Abstract:
Circuits and methods that improve the performance of reference circuits are provided. A reference generator circuit maintains a substantially constant output current over an extended temperature for use as a reference. Output current fluctuations caused by a poorly specified power source or process variations are minimized or eliminated.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to electronic reference circuitry. More particularly, the invention relates to bandgap references that provide a substantially constant output current which may be used as voltage or current references. 
         [0002]    Bandgap voltage references have been widely used in electronic applications for many years. The purpose of a bandgap voltage reference is to provide a substantially constant and stable voltage over a fairly wide temperature range. Such references form a vital part of numerous commonly used circuits such as analog-to-digital and digital-to-analog converters, phase-locked loops, voltage regulators, comparison circuits, etc. 
         [0003]    The basic principle behind the bandgap reference is the well-known voltage drop associated with certain semiconductor junctions. For example, a silicon p-n junction such as the emitter-base junction bipolar transistor may have a forward conduction characteristic (i.e., voltage drop) of about 0.6 volts. It is possible to construct a basic voltage reference circuit based on this known physical conduction property. For example, one or more such p-n junctions may be connected in series to form a voltage reference circuit that has a predetermined and stable output voltage. For example, connecting two silicon diodes in series provides a regulated 1.2 volt output, three silicon diodes connected in series provide a regulated 1.8 volt output, etc. 
         [0004]    Although the configuration proposed above does provide a stable reference voltage, it is well known that the forward conduction characteristics of semiconductor junctions change with temperature. As temperature rises, the forward voltage drop is altered, resulting in a negative temperature coefficient, which undesirably changes the output voltage. Similarly, as temperature falls, the forward voltage is also altered, resulting in a positive temperature coefficient, which also undesirably alters the output voltage, albeit with an opposite effect. 
         [0005]    Improved bandgap voltage references have been proposed which employ various compensation schemes that attempt to normalize output voltage over a wide temperature range. Such bandgap reference circuits are transistor-based and operate on the principle of compensating the negative temperature coefficient of a base-emitter voltage (V BE ) of a bipolar transistor with the positive temperature coefficient of the thermal voltage, (i.e., with V Thermal =k*(T/q), where k is Boltzmann&#39;s constant, T is the absolute temperature in degrees Kelvin, and q is the electronic charge). In general, the negative temperature coefficient of the base-emitter voltage V BE  is summed with the positive temperature coefficient of the thermal voltage V Thermal , which is appropriately scaled such that the resultant summation provides a small or negligible temperature coefficient over a fairly wide temperature range. 
         [0006]    More specifically, a reference voltage is typically obtained by combining two generated voltages having equal and opposite temperature coefficients (TC). One is the base-emitter voltage (V BE ) of a forward biased bipolar transistor Q REF  with a TC of about −2 mV/° C. This voltage is said to be complementary to absolute temperature voltage (V CTAT ) and can be expressed as: 
         [0000]        V   CTAT   =V   BE ( T   R )− V   G0 −[( V   G0   −V   BE ( T   0 ))*( T/T   0 )]+[( kT/q )*( n−m )*ln { T/T   0 )]  (1) 
         [0000]    where V G0  is the extrapolated bandgap voltage at 0 degrees K, and n and m are process related parameters representing, respectively, the temperature variation of mobility and collector current. T 0  is the temperature at which V BE  is measured, T is the Kelvin temperature, k is Boltzmann&#39;s constant, q is the charge on the electron, and V BE (T R ) is the base-emitter voltage at the reference temperature T R . 
         [0007]    To generate the bandgap, reference circuits typically employ two groups of transistors running at different current densities. For example, one group of transistors will typically run at about ten times the current density of the other group. This causes a 60 mV difference between the base-emitter voltages of the two groups. This difference in voltage is usually amplified by a factor of about ten and is added to the base-emitter voltage. The total of these two voltages typically adds up to about 1.22 volts, which is essentially the bandgap of silicon. 
         [0008]    A typical prior art bandgap circuit  100  is shown in  FIG. 1 . Bandgap circuit  100  generally includes an NPN transistor  160  that runs at a relatively high density. NPN transistor  170  is operated at a lower density, thus the voltage at the emitter of transistor  170  is approximately 60 mV. This voltage is applied across resistor  150  and is increased by the ratio of resistor  140  to resistor  150 . If the ratio is approximately ten to one, the voltage level moves up to approximately 600 mV. This voltage is added to the base-emitter voltage of NPN transistor  180 , producing a total voltage of about 1.22 volts. Transistor  180  then amplifies the error signal through transistors  125  and  190 , which provides enough gain to shunt regulate the output voltage between nodes V+ and V− at 1.22 volts. 
