Abstract:
A voltage reference circuit receives an input voltage through a first port and a time varying input signal through a second port. The voltage reference circuit includes a switching circuit that is responsive to the first and the second ports and that generates an AC signal from the input voltage. The voltage reference circuit further includes a voltage multiplier circuit, coupled to switching circuit that receives the AC signal and creates a DC signal with a selected voltage level. The voltage reference circuit further includes a voltage regulator, coupled to the voltage multiplier circuit that regulates the DC signal from the voltage multiplier circuit. An output voltage is provided through an output port.

Description:
TECHNICAL FIELD  
       [0001]     The following description relates generally to the field of electronic circuits, and more particularly to a precision, low drift, closed loop voltage reference.  
       BACKGROUND  
       [0002]     Voltage references are used pervasively throughout electronics applications. They are used to supply stable unvarying voltages to other systems and circuits. For circuits such as instrumentation and test equipment, data acquisition systems, portable devices, medical equipment, analog to digital converters, digital to analog converters, and sensors, precision voltage references constitute critical circuit elements.  
         [0003]     Common problems that voltage references encounter comprise shifts in the output voltage. For example, a voltage reference may shift due to changing thermal conditions. Furthermore, a voltage reference is also susceptible to lifetime shifts, known as long term voltage drift, that occur gradually as systems and circuits are used. These problems can be exacerbated if the reference voltage circuit is designed to consume a minimum of power supply current. Many reference voltage circuits are also susceptible to voltage changes due to radiation. Where a voltage reference circuit is to be used in a radiation rich environment, maintaining the stability of the reference voltage becomes increasingly problematic. One approach to ameliorate inaccuracy caused by radiation is to provide radiation shielding. However, this solution can be costly and uses valuable space in systems wherein such space might be limited.  
         [0004]     Voltage references also suffer from other common limitations. For example, many voltage references are normally only used with small stable loads. Other voltage references are incapable of providing a stable high voltage output. Few voltage references can provide a load above a few milliamps and still maintain a stable precision output voltage. This introduces problems where larger loads are desirable or necessary. It also restricts or prohibits the use of reference voltage circuits where a changing load is necessary.  
         [0005]     One application in which the mentioned problems are encountered is that of sensor equipment used where the atmosphere is thin or nonexistent, such as in space. Sensors require tight tolerances for their reference voltages to accurately detect the required phenomena. Therefore, even slight variations in the reference voltage may be unacceptable. A combination of the above design problems makes the use of reference voltages in high radiation environments problematic, particularly when used in sensor circuits. Therefore, there exists a need in the art for extremely stable, precise, high voltage references. This need is magnified in certain applications such as those used in space.  
       SUMMARY  
       [0006]     In one embodiment, the invention advantageously provides a voltage reference circuit. The voltage reference circuit includes a first port, adapted to receive an input voltage; a second port, adapted to receive a time varying input signal; and a switching circuit, responsive to the first and the second ports. The switching circuit generates an AC signal from the input voltage. The voltage reference circuit further includes a voltage multiplier circuit, coupled to switching circuit to receive the AC signal and to create a DC signal with a selected voltage level; a voltage regulator, coupled to the voltage multiplier circuit, that regulates the DC signal from the voltage multiplier circuit; and an output port that is adapted to provide an output voltage.  
         [0007]     In accordance with another aspect of the invention, it provides an electronic device attached to a voltage reference. The voltage reference includes a first port adapted to receive an input voltage; a second port adapted to receive a time varying input signal; and a switching circuit, responsive to the first and second ports. The switching circuit generates an AC signal from the input voltage. The voltage reference further includes a voltage multiplier circuit, coupled to switching circuit to receive the AC signal and to create a DC signal with a selected voltage level; a voltage regulator, coupled to the switching signal, that regulates the DC signal from the switching circuit; and an output port that is adapted to provide an output voltage.  
         [0008]     In accordance with another aspect of the invention, it provides a method for supplying a reference voltage. The method includes generating an alternating current signal from a direct current input voltage and a time varying input signal; generating a DC voltage signal from the alternating current signal that is greater in magnitude than the direct current input voltage; regulating the generated DC voltage; and delivering the regulated DC voltage.  
