Abstract:
The present invention provides a bias generation circuit in which the voltage of electrically isolated circuits are stabilized by providing a photovoltaic diode in each circuit, a common light source uniformly positioned to provide equivalent energy to each photovoltaic diode and an operational amplifier, configured with a capacitor as an integration circuit, driving the common light source, wherein one isolated circuit provides feedback to the amplifier, such that variations in the voltage in the isolated circuit causes the amplifier to provide an adjusted signal to the common light source, adjusting the energy output to compensate for voltage variations simultaneously, yet independently occurring in each isolated photovoltaic diode circuit. Such bias voltage circuit may be used with chromatographic ionization detectors as well other devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not Applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable.  
       BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The invention is generally related to electrical bias voltage generation and more specifically to the optical generation of an adjustable, stable, low-noise, electronically isolated bias for use with precision analytical equipment.  
         [0005]     2. Description of the Related Art  
         [0006]     The generation of bias voltages is widely known in the field of analytical chemistry. Equipment used to detect very small levels of charge use a bias voltage to produce an accelerating field in ion detectors, such as chromatographic ionization detectors.  
         [0007]     A chromatographic ionization detector operates by applying a high voltage across discharge electrodes that are located in a gas-filled source chamber. In the presence of a detector gas such as helium, a characteristic discharge emission of photons occurs. The photons irradiate an ionization chamber receiving a sample gas that contains an analyte of interest. Ions are produced in the ionization chamber as a result of photon interaction with ionizable molecules in the sample gas. Such detectors are well known in the art and include U.S. Pat. No. 5,767,683 issued Jun. 16, 1998 to Stearns, Cai and Wentworth, U.S. Pat. No. 5,594,346 issued Jan. 14, 1997 to Stearns and Wentworth, and U.S. Pat. No. 5,541,519 issued Jul. 30, 1996 to Steams and Wentworth.  
         [0008]     The sensitivity and resolution of detection equipment may be limited by the stability of the bias voltage and the extraneous electrical variations, or noise, created by associated electrical circuits. Voltage variations in the bias and/or leakage currents produced by the bias may mask the desired occurrences to be measured.  
         [0009]     Simple bias voltage may be generated from a 12V DC power supply. Transistors and integrated circuit converters are used to modify the frequency and voltage of the current from the power supply to obtain a desired bias. Further transistorized circuitry may be used to filter and monitor the current and voltage in order to achieve a useable degree of stability.  
         [0010]     Bias generation in the prior art has typically involved the use of transformer-coupled circuits in which a first transformer, driven by an alternating-current source, is connected to a second transformer whose isolated output is then rectified, filtered, and regulated at a predetermined voltage by additional circuitry. Disadvantages of this scheme include: the output bias voltage is not adjustable without additional feedback circuitry; variations in the output bias voltage are not sensed and regulated without additional feedback circuitry; AC electromagnetic fields may be coupled to the detecting circuitry, causing instability in the measurement process without additional shielding; and the number of components required may increase the cost and reduce the reliability of the employing device.  
         [0011]     Diodes are known to be able to produce light when a current is passed through, or to generate a current when excited by a light source. In both cases, the intensity of the light is proportional to the magnitude of the current.  
         [0012]     Incident with a current flow through a diode is a voltage drop across the diode. The relationship between the current and the voltage is given by the well-known diode equation: 
 
 I   D   =I   S   e   K(T−T     0     )   [e   VλV     t   −1]
 
 where 
        I S  is the saturation current, fixed by the materials and fabrication of the diode (amps);     K is a constant for the material used for the diode, approximately 0.045 for silicon;     T is the diode temperature (°K);     T 0  is the diode reference temperature (°K);     V t  is the threshold voltage, 0.026 volts (V);     V is the voltage through the diode (V);     e is the electron charge (1.602×10 −19  C);     K is Boltzmann&#39;s Constant (1.380×10 −23  J/K); and     λ is a constant for the material used for the diode, approximately 2 for silicon. 
 
 Of importance is that diode current and voltage drop are not linearly proportional and are influenced by temperature. For illustration of the influence of temperature, where I S =1.0E-9 amperes, T−T 0 =0 and V=0.036 volts, then I D =1.0E-9 amperes; in the same example where V=0.36 volts, then I D =1.0E-6 amperes. If at V=0.36 Volts, diode temperature, T, rises such that T−T 0 =10° C., then I D =1.57E-6 amperes. 
       
