Abstract:
Voltage dividing resistors (R 1   a , R 1   b , R 2   a , R 2   b ) are connected in parallel with diode connected bipolar transistors (Q 1 , Q 2 ) for splitting the voltage to the inputs of an operational amplifier ( 62, 82 ). Current is provided to this arrangement by current sources (I 1 , I 2 ). When the supply voltage is about 0.85 volts, a temperature insensitive reference voltage of about 200 millivolts is available at the drain of a second transistor (M 2 , M 2 ).

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates to a voltage reference circuit with low sensitivity to temperature, and more specifically, to a low-voltage bandgap reference circuit. 
   2. Description of the Prior Art 
   Reference voltage generators are widely used in both analog and digital circuits such as DRAM and flash memories. A bandgap reference (also termed BGR) is a circuit that provides a stable output voltage with low sensitivity to temperature and supply voltage. 
   A conventional bandgap reference output is about 1.25 V, which is almost equal to the silicon energy gap measured in electron volts. However, in modern deep-submicron technology, a voltage of around 1 V is preferred. As such, the conventional bandgap reference is inadequate for current requirements. 
   The 1 V minimum supply voltage is constrained by two factors. First, the reference voltage of about 1.25 V exceeds 1 V. Second, low voltage design of proportional-to-absolute-temperature (PTAT) current generation loops is limited by the input common-mode voltage of the amplifier. The effects of these constraints can be reduced by resistive subdivision methods and by using low threshold voltage devices or BiCMOS processes. However, both of these solutions require costly special process technology. 
   Bandgap references can be divided into two groups: type-A and type-B. Type-A bandgap references sum voltages of two elements having opposite temperature components. Type-B bandgap references combine the currents of two elements. Both type A and type B bandgap references can be designed to function with a normal supply voltage of greater than 1 V and a sub-1-V supply voltage. 
     FIG. 1  illustrates a conventional type-A bandgap reference circuit  10 . The bandgap reference circuit  10  includes an operational amplifier  12 , two transistors M 1  and M 2 , two resistors R 1  and R 2 , and two diodes Q 1  and Q 2 . The sources of the transistors M 1 , M 2  are connected to a supply voltage V DD . The drain of the transistor M 1  is connected to the emitter of the diode Q 1  through the resistor R 1  and to the positive input of the amplifier  12 . Similarly, the drain of the transistor M 2  is connected to the emitter of the diode Q 2  through the resistor R 2  and to the negative input of the amplifier  12 . The gates of the transistors M 1 , M 2  are connected to the output of the amplifier  12 . In CMOS applications, each diode Q 1 , Q 2  is formed with a parasitic vertical bipolar transistor having a collector and base connected to ground. 
   Neglecting base current, the emitter-base voltage of a forward active operation diode can be expressed as: 
                 V   EB     =       kT   q     ⁢     ln   ⁡     (       l   c       l   s       )           ,           (   1   )             
 
where:
         k is Boltzmanns constant (1.38×10 −23 J/K),   q is the electronic charge (1.6×10 −19 C),   T is temperature,   I C  is the collector current, and   I S  is the saturation current.       

   When the input voltages of the amplifier  12  are forced to be the same, and the size of the diode Q 1  is N times that of the diode Q 2 , the emitter-base voltage difference between diodes Q 1  and Q 2 , ΔV EB , becomes: 
                 Δ   ⁢           ⁢     V   EB       =         V   EB2     -     V   EB1       =       kT   q     ⁢   ln   ⁢           ⁢   N         ,           (   2   )             
 
where:
         V EB1  is the emitter-base voltage of diode Q 1 , and   V EB2  is the emitter-base voltage of diode Q 2 .       

   Finally, when the current through resistor R 1  is equal to the current through resistor R 2  and is set to be PTAT, an output reference voltage, V REF , can be obtained by: 
                 V   REF     =         V   EB2     +         R   2       R   1       ⁢   Δ   ⁢           ⁢     V   EB         ≡     V     REF   -   CONV           ,           (   3   )             
 
where:
         R 1  is the resistance of resistor R 1 ,   R 2  is the resistance of resistor R 2 , and   V REF-CONV  is the reference voltage (conventional).       