         [0009]     In accordance with still another aspect of the invention, it provides a voltage reference apparatus. The voltage reference apparatus includes a switching circuit that generates an AC signal from a DC input and a time varying input signal; a voltage multiplier circuit, coupled to the output of the switching circuit, that receives the AC signal and generates a DC signal with a selected voltage level; and a voltage regulator circuit, coupled to the voltage multiplier circuit, that regulates the DC signal from the voltage multiplier circuit.  
         [0010]     In accordance with yet another aspect of the invention, it provides a method for supplying a reference voltage. The method for includes introducing a direct current input voltage; filtering the direct current input voltage; introducing a time varying input signal; generating an alternating current signal from the filtered direct current input voltage and the time varying input signal; generating a DC voltage signal from the alternating current signal that is greater in magnitude than the direct current input voltage; regulating the generated DC voltage; filtering the regulated DC voltage; and delivering the regulated DC voltage. 
     
    
     DRAWINGS  
       [0011]      FIG. 1  is a block diagram of a voltage reference circuit in accordance with an embodiment of the present invention;  
         [0012]      FIG. 2  is a block diagram of an electronic device in accordance with an embodiment of the present invention;  
         [0013]      FIG. 3  is a circuit diagram of a switching circuit in accordance with an embodiment of the present invention;  
         [0014]      FIG. 4  is a circuit diagram of a voltage multiplier circuit in accordance with an embodiment of the present invention;  
         [0015]      FIG. 5  is a circuit diagram of a voltage regulator in accordance with an embodiment of the present invention;  
         [0016]      FIG. 6   a  is a circuit diagram of an input filter in accordance with an embodiment of the present invention;  
         [0017]      FIG. 6   b  is a circuit diagram of an output filter in accordance with an embodiment of the present invention; and  
         [0018]      FIG. 7  is a circuit diagram of a voltage reference circuit in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]     The primary embodiments of the invention will now be discussed in detail, examples of which are illustrated in the accompanying figures. Illustrated embodiments are presented by way of example and are not to be construed as limitations. All alternatives, modifications, and equivalents that fall within the scope and spirit of the invention are incorporated herein. For example, it is understood by a person of ordinary skill in the art that a transformer may be used in place of a cascade voltage multiplier to achieve the same increased voltage output.  
         [0020]     Embodiments of the present invention may be employed in combination with other circuit designs, such as those that require a stable high voltage reference. This description is presented with enough detail to provide an understanding of the present invention, and to enable one of ordinary skill in the art to build a precision, low drift, closed loop voltage reference. This detailed description should not be construed to encompass all necessary materials in circuit production or operation.  
         [0021]     Referring more particularly to the drawings,  FIG. 1  illustrates a block diagram of a voltage reference circuit  100  in accordance with an embodiment of the present invention. The voltage reference circuit  100  comprises an input voltage  102 , a time varying input signal  104 , an input filter  106  and output filter  108 , a switching circuit  111 , a voltage multiplier circuit  113 , a voltage regulator  115 , and a voltage output  117 . In one embodiment, the input voltage  102  comprises a standard constant voltage source. In one embodiment the voltage source provides a constant ±15 volts. Standard voltage sources have a five percent error, therefore in certain embodiments the voltage provided is 15±0.75 volts.  
         [0022]     The input voltage  102  travels through the input filter  106  to attenuate or eliminate any initial noise or irregularities generated by the voltage source. The filtered input voltage  102  and time varying input signal  104  are both received by the switching circuit  111 . The switching circuit  111  generates an AC output based on these DC signals and sends it to the voltage multiplier circuit  113 . A voltage multiplier circuit  113  is a passive circuit, for example, a collection of passive devices such as capacitors, and diodes, arranged to produce a multiple of the input voltage at the output. The voltage multiplier circuit  113  steps up the voltage above the level of the input voltage  102  according to the circuit design, and outputs the multiplied voltage. The multiplied voltage output constitutes a direct current voltage. A voltage regulator  115  monitors the voltage being output by the voltage multiplier circuit  113  and modifies it to maintain a constant value. The voltage is finally sent through an output filter  108  to reduce any ripple or noise that was generated by the circuit or by interactions with nearby circuits. The result is a stable, precise, high voltage output  117  that can be used as a reference voltage.  