 
         [0024]     In a practical photovoltaic diode circuit, some type of device or load will be externally connected to the photovoltaic diode. When the effect of such a load is added to the diode equation the equation becomes: 
 
 I   D   =I   S   e   K(T−T     0     )   [e   vλV     t   −1 ]+V/R   L  
 
 where I D  is the total generated current and R L  is the value of the load, in ohms. 
 
         [0026]     Anomalies in a power supply and environmental conditions, such as temperature and humidity affect the electrical current produced by an electrical circuit. The voltage supplied to the load is subject to such anomalies. A practical photovoltaic diode circuit requires some means of control and stabilization of the generated voltage. Some examples of prior art circuits designed to compensate for voltage variations in circuits include:  
         [0027]     U.S. Pat. No. 4,375,596, issued on Mar. 1, 1983, to Hoshi, discloses a reference voltage generator circuit, which overcomes variations in a power supply by dividing the power supply voltage to create two output signals, uniformly modifying the signals in opposite polarity, then averaging the resulting signals to generate a constant value of reference voltage.  
         [0028]     U.S. Pat. No. 4,380,706, issued on Apr. 19, 1983, to Wrathall, discloses a temperature stable voltage reference source, which uses a differential amplifier with an output coupled to an additional amplifying stage, involving two bipolar transistors, wherein the emitter of one transistor is larger than the emitter of the other transistor. Cascaded emitter followers are used between the two amplifying stages to develop a higher voltage, which is fed back into the inputs of the differential amplifier, thereby establishing a more independently stable reference voltage circuit.  
         [0029]     U.S. Pat. No. 4,471,290, issued on Sep. 11, 1984, to Yamaguchi, discloses a substrate bias generating circuit responsive to the output signal of the oscillator circuit, which includes a voltage divider connected between the output terminal of the bias generating circuit and a ground terminal, and a level sensor for producing a control signal to the oscillator circuit when it is detected that the output voltage of the voltage divider reaches a predetermined value, to thereby stop the oscillating operation of the oscillator circuit  
         [0030]     U.S. Pat. No. 5,262,989, issued on Nov. 16, 1993, to Lee et al., discloses a circuit for sensing back-bias levels in a semiconductor device that causes the voltage pump circuit to adjust output to reach and maintain a desired voltage level.  
         [0031]     U.S. Pat. No. 3,975,649, issued on Aug. 17, 1976, to Kawagoe et al., discloses a temperature compensation circuit that uses a high value resistor and at least one field-effect transistor for connection between a circuit to be compensated and the power source, such that the when ambient temperature of the circuit increases the current flowing through the field-effect transistor decreases. However, the decreased current from the field-effect transistor causes voltage drop across the resistor to decrease. With the opposite end of the resistor connected to the gate of the field-effect transistor, the relative increase in voltage causes an increased current flow through the field-effect transistor, compensating for the temperature fluctuation to stabilize the output voltage.  
         [0032]     U.S. Pat. No. 4,794,247, issued on Dec. 27, 1988, to Stineman, Jr., discloses using an integrating amplifier with a feedback capacitor, to stabilize the bias signal from a photovoltaic detector, while reducing the noise effect.  
         [0033]     U.S. Pat. No. 4,843,265, issued on Jun. 27, 1989, to Jiang, discloses a temperature compensating circuit that generates inverse variations in a field-effect transistor, achieved by charging a capacitor to a voltage and discharging the capacitor through a field-effect transistor in response to the fluctuations.  
         [0034]     Also known to the field of art is the use of photovoltaic diodes to produce a current isolated from the current of the light source. Light sources capable of exciting current in photovoltaic diodes include light-emitting diodes. Prior art that demonstrates these uses include:  
         [0035]     U.S. Pat. No. 5,805,062, issued on Sep. 8, 1998, to Pearlman, discloses an isolation amplifier that transmits data to a receiver via a current loop, where the isolated portion of the circuit is powered by a photovoltaic array illuminated by a light source, optionally an array of same frequencied light-emitting diodes.  
         [0036]     A device is commercially available, referred to as an optically coupled floating power source, that is composed of one or more light-emitting diodes and one or more photovoltaic diodes, disposed within an opaque package in such a way that light from the light-emitting diodes impinge on the photovoltaic diodes, thereby generating a current in the photovoltaic diodes in response to the current supplied to the light-emitting diodes.  
         [0037]     It would be an improvement to the field to create a bias voltage from a power source comprised of at least one light-emitting diode stimulating matched currents in at least two electrically isolated photovoltaic diodes, such that the circuit of one diode is used to provide a feedback voltage to an operational amplifier driving the light-emitting diode, thereby stabilizing the output voltage in both of the photovoltaic diode circuits.  
         [0038]     It would be a further improvement to provide a distance between the bias source and detector due to the temperature of the detector. Such distance however typically requires shielding of the connection between the electronics and the detector, typically by coaxial cabling. The use of such shielding introduces capacitance which creates a pathway into the electronics for current noise resulting from the voltage noise in the bias generator. Low noise in the bias generator therefore becomes more critical under these circumstances.  
       BRIEF SUMMARY OF THE INVENTION  
       [0039]     Accordingly, the objects of this invention is to provide, inter alia, an electrical circuit for generating a bias voltage that: 
        provides sufficient voltage stability for highly precise analytical measuring equipment, including chromatographic ionization detectors has low noise production;     has the output circuit electrically separated from the drive and feedback circuit;     provides a stabilizing feedback voltage to a drive amplifier; and     provides the ability to set and vary the generated voltage of the circuit.        
 