         [0009]    Such conventional bandgap circuits however, are typically concerned with providing a substantially constant output voltage. Moreover, output voltage in conventional bandgap circuits is dependent on certain transistor conduction characteristics, current gain (i.e., beta), and therefore subject to change due to process and other variations associated with physical implementation. Moreover, the minimum output voltage of such references is about one bandgap, or 1.22 volts. 
         [0010]    Accordingly, in view of the foregoing, it would be desirable to provide improved reference circuitry that overcomes these and other drawbacks. 
       SUMMARY OF THE INVENTION 
       [0011]    It is therefore an object of the present invention to provide circuits and methods that improve the performance of electronic reference circuitry, at least in part, by providing a substantially constant output current instead of voltage and reducing or eliminating output current variance based on certain physical process characteristics. 
         [0012]    In one embodiment of the present invention, the bandgap reference circuit is configured to provide a substantially constant output current, the bandgap reference including a reference generator circuit, the reference generator circuit including a first transistor running at a first predefined current, a second transistor running at a second predefined current, wherein the first current is substantially defined by the second current minus a third predefined current; and an output circuit coupled to the reference generator circuit that provides the substantially constant output current proportional to the second predefined current. 
         [0013]    In another embodiment of the present invention, a bandgap reference circuit is provided that generates a substantially constant output current and includes a reference generator circuit that generates a substantially constant output current as temperature changes, an output circuit that provides the substantially constant output current based on the output current of the reference generator, and a regulator circuit coupled to the reference generator circuit and the output circuit, the regulator circuit forming a feedback loop that controls the output current of the output circuit to be substantially constant and proportional to the output current of the reference generator circuit. 
         [0014]    Another embodiment of the present invention is directed toward a method of providing a substantially constant output current, including generating a first predefined current with a first transistor in a reference generator circuit, generating a second predefined current with a second transistor in a reference generator circuit, wherein the first predefined current is substantially defined by the second current minus a third predefined current, and providing the substantially constant output current with an output circuit based on the second predefined current. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The above and other objects and advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
           [0016]      FIG. 1  is a schematic diagram of a prior art bandgap reference circuit; 
           [0017]      FIG. 2  is a generalized block diagram of one embodiment of a reference circuit constructed in accordance with the principles of the present invention; 
           [0018]      FIG. 3  is a schematic diagram of another embodiment of a reference circuit constructed in accordance with the principles of the present invention; 
           [0019]      FIG. 4  is a diagram of another embodiment of a reference circuit constructed in accordance with the principles of the present invention; and 
           [0020]      FIG. 5  is a more detailed schematic diagram of another embodiment of a reference circuit constructed in accordance with the principles of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    A block diagram of one embodiment of a bandgap reference circuit  200  constructed in accordance with the principles of present invention is shown in  FIG. 2 . As shown, reference circuit  200  generally includes a bias circuit  202 , a reference generator circuit  208  and regulation circuit  209 . In operation, bias circuit  202  may be activated such that it provides current to reference generator circuit  208  and regulation circuit  209 , turning reference circuit  2000 N. Such activation may be automatic based on a power connection or may be selectively enabled as desired. 
         [0022]    After a startup period, reference circuit  200  reaches steady state and may operate as follows. Reference generation circuit  208  receives current from bias circuit  202  and provides one or more outputs (e.g., current) to regulator  209  that remain substantially constant over an extended temperature range. This is achieved through the use of various temperature compensation techniques described herein. Regulator  209  may compare or otherwise evaluate this signal with respect to a rail current or bias current provided by bias circuit  202  in order to generate a difference signal or other control signal that regulates the output of reference circuit  200 . 
         [0023]    This control signal may be used as a part of a feedback loop to regulate the output signal of regulator  200  (and/or to drive other components that produce an output signal which is equal or proportional to the output signal of reference generator circuit  208  with respect to a bias signal). This arrangement allows reference circuit  200  to maintain a constant output signal despite a poorly specified or fluctuating power source or bias signal. 
         [0024]    In addition, regulator  209  may be configured to provide certain correction functions for circuitry in reference generator circuit  208 . For example, as shown, reference generator circuit  208  may be constructed using one or more semiconductor devices such as bipolar transistors. Generating the substantially constant output signal described above may involve the use of components or networks in reference generator circuit  208  that experience voltage or current drops associated with operating conditions of such components. These changes may introduce errors to certain portions to circuit  208 . Regulator  209  may be coupled with such circuitry to correct or otherwise compensate for such errors. 