         [0023]      FIG. 2  is a block diagram of an electronic device, shown generally at  90 . The electronic device  90  comprises an electronic circuit  95  connected to a voltage reference circuit  100 . The voltage reference circuit  100  receives a voltage input  102  and a time varying input signal  104 , from which it generates a stable, precise, high voltage output  117  that is input into the electronic circuit  95 . In one embodiment the electronic circuit  95  comprises a sensor circuit and the electronic device  90  comprises a sensor, such as a MEMS inertial sensor.  
         [0024]      FIG. 3  is a circuit diagram of a switching circuit  111  in accordance with an embodiment of the present invention. The switching circuit  111  is used to generate an AC voltage output from a DC voltage input. This process is referred to as voltage conversion. In one embodiment, the switching circuit  111  achieves voltage conversion through the use of a first NPN junction transistor  120  and a second NPN junction transistor  122 . A time varying input signal  104  and input voltage  102  are introduced into the circuit as shown. In one embodiment, the time varying input signal  104  comprises a square wave input. In other embodiments, the time varying input signal  104  comprises a triangle wave input, or a sinusoidal input. The time varying input signal  104  is a pulse train or clock whose frequency is selected from a range of frequencies, for example, between 125 kHz and 500 KHz. At 500 KHz the voltage multiplier circuit  113  provides better voltage regulation, that is, the voltage does not drop rapidly as a function of output current. However, the higher frequency has the drawback of generating additional noise that can couple into surrounding electronics, having an adverse affect on the precision of the reference voltage. At 125 KHz, the voltage multiplier circuit  113  generates less noise, but provides less stable voltage regulation. These competing features create a system tradeoff, wherein the user must balance the need for precision of the voltage reference and the need to regulate the voltage emerging from the voltage regulator  115 . In one embodiment, the pulse train has a frequency of 250 KHz.  
         [0025]     The time varying input signal  104  may alternate, for example, between a negative and a positive voltage, or between zero volts and a positive voltage. In one embodiment the time varying input signal  104  comprises a square wave input that alternates between zero and five volts. When the time varying input signal  104  is at zero volts, a higher potential is applied to the collector than the base of the first NPN junction transistor  120 , and it is activated, inserting fifteen volts into the voltage multiplier circuit  113 . Meanwhile the second NPN junction transistor  122  remains off. When the time varying input signal  104  is at five volts, the potential at the base of the first NPN junction transistor  120  is greater than the potential at its collector, turning it off. Simultaneously, the second NPN junction transistor  122  is activated, and the input voltage  102  is grounded. Thus no charge is applied to the voltage multiplier circuit  113 . A first diode  124  ensures that the second NPN junction transistor  122  remains inactive while the time varying input signal  104  is at zero, and a second diode  126  ensures that the voltage multiplier circuit  113  does not receive any charge when the time varying input signal  104  is at five volts.  
         [0026]      FIG. 4  depicts a circuit diagram of one embodiment of a voltage multiplier circuit  113  in connection with the switching circuit  111 . In the illustrated embodiment, the voltage multiplier circuit  113  comprises a first stage  115 , a second stage  118 , through an nth stage  120 . Each stage comprises a primary capacitor  122 , a primary diode  124 , a secondary capacitor  126 , and a secondary diode  128 . The alternating current introduced by the switching circuit  111  successively charges the primary capacitor  122  to the input voltage  102  through the primary diode  124 , then the secondary capacitor  126  to twice the input voltage  102  through the secondary diode  128 . The charges of the secondary capacitors  126  are then added in series to generate a theoretical  2 n times the input voltage  102 , where n is the number of stages. Depending on the arrangement of the diodes and capacitors, the output voltage is either the same polarity or the opposite polarity as the input voltage  102 . In the arrangement shown, the output voltage is the opposite polarity to the input voltage. In one embodiment, the value of capacitance of each capacitor in the voltage multiplier circuit  113  is the same. In other embodiments, the value of at least some capacitors differs. For example, advantageously, in one embodiment, capacitors with a higher capacitance are used in earlier stages of the voltage multiplier circuit  113 , and capacitors with lower capacitance are used in the later stages. This varying of capacitor values is designed to reduce ripple in the output voltage, which is commonly produced by voltage multiplier circuits  113 .  