         [0044]     Other objects of my invention will become evident throughout the reading of this application.  
         [0045]     The current invention is an electrical circuit for detection equipment, such as chromatographic ionization detectors, for the generation of a stable, low-noise bias, having at least one set of one or more light-emitting diodes (LED) and at least two photovoltaic diode sets disposed in such a way that light from each light-emitting diode impinges on at least two photovoltaic diode sets, thereby generating a current in the photovoltaic diode sets in response to current supplied to the light-emitting diode. The photovoltaic diode set may include two or more photovoltaic diodes. The current from one photovoltaic diode set produces the output voltage, while the current of the other photovoltaic diode set feeds into an amplifier, which regulates the drive current to the light-emitting diode set. Fluctuations in the current produced in the photovoltaic diode set in the output circuit are identically, though independently, represented in the other photovoltaic diode, which in turn causes a corresponding adjustment in the drive current to the light-emitting diode to correct the fluctuation. The result is an essentially stable output voltage. (The term “essentially”, as used herein, means closely approximating to a degree sufficient for practical purposes.) 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0046]      FIG. 1  is a simplified schematic of a bias generation circuit in accordance with the present invention.  
         [0047]      FIG. 2  is a simplified schematic of a bias generation circuit in accordance with the present invention, having multiple light-emitting diodes in series.  
         [0048]      FIG. 3  is a dissected simplified schematic of the electrically isolated controlled circuit of the bias generation circuit of  FIG. 2 .  
         [0049]      FIG. 4  is a dissected simplified schematic of the electrically isolated controlling circuit of the bias generation circuit of  FIG. 2 .  
         [0050]      FIG. 5  is a simplified schematic of a bias generation circuit in accordance with the present invention having a potentiometer for equalization of current output from the two photovoltaic diode sets to correct for any differences in output of the two photovoltaic diode sets. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0051]     Referring to  FIG. 1 , the present invention provides a bias generation circuit  10  in which an optically coupled power source  11  generates identical currents within electrically isolated circuits. Optically coupled power source  11  comprises light emitting diode  35 , connected to ground  29  at its anode end and to resistor  15 , then on to output  80  of operational amplifier  13  on its cathode end. Light emitting diode  35  is disposed in such a way that the light from light emitting diode  35  impinges equally on controlled photovoltaic diode set  36  and controlling photovoltaic diode set  37 . Controlled photovoltaic diode set  36  and controlling photovoltaic diode set  37  thereby respectively generate essentially equivalent, electrically isolated controlled current  60  and controlling current  70 . In the preferred embodiment, optically coupled power source  11  is a commercially available circuit chip, DIG-12-8-30, by Dionics, Inc.  
         [0052]     Controlled photovoltaic diode set  36  is connected into controlled circuit  30 . Output node  25  connects to the anode end of controlled photovoltaic diode set  36  and input node  26  connects to the cathode end of controlled photovoltaic diode set  36 . Also connected between input node  26  and output node  25 , parallel with controlled photovoltaic diode set  36  are resistors  24  and  12 . In the exemplary embodiment, resistor pairs  24  and  12 , and  22  and  23 , possess equivalent resistance.  
         [0053]     Controlling photovoltaic diode set  37  is connected into controlling circuit  32 . Positive output node  18  connects to the anode end of controlling photovoltaic diode set  37 . Positive output node  18  is also connected to a reference voltage source  16 , which is adjustable. In the exemplary embodiment, reference voltage source  16  is set to +10 volts. Node  17  connects to the cathode end of controlling photovoltaic diode set  37 . Node  17  also connects to resistor  23 , which in turn connects to node  20 . Resistor  22  connects to node  20  on one end and to node  18  on the other. Resistors  23  and resistor  22  possess equivalent resistance.  
         [0054]     Non-inverting input  84  of operational amplifier  13  is connected to ground  19 . Inverting input  82  of operational amplifier  13  is connected to node  20  and to one side of capacitor  14 . Output  80  of operational amplifier  13  is connected to resistor  15  and the other side of capacitor  14 .  
         [0055]     Referring to  FIG. 1 , operational amplifier  13 , well known to those skilled in the art, produces an output voltage proportional to the difference between the voltages at the input nodes as: 
 