         [0025]    In certain embodiments, regulator circuit  209  may be configured as a buffer or other amplifier, with its output signal used as the output of reference  200  (not shown). In this case, the input signal to regulator  209  may or may not be compared to a bias or other power signal. Moreover, in such embodiments, the output of regulator  209  may not be provided to bias circuit  202  and may be used to directly drive other circuitry or components (e.g., such as external circuitry or other components). 
         [0026]    In other embodiments, however, regulator  209  may provide its output to bias circuit  202 , which may be used to drive bias circuitry and/or certain output circuitry to generate a substantially constant reference signal I OUT  (discussed in more detail below). In such embodiments, bias circuitry and output circuitry may share a common drive signal which may result in the same or similar operating conditions of the circuits, allowing reference  200  to maintain a substantially constant output signal despite power supply fluctuation. Furthermore, reference circuit  200  may, in certain embodiments, include an optional precision resistor  250  if it is desired to generate a reference voltage V OUT  based on I OUT . 
         [0027]    Referring now to  FIG. 3 , one possible specific implementation  300  constructed in accordance with the principles of the present invention is shown. Circuit  300  is similar in certain respects to the circuit described in  FIG. 2  and generally includes components and functional blocks which have been numbered similarly to denote similar functionality and general correspondence. For example, circuit  300  includes a bias circuit  302  (bias circuit  202  in  FIG. 2 ), a reference generator circuit  308  (reference circuit  208  in  FIG. 2 ) and regulator circuit  309  (regulator  209  in  FIG. 2 ). 
         [0028]    As shown, bias circuit  302  may include PNP transistors  325 ,  330 ,  335 ,  340  and  345 . In this example, the transistors are illustrated as bipolar junction transistors (BJTs), however, other suitable semiconductor devices, such as p-channel FETS, may be used if desired. In this embodiment, transistor  340  is depicted as a diode connected transistor, which is connected to ground through resistor  316 , and is used as “start up circuitry” to begin conduction within circuit  300 . However, other suitable start up circuitry may be used if desired. As shown, the bias transistors may be base connected to one another forming a current mirror and be of similar size. In other embodiments, however, transistor  325  may be somewhat larger than the others (e.g., four times larger in area) in order to provide additional current to portions of reference generator circuit  308 . 
         [0029]    In operation, when current source  301  is applied to the common emitter node of the PNP transistors in bias circuit  302 , diode connected transistor  340  turns ON, and applies a drive signal to the common base of transistors  325 ,  330 ,  335  and  345  turning them ON. This turns bias circuit  302  ON, thus providing current to reference generator circuit  308  and regulator circuit  309 , turning them ON as well. Because output transistor  345  is connected to the base of other transistors in bias circuit  302 , its collector output will mirror the current provided by other similarly sized transistors in circuit  302 . 
         [0030]    As shown in  FIG. 3 , reference generator circuit  308  may include NPN transistors  305  and  306 , resistors  303 ,  304  and  307 . Generally speaking, transistors  305  and  306  operate such that they produce a substantially constant voltage at the emitter of transistor  306 , which in turn generates a substantially constant current across resistor  307 . As a result, the current flowing through PNP transistor  330  is regulated to substantially the same as the current flowing through transistor  306  (plus a base current correction factor). This causes the current flowing through transistor  345  to mirror the current developed across resistor  307 , thus producing the substantially constant output current I OUT  at its collector. 
         [0031]    More specifically, reference generator circuit  308  operates as follows. Transistors  305  and  306  may be constructed such that there is a significant size difference between the two and thus a significant difference in their respective current densities (e.g., transistor  306  may be ten times the size of  305 ). This difference provides a component which is proportional to absolute temperature or exhibits a positive temperature coefficient. This may be represented by the difference in the base-emitter voltage of transistors  305  and  306  and may be expressed as equation (2): 
         [0000]      Δ V   BE =( kT/q )*ln( J   1   /J   2 )  (2) 
         [0000]    where k is Boltzmann&#39;s constant, T is the absolute temperature in degrees Kelvin, q is the electronic charge, J 1  is the current density of transistor  305 , and J 2  is the current density of transistor  306 . 
         [0032]    Another portion of the reference generator circuit  308  may include an amplification component which may be constructed with NPN transistor  305  and resistors  303  and  304 . This amplification portion may be constructed as a V BE  multiplier based on the ratio of resistors  303  and  304  and the V BE  Of transistor  305 . 