         [0027]     Due to their nature, voltage multiplier circuits  113  are generally used to generate high voltages with low currents. As the voltage is stepped up, the current is necessarily decreased. The fewer stages that are used to increase the voltage, the less the current drops. Therefore, there is a design tradeoff between necessary voltage and necessary current. The capacitors of different stages may be arranged in series, or they may be arranged such that they share common connections. Arranging the capacitors of different stages in series maximizes the voltage multiplication consequently the current drop. On the other hand, capacitors of different stages configured to share common connections produce a voltage multiplier circuit  113  that is better suited to applications where lower output voltages and higher currents are needed. When the capacitors are arranged to share common connections, they must have higher voltage ratings.  
         [0028]     Voltage multiplier circuits  113  are generally small and light, and composed of relatively inexpensive components. They have the advantage that the voltage across each stage of the multiplier is at most only two times the input voltage. In addition to making the multiplier easy to insulate, and permitting the use of low cost components, this offers particular advantages in radiation rich environments. High voltage components are more susceptible to output changes due to radiation. For example, high voltage diodes and transistors are lightly doped, whereas low voltage diodes and transistors are heavily doped. Over time, exposure to radiation causes impurities to be introduced into the junctions of both the diodes and transistors. Where the diode or transistor is heavily doped, the percentage of impurities is relatively low, and therefore has a minimal effect. However, where the diode or transistor is lightly doped, the percentage of impurities is high, which causes leakage across the junction, and decreases the effectiveness of the component. Therefore low voltage components are preferable in devices that will be exposed to high levels of radiation.  
         [0029]     Ideally a voltage multiplier circuit  113  doubles the input voltage  102  at each stage. For example, a single stage voltage multiplier circuit  113  would generate two times the input voltage  102 , a two stage multiplier would generate four times the input voltage  102 , a three stage multiplier would generate six times the input voltage  102 , and so on. However, once a load is attached, the output voltage is reduced. Minor fluctuations in the load impedance also produce large fluctuations in the output voltage. Additionally, electrical components are not ideal, and therefore each component introduces its own impedance, further reducing the generated voltage. For example a standard diode drops the voltage across it by six tenths of a volt. The voltage loss within the multiplier becomes increasingly large as further stages are added. The possibility of voltage arcing also increases with the introduction of additional stages. Therefore, in some embodiments, the actual output from the voltage multiplier circuit  113  is less than  2 n times the voltage input  102 .  
         [0030]     In an alternative embodiment, the voltage reference circuit  100  is adapted such that voltage can be drawn from any stage of the voltage multiplier circuit  113 . Depending on the present needs of the circuit, it can draw voltage from, for example, the first stage  115  if only twice the input voltage  102  is required, or the second stage  118  if four times the input voltage  102  is required. This increases the versatility of a single circuit.  
         [0031]     In further alternative embodiments, a step up transformer is used as the voltage multiplier circuit  113  to increase the input voltage to the necessary value. Advantageously, a transformer typically produces less ripple than other multiplier circuits, and thus certain circuits using a transformer require less complex filters.  
         [0032]     To enable the voltage reference circuit  100  to be able to maintain a constant voltage even where changes occur in the load impedance, or in individual component characteristics over time, a voltage regulator  115  is implemented in some embodiments. A voltage regulator maintains a constant voltage by adjusting its internal resistance in relation to changes, for example, in the load resistance. Voltage regulators  115  are divided into two broad categories comprising switching regulators and linear regulators. Linear voltage regulators are further divided into shunt regulators and series regulators. In a shunt regulator the regulator is in parallel with the load, as compared to a series regulator wherein it is in series with the load. Voltage regulators  115  are also divided into open loop regulators and closed loop regulators. In an open loop regulator, the voltage control is inherent in an element of the regulator, whereas in a closed loop regulator a feedback control system is used to maintain a constant voltage.  
         [0033]      FIG. 5  illustrates a circuit diagram of a voltage regulator  115 , wherein the voltage regulator  115  is a linear, closed loop, shunt voltage regulator, in accordance with one aspect of the present invention. In the illustrated embodiment, the voltage regulator  115  comprises an error amplifier  135 , a pass element  137 , a reference voltage  140 , and a feedback network  142 . The voltage output from the voltage multiplier circuit  113  is connected to one terminal of the error amplifier  135 . The actual voltage received by the error amplifier  135  is a fraction of the output from the voltage multiplier  113  based on, for example, resistors  145  and  147 . Depending on design considerations, the output may be connected to either the positive or negative terminal of the error amplifier  135 . As shown, the output is connected to the positive terminal. The reference voltage  140  is connected to the other terminal. The voltage that is output from the multiplier should be sufficiently higher than the regulated voltage (error amplifier  135  output) to assure proper operation of the voltage regulator  115 . The error amplifier  135  compares the two inputs, and outputs an error based on their difference. The error that is output is a multiple of the reference voltage  140 , whose value is based on the arrangement and selection of the elements in the voltage regulator  115 . This error is combined with the voltage multiplier output to generate a regulated voltage through the feedback network  142 . The regulated voltage maintains a constant value, determined by the configuration of the voltage regulator  115 .  