 V   0   =A ( V   + −V − ) 
 
 where V 0  is the output voltage, V +  is the non-inverting input node voltage, V − is the inverting input node voltage, and A is the gain factor, usually on the order of 10 6 . Under conditions of stable operation, the magnitude of V 0  will be less than a few volts (e.g., &lt;10 volts), and the input voltage difference, V + −V − , will therefore be less than V 0 /A (e.g., &lt;10 micro-volts). For practical purposes, the input voltage difference may then be considered to be zero. 
 
         [0057]     Introducing drive current  50 , through light emitting diodes  35 , activates circuit  10 . The light emitted by light emitting diodes  35 , induces driven output current  60  and driven feedback current  70 .  
         [0058]     Under stable operating conditions, equal currents  60  and  70  are produced by photovoltaic diode sets  36  and  37 , respectively, the voltage across resistors  12  and  24 , is equal to the voltage across resistors  22  and  23 , the voltage at node  17 , is equal in magnitude and opposite in sign to the voltage at node  18 , and the voltage at node  20 , (since the resistors  22  and  23 , are of equal value) is essentially zero. The electrically isolated voltage source at nodes  25  and  26  is used as the desired stable generated bias.  
         [0059]     The equality of current  60  and  70  contains natural variations, possibly due to non-uniform transmission of light energy simultaneously to diode  36  and  37  from diode  35 , the physical characteristics of diodes  36  and  37  not being completely identical, or other variation sources. In the exemplary embodiment shown in  FIG. 5  these variations are adjusted by inserting the adjustable contact of potentiometer  201  to node  20 , between resistors  22  and  23 , which alters the ratios of values of resistors  22  and  23  while keeping the sum of their values constant. The total resistance through the potentiometer  201  at node  20 , and resistors  22  and  23  would equal the total resistance through resistors  12  and  24 . Alternatively, the ratio of resistors  22  and  23  could be left constant and the configuration of resistors  12  and  24  could be altered to adjust the sum of resistors  12  and  24 , in order to correct the imbalances as they occurred. As a further alternative, potentiometer  201  could be replaced with a resistor of resistance equal to potentiometer  201  (not shown) Other equivalent solutions are know to the field, which may be employed to manipulate the ratio and sum of the resistance values between nodes  17  and  18  with the resistance values between nodes  25  and  26 .  
         [0060]     Referring to  FIG. 1 , bias generation circuit  10  is configured to seek a stable condition. Since both photovoltaic diode pairs  36  and  37  are subject to the same conditions of loading—illumination, temperature, etc.—the voltage difference between nodes  25  and  26  will be the same as the voltage difference between nodes  18  and  17 . Although the voltage at node  18  is set by reference source  16  to be +10 volts in the following examples, the condition for stability is not dependent on the magnitude of that voltage, within the operational limits of the circuit.  
       EXAMPLE ONE  
     Stability  
       [0061]     Assume that resistors  12 ,  22 ,  23  and  24 , have equal values of 1.0×10 6  ohms (1.0M ohms); the amplifier gain A, is 1.0×10 6 ; the voltage at node  18 , set by reference source  16 , is +10 volts; the current generated by the photovoltaic diodes is 10 microamperes; the voltage at node  17  is −10 volts; the voltage difference between nodes  25  and  26  is 20 volts; and the voltage at node  21  is −5 volts. The voltage at node  20  is then +5×10 −6  volts, essentially zero for practical purposes. Since the current through resistor  22  into node  20  is equal to the current through resistor  23  out of node  20 , no net current flows into (out of) inverting input  82  of amplifier  13 , or through capacitor  14 , via node  20 . Since no current flows through capacitor  14  the voltage across capacitor  14  does not change and driving current  50  through resistor  15  does not change.  
       EXAMPLE TWO  
     Variation Correction  
       [0062]     Assume that an instantaneous variation in ambient conditions, e.g., temperature, occurs such that the voltage drop across resistors  22  and  23  (and thereby across resistors  12  and  24 ) is reduced by 1.0 volt. Since the voltage at node  18  is fixed at +10 volts by reference source  16 , and the voltage at node  20  is essentially zero, the voltage at node  17  will thereby be −9 volts. The current through resistor  22 , into node  20 , will still be 10 microamperes; the current through resistor  23 , out of node  20 , will be 9 microamperes, and the net current into node  20 , through capacitor  14 , will thereby be 1 microampere. Since the voltage across a capacitor is proportional the integral of the current through it as: 
 