   The emitter-base voltage, V EB , has a negative temperature coefficient of −2 mV/° C., while the emitter-base voltage difference, ΔV EB , has a positive temperature coefficient of 0.085 mV/° C. Hence, if a proper ratio of resistances of resistors R 1  and R 2  is selected, the output reference voltage, V REF , will have low sensitivity to temperature. In general, the supply voltage, V DD , is set to about 3-5 V and the output reference voltage, V REF , is about 1.25 V, as the conventional bandgap circuit  10  cannot be used at a lower voltage such as 1 V. 
     FIG. 2  illustrates a conventional type-B bandgap reference circuit  20 . Elements in  FIG. 2  having the same reference numbers of those in  FIG. 1  are the same. The bandgap reference circuit  20  includes an operational amplifier  22 ; three transistors M 1 , M 2 , and M 3 ; four resistors R 1 , R 2 , R 3 , and R 4 ; and two diodes Q 1  and Q 2  interconnected as illustrated in FIG.  2 . 
   Compared with the type-A circuit  10 , the type-B circuit  20  is more suitable for operating with a low supply voltage. Instead of stacking two complementary voltages, the type-B bandgap reference  20  adds two currents with opposite temperature dependencies. In the bandgap reference of  FIG. 2 , the current through the resistor R 3  is PTAT. If the resistances of the resistors R 1  and R 2  are equal, then the current through the MOS transistor M 3  mirrored from transistors M 1  and M 2  can be expressed as: 
                 I   M3     =       1     R   1       ⁢     (       V   EB2     +         R   1       R   3       ⁢     kT   q     ⁢   ln   ⁢           ⁢   N       )         ,           (   4   )             
 
with the reference voltage being expressed as: 
               V   REF     =           R   4       R   1       ⁢     (       V   EB2     +         R   1       R   3       ⁢     kT   q     ⁢   ln   ⁢           ⁢   N       )       =         R   4       R   1       ·     V     REF   -   CONV                   (   5   )             
 
   Thus, in the bandgap reference circuit  20  of  FIG. 2 , as ratios of resistances are key, the variations in individual resistances due to process conditions does not greatly affect the reference voltage. In general, the supply voltage, V DD , is set to about 1.5 V and the output reference voltage, V REF , is about 1.2 V. 
     FIG. 3  illustrates a conventional type-B bandgap reference circuit  30  capable of sub-1-V operation. Elements in  FIG. 3  having the same reference numbers of those in  FIG. 2  are the same. The bandgap reference circuit  30  includes an operational amplifier  32 ; three transistors M 1 , M 2 , and M 3 ; six resistors R 1   a , R 1   b , R 2   a , R 2   b , R 3 , and R 4 ; and two diodes Q 1  and Q 2  interconnected as illustrated in FIG.  3 . The supply voltage is limited by the input common-mode voltage of the amplifier  32 , which must be low enough to ensure that the input pair operate in the saturation region. 
   The improvement of low supply voltage realized with the bandgap reference circuit  30  is based on the positions of the input pair of the operational amplifier  32 . The established feedback loop produces a PTAT voltage across the resistor R 3 . The resistance ratio of the resistors R 1   a  and R 2   a  causes the voltage between the supply voltage and the input common voltage of the operational amplifier  32  to become increased. This makes the p-channel input pair operate in the saturation region even when the supply voltage is under 1V. The sub-1-V reference voltage output by the circuit  30  can be expressed as: 
                 V     REF   -   SUB1V       =           R   4       R   1       ⁢     (       V   EB2     +         R   1       R   3       ⁢     kT   q     ⁢   ln   ⁢           ⁢   N       )       =         R   4       R   1       ·     V     REF   -   CONV             ,           (   6   )             
 
which is similar to the circuit  20  of FIG.  2 . During operation of the circuit  30 , the supply voltage, V DD , is set to about 1.0-1.9 V and the output reference voltage, V REF , is about 0.6 V.
 
   Given the state-of-the-art bandgap reference circuits  10 ,  20 , and  30  described above, it is clear that an improved and inexpensive low-voltage bandgap reference circuit is required. 
   SUMMARY OF INVENTION 
   It is therefore a primary objective of the claimed invention to provide a low-voltage bandgap reference circuit having low sensitively to temperature. 
   Briefly summarized, the claimed invention includes an operational amplifier, a first transistor having a source connected to a first voltage and a drain connected to a positive input end of the operational amplifier and separately to a first node through a first resistor, a second transistor having a source connected to the first voltage and a drain connected to a second node through a second resistor, the second node being connected to the negative input end of the operational amplifier, and a gate of the second transistor being connected to a gate of the first transistor and the output of the operational amplifier. The claimed invention further includes a third resistor and a fourth resistor connected in series at the first node, the third and fourth resistors connected in parallel with a first diode between a first current source and a second voltage; and a fifth resistor and a sixth resistor connected in series at the second node, the fifth and sixth resistors connected in parallel with a second diode between a second current source and the second voltage. 
   It is an advantage of the claimed invention that a temperature insensitive reference voltage of less than 1 volt can be obtained at the drain of the second transistor when the first voltage is set appropriately relative to the second voltage. 
   It is a further advantage of the claimed invention that the bandgap reference circuit is compatible with established CMOS technology. 
   It is a further advantage of the claimed invention that no low-threshold voltage or BiCMOS devices are required. 
   These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a circuit diagram of a conventional bandgap reference. 
       FIG. 2  is a circuit diagram of a conventional low-voltage bandgap reference. 
       FIG. 3  is a circuit diagram of a conventional low-voltage bandgap reference. 
       FIG. 4  is a graph of base-emitter voltage versus temperature of two diodes of a bandgap reference. 
       FIG. 5  is a graph of the difference of the diode base-emitter voltages of  FIG. 4  versus temperature. 
       FIG. 6  is a circuit diagram of a first embodiment of the present invention bandgap reference. 
       FIG. 7  is a circuit diagram of an operational amplifier used in the bandgap reference of FIG.  6 . 
       FIG. 8  is a circuit diagram of a second embodiment of the present invention bandgap reference. 
       FIG. 9  is a circuit diagram of an operational amplifier used in the bandgap reference of FIG.  8 . 
       FIG. 10  is a graph of reference voltage versus temperature showing lines of constant supply voltage for the bandgap reference of FIG.  6 . 
       FIG. 11  is a graph of reference voltage versus supply voltage showing lines of constant temperature for the bandgap reference of FIG.  6 . 
   