         [0034]     It is very important that the reference voltage  140  remain stable. Since the voltage output of the voltage multiplier circuit  113  is compared to the reference voltage  140 , any change in the reference voltage  140  will significantly alter the error value measured by the error amplifier  135 . The reference voltage  140  may be a band gap-type reference or, for example, a zener diode.  
         [0035]     Another component that is added to a voltage regulator  115  in some embodiments is a pass element  137 . The pass element  137  serves as a voltage controlled resistance and helps regulate the output voltage going to the load. An effective device to use as a pass element is a PNP junction transistor  149 , as illustrated.  
         [0036]     In alternative embodiments series voltage regulators are used. Series regulators are the most common type of linear voltage regulator, and share a number of advantages over shunt voltage regulators. Since series regulators are more common, they may be found in monolithic form, built into integrated circuits. In some applications, series regulators are also more efficient than shunt regulators.  
         [0037]     The regulated voltage finally passes through an output filter  108  in some embodiments. A filter is a circuit or circuit element that alters the amplitude and/or phase characteristics of an electronic signal with respect to frequency. The output filter  108  is used generally to filter out noise generated by the circuit. More specifically, voltage multipliers commonly emit a ripple, whose effects increase as further stages are added to the multiplier. To ameliorate or eliminate this ripple, a filter is necessary in some embodiments. In some embodiments, an input filter  106  is also inserted to filter the input voltage  102  before it enters the switching circuit  111 . In one embodiment, the input filter  106  and output filter  108  are substantially the same, and in other embodiments they use different configurations.  FIGS. 6   a  and  6   b  illustrate circuit diagrams of one possible configuration of an input filter  106  and an output filter  108  respectively, in accordance with an embodiment of the present invention. The illustrated embodiments depict passive filters, made up of only passive elements such as capacitors  152 , inductors  154 , and resistors  156 . Passive filters share a number of advantages. Because passive filters have no active elements, they do not require a power supply. They may also be used at high frequencies and at high voltage and current. However, passive filters are not capable of supplying any gain, which may be required in certain circuit configurations.  
         [0038]     In alternative embodiments, the filters comprise active filters. Active filters use amplifying elements such as op amps, along with capacitors and resistors, to perform substantially the same function as passive filters. Active filters have the advantage that they can introduce gain into the signal, and they are generally easier to design than passive filters.  
         [0039]      FIG. 7  shows a circuit diagram of one possible configuration of a voltage reference circuit  100  in accordance with an embodiment of the present invention. Where possible, the same reference numbers are used for the same or like components as in previous figures. An input voltage  102  of fifteen volts first travels through an input filter  106  to a switching circuit  111 . The switching circuit  111  receives the filtered input voltage  102  and a 250 kHz square wave input signal  104  that may be generated by a field programmable gate array, and generates an alternating current. The alternating current is received by the voltage multiplier circuit  113 , comprising an eight stage voltage multiplier circuit  113 . The voltage multiplier circuit  113  then outputs a voltage at approximately negative seventy-five volts. The voltage regulator  115  monitors the output of the voltage multiplier circuit  113  and modifies it such that a constant voltage of negative sixty volts is sent to the voltage output  117  after being filtered by the output filter  108 . In one embodiment, the output voltage has a tolerance of three hundred millivolts. The illustrated voltage regulator  115  comprises a shunt closed loop voltage regulator, and the illustrated filters comprise passive filters. Any changes in output impedance or component characteristics are rectified by the voltage regulator  115 , thus creating a constant, precise, temperature stable voltage reference.  
         [0040]     In view of the foregoing, it will be understood by those skilled in the art that the methods of the present invention can be used in conjunction with other electronic circuits and networks. The above embodiments have been presented by way of example and not by way of limitation. Variations and modifications may occur, which fall within the scope of the present invention, as set forth in the following claims.