 V =(1 /C )∫ dt  
        the voltage across capacitor  14 , will begin to change at a rate that satisfies the relation: 
 
 i=CdV/dt  
 
 where i is the current flowing through the capacitor, C, is the capacitance in Farads, and V is the voltage across the capacitor. (E.g., let the capacitance, C, be 1×10 −6  farad and the current be 1 microampere, as above. The voltage across the capacitor  14  will then instantaneously begin to increase at the rate of 1 volt/second.) As the voltage across capacitor  14  increases, the voltage at node  21  becomes increasingly more negative and the driving current  50  increases until a new stable condition exists, such that driving current  50  is of a magnitude to sustain the conditions assumed above in Example One. 
       
 
         [0065]      FIG. 2  depicts an alternate exemplary embodiment wherein bias generation circuit  100  comprises multiple optically coupled power sources  111 A,  111 B and  111 C, connected in series. Such configuration provides the potential to develop greater levels of voltage across output node  125  and input node  126  than would be generated by a single similar optically coupled power source (not shown).  
         [0066]     Referring to  FIGS. 2, 3  and  4 , optically coupled power source  111 A is comprised of light emitting diode  135 A, and photovoltaic diodes  136 A and  137 A. Light emitting diode  135 A is disposed in such a way that the light from light emitting diode  135 A impinges equally on controlled photovoltaic diode set  136 A and controlling photovoltaic diode set  137 A. Optically coupled power source  111 B is comprised of light emitting diode  135 B, and photovoltaic diodes  136 B and  137 B. Optically coupled power source  111 C is comprised of light emitting diode  135 C, and photovoltaic diodes  136 C and  137 C. Optically coupled power sources  111 B and  111 C are configured similarly to optically coupled power source  111 A, such that light emitting diode  135 B is disposed in such a way that the light from light emitting diode  135 B impinges equally on controlled photovoltaic diode set  136 B and controlling photovoltaic diode set  137 B, and light emitting diode  135 C is disposed in such a way that the light from light emitting diode  135 C impinges equally on controlled photovoltaic diode set  136 C and controlling photovoltaic diode set  137 C.  
         [0067]     Light emitting diodes  135 A,  135 B and  135 C are connected in series. The anode end of light emitting diode  135 C is connected to ground  129 , and the cathode end of light emitting diode  135 C is connected to the anode end of the next light emitting diode  135 B in series. The cathode end of light emitting diode  135 B is connected to the anode end of the next light emitting diode  135 A in series. The cathode end of light emitting diode  135 B is connected to resistor  115 , which is then connected to output  180  of operational amplifier  113 .  
         [0068]     Controlled photovoltaic diode sets  136 A,  136 B and  136 C generate an electrically isolated controlled current  160 , which is essentially equivalent to an electrically isolated controlling current  170  generated by respective, controlling photovoltaic diode sets  137 A,  137 B and  137 C.  
         [0069]     Controlled photovoltaic diode sets  136 A,  136 B and  136 C are connected in series into controlled circuit  130 . Output node  125  connects to the anode end of controlled photovoltaic diode set  136 C. The cathode end of photovoltaic diode set  136 C connects to the anode end of the next photovoltaic diode set  136 B in series. The cathode end of photovoltaic diode set  136 B connects to the anode end of the next photovoltaic diode set  136 A in series. Input node  126  connects to the cathode end of controlled photovoltaic diode set  136 A.  
         [0070]     Also connected between input node  126  and output node  125 , parallel with controlled photovoltaic diode sets  136 A,  136 B and  136 C are resistors  124  and  112 . In the exemplary embodiment, resistors  124  and  112  possess equivalent resistance.  