         [0033]    Thus, in operation, current provided to the collector of NPN transistor  305  causing its emitter-base voltage to be impressed across resistor  304 . The current through resistor  304  flows through resistor  303 , which generates a voltage across resistor  303  proportional to the ratio of resistor  303  to resistor  304  and the V BE  of NPN transistor  305 . As shown, this voltage is applied to the base of transistor  306  and thus the resultant voltage across resistor  307  is a combination of the V BE  voltage of transistor  306  plus the difference in emitter-base voltage due to the area ratio of transistor  305  to transistor  306 . The current in the collector of NPN transistor  306  is thus equal to the current flow in its emitter minus its base current. 
         [0034]    With this configuration, if the value of resistor  303  is selected properly using known techniques (e.g., in view of the area ratio of transistors  303  and  304 ) the voltage across resistor  307  will remain constant over an extended temperature range. As explained above, transistors  305  and  306  do not operate with a fixed current density ratio as temperature changes. Transistor  305  operates at substantially the same current as transistor  325  minus the current flow through resistors  303  and  304 . This decrease in current (which is proportional to V BE ) varies with temperature (as does the current through the resistors) and thus provides compensation for second order errors (sometimes referred to as “curvature compensation”). If desired, the amount of compensation provided can be altered by adjusting the proportion of current flowing through resistors  303  and  304  compared to the current flowing through transistor  305 . 
         [0035]    Because the voltage across resistor  307  is substantially constant, the current through resistor  307  is substantially constant as well. However, the current at the collector of transistor  306  is lower than the current at its emitter by the value of its base current. As a result, the current drawn from transistor  330  by transistor  306  does not fully reflect the current in resistor  307  and therefore introduces an error factor into reference circuit  300 . 
         [0036]    This error factor may be corrected by coupling the base of transistor  315  in regulator circuit  309  to the collector of transistor  306 . If transistors  306  and  315  are constructed such that they are substantially the same size, and operate at substantially the same current, the base current missing from the collector of transistor  306  may be added into the circuit by transistor  315 . With this correction, the current drawn from transistor  330  is substantially equal to the current through resistor  307 . This causes current mirrored to transistor  345  from transistor  306  to be substantially equal to the current in resistor  307 . 
         [0037]    As shown in  FIG. 3 , regulator circuit  309  may include NPN transistor  315  and PNP transistor  320 . In operation, this circuit may act as a feedback loop with transistor  315  driving the base of transistor  320  as a shunt regulator to maintain the current of transistor  330  substantially equal to the current at the collector of transistor  306 . As a result, the mirror current through transistors  325 ,  335 ,  340  and  345  also remain substantially constant with temperature variation. The current through the mirror in bias circuit  302  is varied to match the current in transistor  330  because the current in resistor  316  varies due the voltage across the regulation loop. Regulator  309  may also establish the voltage at the collector of transistor  306 . In this way, reference  300  provides a robust rejection of bias fluctuation and provides an output current that is substantially constant over an extended temperature range. 
         [0038]    In some implementations of the present invention, it may be desirable to trim certain components to ensure that the output current of reference  300  is within acceptable tolerances. In this case, it may be desirable at some point in the manufacturing process and test reference  300 , and if necessary, trim the value of resistor  307  to ensure output accuracy or establish a desired current value. Furthermore, trimming resistor  307  to set the output current in feedback loop created by regulator circuit  309  also changes the current in transistors  305  and  306 . This change in current as a function of trimming resistor  307  helps keep the transistors operating at approximately the same current density so that trimming the output current has a minimal effect on temperature coefficient of reference  300 . 
         [0039]    An additional advantage of reference  300  is that transistors  325 ,  330 , and  335  may operate at substantially the same collector voltage. Because transistors  325  and  330  operate at substantially the same collector to base voltage, better matching is achieved. Moreover, if the collector of transistor  345  is used to drive a resistor to ground to obtain a constant output voltage, then the collector to base voltage of transistors  330  and  345  are also approximately equal. It will be appreciated that although transistor  345  is depicted as part of bias circuit  202 , that its primary function is to provide output current for reference  300  and thus may be viewed as an output circuit. 