   DETAILED DESCRIPTION 
   As a basis for the explaining the present invention, please refer to FIG.  4  and FIG.  5 .  FIG. 4  illustrates base-emitter voltage of two diodes Q 1 , Q 2  (discussed later) with respect to temperature.  FIG. 5  illustrates the difference of the diode base-emitter voltages with respect to temperature. It can be seen that the base-emitter voltage, V EB , has a negative temperature coefficient of about −2 mV/° C. with V EB =0.55 V and T=300 K. The difference of the diode base-emitter voltages, ΔV EB , with respect to temperature, as shown in  FIG. 5 , is used in the present invention to produce a PTAT to eliminate the effect of the negative temperature coefficient. 
   Please refer to  FIG. 6  which illustrates a circuit diagram of a bandgap reference circuit  60  according to a first embodiment of the present invention. The circuit  60  is a CMOS circuit and includes a p-channel operational amplifier  62 , and a first PNP transistor M 1  having a source connected to a positive voltage V DD  and a drain connected to the positive input end of the amplifier  62 . The drain of the transistor M 1  is further connected to a first node n 1  through a first resistor R 3 . A second PNP transistor M 2  has a source connected to the voltage V DD  and a drain connected to a second node n 2  through a second resistor R 4 . The gate of the transistor M 2  is connected to the gate of the transistor M 1 , the gates of the transistors M 1 , M 2  both being connected to the output of the operational amplifier  62 . The second node n 2  is connected to the negative input end of the amplifier  62 . The circuit  60  further includes a third resistor R 1   a  and a fourth resistor R 1   b  connected in series at the first node n 1 . The resistors R 1   a , R 1   b  are connected in parallel with a first bipolar PNP diode Q 1 , which has a collector and a base connected to ground and an emitter connected to the current source I 1 . Finally, the circuit  60  includes a fifth resistor R 2   a  and a sixth resistor R 2   b  connected in series at the second node n 2 . The fifth and sixth resistors R 2   a , R 2   b  are connected in parallel with a second diode Q 2 , which has a collector and a base connected to ground and an emitter connected to the current source I 2 . 
     FIG. 7  illustrates one possible circuit for the p-channel operational amplifier  62 . The operational amplifier  62  comprises a third PNP transistor M 4  having a source connected to the voltage V DD  and a drain connected to sources of fourth and fifth PNP transistors M 5 , M 6 . The drains of the transistors M 5 , M 6  are respectively connected to drains of sixth and seventh NPN transistors M 7 , M 8 . The NPN transistors M 7 , M 8  have sources grounded and gates mutually connected and also connected with the drain of the transistor M 7 . 
   Given the amplifier  62 , the minimum supply voltage is ex-pressed as:
 
 V   DD(min)   =V   IN(max)   +|V   TP |+2 ·|V   DSsat |  (7) 
 
where the voltages V TP  and V DSsat  are as illustrated in FIG.  7 . Thus, the reference voltage, V REF , of the present invention bandgap reference circuit  60  is: 
                 V   REF     =         R     1   ⁢   b           R     1   ⁢   a       +     R     1   ⁢   b           ⁡     [         (       R   4     +         R     1   ⁢   a       ⁢     R     1   ⁢   b             R     1   ⁢   a       +     R     1   ⁢   b             )     ⁢       Δ   ⁢           ⁢     V   EB         R   3         +     V   EB2       ]         ,           (   8   )             
 
where:
         R 1a , R 1b , R 3 , and R 4  are the resistances of the resistors R 1   a , R 1   b , R 3 , and R 4 , respectively,   ΔV EB  is the emitter-base voltage difference between the diodes Q 1  and Q 2 , and   V EB2  is the emitter-base voltage of the diode Q 2 .       