         [0071]     Also connected between input node  126  and output node  125 , parallel with controlled photovoltaic diode sets  136 A,  136 B and  136 C, and resistors  124  and  112 , is capacitor  127 . One operational side of capacitor  127  is connected to input node  126  and the other operational side of capacitor  127  is connected to output node  125 . In the exemplary embodiment, resistor  128  is also connected to node output  125  intermediate the device intended to use the generated bias voltage.  
         [0072]     Controlling photovoltaic diode sets  137 A,  137 B and  137 C are connected into controlling circuit  132 . Positive output node  118  connects to the anode end of controlling photovoltaic diode set  137 C. The cathode end of photovoltaic diode set  137 C connects to the anode end of the next photovoltaic diode set  137 B in series. The cathode end of photovoltaic diode set  137 B connects to the anode end of the next photovoltaic diode set  137 A in series. The cathode end of controlling photovoltaic diode set  137 A connects to node  117 .  
         [0073]     Positive output node  118  is also connected to a reference voltage source  116 . In the exemplary embodiment, reference voltage source  16  is set to +10 volts. Node  117  also connects to resistor  123 , which in turn connects to node  120 . Resistor  122  connects to node  120  on one end and to node  118  on the other. Resistors  123  and resistor  122  possess equivalent resistance.  
         [0074]     Non-inverting input  184  of operational amplifier  13  is connected to ground  19 . Inverting input  182  of operational amplifier  113  is connected to node  120  and to the one operational side of capacitor  114 . Output  180  of operational amplifier  113  is connected to node  121 , which also connects to resistor  115  and the other operational side of capacitor  114 .  
         [0075]     Introducing drive current  150 , as sub-currents  150 A,  150 B and  150 C, through light emitting diodes  135 A,  135 B and  135 C, respectively, activates circuit  100 . The light emitted by light emitting diodes  135 A,  135 B and  135 C, induces currents  160 A,  160 B and  160 C, in photovoltaic diodes  136 A,  136 B and  136 C, respectively, which in series form driven output current  160 . At the same time the light emitted by light emitting diodes  135 A,  135 B and  135 C, induces currents  170 A,  170 B and  170 C, in photovoltaic diodes  137 A,  137 B and  137 C, respectively, which in series form driven feedback current  170 .  
         [0076]     Under stable operating conditions, driven output current  160 , generated by photo-voltaic diode sets  136 A,  136 B and  136 C, is essentially equivalent to driven feedback current  170 , generated by photovoltaic diode sets  137 A,  137 B and  137 C. Additionally, the voltage across resistors  112  and  124  is equal to the voltage across resistors  122  and  123 ; the voltage at node  117  is equal in magnitude and opposite in sign to the voltage at node  118 ; and the voltage at node  120 , (since the resistors  122  and  123 , are of equal value) is essentially zero. The electrically isolated voltage source at nodes  125  and  126  is used as the desired generated bias.  
         [0077]     Bias generation circuit  100  is configured to seek a stable condition. Since controlled circuit  130  and controlling circuit  132  are subject to the same conditions of loading—e.g., illumination, temperature, etc.—the voltage difference between nodes  125  and  126  will be the same as the voltage difference between nodes  118  and  117 .  
         [0078]     The foregoing disclosure and description of the invention is illustrative and explanatory thereof. Various changes in the details of the illustrated construction may be made within the scope of the appended claims without departing from the spirit of the invention. The present invention should only be limited by the following claims and their legal equivalents.