         [0040]    Furthermore, in some embodiments, it may desirable to introduce additional components to reduce or eliminate certain undesirable effects associated with process variations such as variation of transistor base width which may cause changes in certain conduction characteristics such as current gain (beta values) and/or V BE . One way this may be accomplished is by the introduction of optional resistor  310  (shown in dotted lines) between the collector of transistor  305  and the base of transistor  306 . If the proper value of optional resistor  310  is obtained, the effects of beta variation may be minimized or substantially cancelled. This, however, may require trimming resistor  310  (or precision fabrication). 
         [0041]    The beta variation mentioned above with respect to transistors  305  and  306  also results in changes to their V BE . Moreover, the base current of transistor  305  flows through the resistance of its associated bias network (e.g., the parallel resistance of resistors  303  and  304 ) adding a temperature drift component to the output current. 
         [0042]    Changes in V BE  alter the temperature drift of reference generator circuit  308 . For many fabrication processes, the base current of an NPN transistor has a negative temperature coefficient (e.g., increasing as temperature decreases). The base current of transistor  305 , and its associated temperature coefficient, may be used to minimize the changes in drift of reference generator  308  as beta varies with process. The change in temperature coefficient due to changes in V BE  are opposite to changes in drift from the base current of transistor  305 . Additional compensation may be obtained by adding an optional resistor  346  in series with the base of transistor  305 . 
         [0043]    Optional resistor  310  has the opposite effect on drift compared with the effects of the base current flow through resistors  303  and  304  as temperature varies. The addition of optional resistor  310  may cause the drift of reference generator circuit  308  to be substantially independent of beta or base current. However, changes in drift V BE  to V BE  variation shall occur. The base current of transistor  305  is substantially canceled by base current of transistor  306 . 
         [0044]    Referring now to  FIG. 4 , another specific implementation  400  constructed in accordance with the principles of the present invention is shown. Circuit  400  is similar in many respects to the circuit described in  FIG. 3  and generally includes components and functional blocks which have been numbered similarly to denote similar functionality and general correspondence. For example, circuit  400  includes a bias circuit  402  (bias circuit  302  in  FIG. 3 ), a reference generator circuit  408  (reference circuit  308  in  FIG. 3 ) and amplifier circuit  409  (amplifier  309  in  FIG. 3 ). 
         [0045]    As shown, reference  400  may operate in substantially the same way as reference  300 , with the exception of amplifier circuit  409  and diode  418 . In operation, diode  418  and resistor  419  may set a collector voltage on transistor  406  when a bias current is applied to its anode. Amplifier  415  drives bias circuit  402  to control the collector current of transistors  425 ,  430 , and  445 . 
         [0046]    As shown, circuit  400  includes an amplifier circuit  409  and does not operate with the shunt topology shown in  FIG. 3 . With this arrangement, the output of reference generator circuit  408  is compared to the collector current of transistor  430  (at the non-inverting input of amplifier  415 ). Amplifier  415  compares the output of reference generator circuit  408  with the current provided by bias circuit  402 . The difference between the collector current of transistors  406  and  430  is used in a feedback loop to regulate the current produced by bias circuit  402 . This arrangement allows reference circuit  400  to maintain a constant output current with changes in supply voltage. 
         [0047]    Referring now to  FIG. 5 , another specific implementation  500  constructed in accordance with the principles of the present invention is illustrated. Circuit  500  is similar in many respects to the circuit described in  FIGS. 3 and 4  and generally includes components and functional blocks which have been numbered similarly to denote similar functionality and general correspondence. For example, circuit  500  includes a reference generator circuit  508  (reference circuit  308  in  FIG. 3 ) and amplifier circuit  509  (amplifier  309  in  FIG. 3 ). 
         [0048]    As shown in  FIG. 5 , reference generator circuit  508  may include NPN transistors  505  and  506  and resistors  503 ,  504 ,  507 ,  510 ,  516  and  546 . Similar to reference generator  308  of circuit  300 , transistors  505  and  506  operate such that they produce a substantially constant voltage at the emitter of transistor  506 , which in turn generates a substantially constant current across resistor  507 . Amplifier circuit  509  matches the collector current of transistor  506  which is used to drive PNP transistors  535  and  545 , which provides the substantially constant output current I OUT  (discussed in more detail below). 
         [0049]    More specifically, transistors  505  and  506  may be constructed such that there is a significant difference in their respective current densities (e.g., transistor  506  operates at a lower current density than transistor  505 ) which provides a component which is proportional to absolute temperature or exhibits a positive temperature coefficient. 