   And finally, the minimum supply voltage of the bandgap reference is effectively reduced as described by combining (7) and (8) such that: 
               V     DD   ⁡     (   min   )         =           R     1   ⁢   b           R     1   ⁢   a       +     R     1   ⁢   b           ⁢     (       V   EB2     +         R     1   ⁢   a         R   3       ⁢       R     1   ⁢   b           R     1   ⁢   a       +     R     1   ⁢   b           ⁢   Δ   ⁢           ⁢     V   EB         )       +          V   TP          +     2   ·          V   DSsat                      (   9   )             
 
   Simulation results for the bandgap reference are as shown in FIG.  10  and FIG.  11 .  FIG. 10  shows reference voltage, V REF , versus temperature showing lines of constant supply voltage, V DD ; while  FIG. 11  shows reference voltage, V REF , versus supply voltage, V DD , showing lines of constant temperature. In view of these results the preferred operating supply voltage, V DD , of the bandgap reference  60  is greater than about 0.85 V, that is, where the reference voltage is least sensitive to temperature. For other embodiments, the preferred supply voltage may be different. 
   In normal operation of the bandgap reference circuit  60 , the voltage V DD  is set to about 0.85 V, a temperature-insensitive reference voltage, V REF , of about 200 mV with an effective temperature coefficient of 58.1 ppm/° C. is output at the drain of the transistor M 2 . 
   A second embodiment of the present invention is illustrated in  FIG. 8 , which shows a bandgap reference circuit  80 . The second embodiment circuit  80  is an NMOS version of the first embodiment circuit  60 . Compared to the circuit  60 , like reference numerals indicate like components, with primed reference numerals indicating NPN rather than PNP or PNP rather than NPN. The circuit  80  is a CMOS circuit and includes an n-channel operational amplifier  82 , and a first NPN transistor M 1  which has a source connected to ground and a drain connected to the positive input end of the amplifier  82 . The drain of the transistor M 1  is also connected to the first node n 1  through the first resistor R 3 . A second NPN transistor M 2  has a source connected to ground and a drain connected to the node n 2  through the second resistor R 4 . The node n 2  is connected to the negative input end of the operational amplifier  82 . Gates of the transistors M 1 , M 2 ″ are mutually connected and further connected to the output of the operational amplifier  82 . The third resistor R 1   a  and the fourth resistor R 1   b  are connected in series at the node n 1 . The resistors R 1   a , R 1   b  are also connected in parallel with the a diode Q 1  between the first current source I 1  and voltage V DD . The diode Q 1  is a bipolar NPN transistor having its collector and base connected to voltage V DD and its emitter connected to the current source I 1 . The fifth and sixth resistors R 2   a , R 2   b  are connected in series at the node n 2 . The resistors R 2   a , R 2   b  are also connected in parallel with a diode Q 2  between a second current source I 2  and the voltage V DD . The diode Q 2  is a bipolar NPN transistor having its collector and base connected to voltage V DD  and its emitter connected to the current source I 2 . 
     FIG. 9  illustrates one possible circuit for the n-channel operational amplifier  82 . The amplifier  82  includes an NPN transistor M 4  having a source grounded and a drain connected to sources of NPN transistors M 5 , M 6 . The drains of the transistors M 5 , M 6  are respectively connected to drains of PNP transistors M 7 , M 8 . The transistors M 7 , M 8  have sources connected to the voltage V DD  and gates mutually connected and connected to the source of the PNP transistor M 7 . 
   In the bandgap reference circuit  80 , the minimum input voltage, V IN(min) , of the amplifier  82  is according to:
 
 V   IN(min)   =V   IN +2 V   DSsat   (10) 
 
where V TP  and V DSsat  are as illustrated in FIG.  9 .
 
   Operation and output of the bandgap reference  80  are similar to those of the bandgap reference  60 . One significant difference between the two present invention bandgap references  60 ,  80  is in the power supply rejection ratio (PSRR). The PNP bandgap reference  60  has a strong rejection to the positive supply, while the NPN bandgap reference  80  has a strong rejection to the negative supply. Furthermore, the NPN bandgap reference  80  has a reduced susceptibility to ground fluctuations. 
   While the bandgap reference circuits  60 ,  80  were previously described as CMOS circuits, there is no reason why they cannot be implemented with other technologies such as with discrete components, BiCMOS, or emerging semiconductor processes. Furthermore, suitable combinations, where a mix of component types are used, of current or new technologies can also be used to realize the present invention. 
   In contrast to the prior art, the present invention provides a temperature insensitive reference voltage of less than 1 V with a circuit compatible with CMOS technology. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only be the metes and bounds of the appended claims.