
unchanged due to the absence of current in the capacitors 52, 28 and 39, 
respectively. This makes it convenient to check the circuits and units of 
the reference voltage source, since a precision digital voltmeter may be 
used to determine the values of output voltages of different polarities 
with specific resistance ratios of the resistors 42, 43, 44, 53 and 54. If 
necessary, the resistance ratios of these resistors may be adjusted while 
the output voltages are within prescribed limits. 
The accuracy of the reference voltage source according to the invention is 
never below 0.01 percent and is only dependent upon the accuracy with 
which the resistance ratios of the resistors 53, 54 and 42, 43 and 44 are 
selected. 
The source of FIGS. 5 and 6 has the following advantages. 
The circuit 60 with a time-varying transfer voltage ratio transforms a 
square signal into a step signal which approximates a sinusoid. The low 
pass filter 85 suppresses the higher harmonic components, so that a 
sinusoidal signal is produced at the output of the source. The step signal 
approximates a sinusoid in such a way that changes occur at equal 
intervals 4 m times during a period, so that the step signal is 
symmetrical with respect to the centers of half periods and crosses zero 
at the same points as the sinusoid being approximated. As a result, the 
output signal is free from even higher harmonic components. By selecting a 
specific ratio between additional step-like voltage variations, the signal 
is rid of "m" first odd harmonic components, as well as of odd higher 
harmonic components whose series number is "4mk" more than the series 
number of the first harmonic components (k=1, 2, 3 . . . ). For example, 
with three additional switches, the first higher harmonic component of the 
signal is the fifteenth, and its amplitude is 1/15 of the fumdamental 
harmonic. With the use of a low pass filter of the third order, the total 
distortion at the output is less than 0.005 percent. Thus the reference 
voltage source of the invention permits the provision of an essentially 
sinusoidal signal at its output.