         [0050]    In operation, current provided to the collector of transistor  505  causes its emitter-base voltage to be impressed across resistor  504 . The current through resistor  504  flows through resistor  503 , which generates a voltage across resistor  503  proportional to the ratio of resistor  503  to resistor  504  and the V BE  of transistor  505 . As shown, optional resistor  546  may be added if desired which causes an additional voltage drop but provides improved rejection in the case of bias current fluctuation (and may be added to circuits  308  and  408 , if desired). 
         [0051]    Thus, the resultant voltage across resistor  507  is the combination of the fractional V BE  voltage of transistor  505  plus the difference in emitter-base voltage due to the area ratio of transistor  505  to transistor  506 . The current in the collector of transistor  506  is equal to the current flow in the emitter minus its base current. Optional resistor  516  may also be added if desired to provide a more stable collector voltage in the case where the bias current varies somewhat. 
         [0052]    With this configuration, if the value of resistor  503  is selected using known techniques, the voltage across resistor  507  will remain constant over an extended temperature range. As explained above, transistors  505  and  506  do not operate with a fixed current density ratio as temperature changes. Transistor  505  operates at substantially the same current as transistor  525  minus the current flow through resistors  503  and  504 . This decrease in current (which is proportional V BE ) varies with temperature and thus provides compensation for second order errors. If desired, the amount of compensation provided can be altered by adjusting the proportion of current flowing through resistors  503  and  504  compared to the current flowing through transistor  505 . 
         [0053]    Because the voltage across resistor  507  is substantially constant, the current through resistor  507  is substantially constant as well. However, the current at the collector of transistor  506  is lower than the current at its emitter by the value of its base current. As a result, the current drawn from transistor  535  by transistor  506  does not fully reflect the current in resistor  507  and therefore introduces an error factor into reference circuit  500 . 
         [0054]    This error factor, however, may be corrected by coupling the base of transistor  511  in amplifier circuit  509  to the collector of transistor  506 . If transistors  506  and  511  are constructed such that are substantially the same size, and operate at substantially the same current, the base current missing from the collector of transistor  506  may be added into the circuit by transistor  511 . With this correction, the current drawn from transistor  535  is substantially equal to the current through resistor  507 . Accordingly, current mirrored to transistor  545  from transistor  506  is substantially equal to the current in resistor  507 . 
         [0055]    As shown in  FIG. 5 , circuit  500  includes an amplifier circuit  509  with transistors  511 - 514 ,  516 - 517 , resistors  531 - 534  and capacitor  536 . In operation, transistors  511  and  512  (which may be the same or similar in size) receive inputs from a bias voltage VB and from the collector of transistor  506 . Transistors  511  and  512  may form a differential amplifier (biased by diode connected transistors  513  and  514 ) which sets the voltage on the collector of transistor  506  substantially equal to the bias voltage applied at the base of transistor  512  (based on a single ended output at the collector of transistor  512 ). 
         [0056]    Diode connected NPN transistor  516  drives the common base of PNP transistors  525 ,  535  and  545  (with respect to the emitter of transistor  517 ) such that the collector current of transistor  535  substantially matches the collector current for transistor  506 . If transistors  506  and  511  run at approximately the same operating current, the base current of transistor  511  compensates for the loss of base current in transistor  506 . This circuit regulates the currents through the PNP transistors  535  and  545  to provide a current I OUT  that is substantially constant despite power supply and temperature changes. In some embodiments, for optimal regulation and accuracy of bandgap circuit  500 , the voltage at the collector of transistor  545  should be about the same as the collector voltage on transistor  535 . 
         [0057]    Moreover, as shown in  FIG. 5 , voltage reference  500  may include the bias circuitry formed by NPN transistors  518 ,  519  and  521  along with current source  522 . In operation, diode connected transistor  521  turns ON when current is provided from current source  522  and provides voltage to the base of transistors  518  and  519 , turning them ON. These transistors act as bias circuitry to amplifier circuit  509  and set the operating range of reference  500 . 
         [0058]    It will be understood that unlike circuit  300 , circuit  500  operates from a voltage source rather than a current source. The circuit shown in  FIG. 3  is a shunt regulator driven by a current  301 . Circuit  500  uses voltage source V IN , and has regulation circuitry that effectively rejects supply variation. 
         [0059]    Although preferred embodiments of the present invention have been disclosed with various circuits connected to other circuits, persons skilled in the art will appreciate that it may not be necessary for such connections to be direct and additional circuits may be interconnected between the shown connected circuits without departing from the spirit of the invention as shown. Persons skilled in the art also will appreciate that the present invention can be practiced by other than the specifically described embodiments. The described embodiments are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims which follow.