Abstract:
A reference circuit contains a PTAT (Proportional To Absolute Temperature) core. In the PTAT core there is a difference between the currents densities flowing through a first and second transistor. This difference results in a difference in junction voltage in the first and second transistor. The currents are adjusted by a local feedback loop in proportion to one another until the difference in junction voltage equals a voltage drop across a resistor. According to the invention the currents to both transistors are supplied by current sources, and the currents are adjusted by deviating a fraction of the supplied current from the transistors. This makes it possible to reference all control voltages for the transistors and the local feedback loop to the same supply connection, which increases the stability and power supply rejection of the circuit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an electronic circuit with a voltage and/or current reference circuit. 
     2. Description of Related Art 
     Such a circuit is known from an article titled “New class of high-performance PTAT current sources”, by H. C.Nauta and E. H.Nordholt, published in Electronics letters Vol. 21 No. 9 pages 384 to 386, April 1985 (the Nauta article). FIG. 1 shows a PTAT reference circuit disclosed in the Nauta article. 
     At the core of this PTAT reference circuit are two transistors and a resistor. Furthermore, the circuit disclosed in the Nauta article uses two (high impedance) current sources. The current sources on the one hand and the transistors and the resistor on the other hand are connected to opposite power supply poles. Thus the current sources are able to supply proportionally adjustable currents I to the transistors and the resistor (that is, the currents are adjusted so that the proportion between these currents remains fixed). 
     The PTAT reference circuit makes use of the logarithmic relation between base emitter voltage Vbe and junction current density i of bipolar transistors: 
     
       
           Vbe=kT/q  log  i/i 0  
       
     
     Here “log” is the natural logarithm and i0 is a standard current density which is substantially the same for any transistor. In the known PTAT reference circuit unequal current densities i1, i2 (where i1=n*i2) are supplied to two transistors by supplying the same current I to two transistors whose junction area differs by a factor n. As a result, there is a fixed difference dV between the base emitter voltages in the two transistors: 
     
       
           dV=kT/q  log  n    
       
     
     At the same time, the current I is fed through a resistor R, so that a voltage drop IR occurs through the resistor. A feedback loop adjusts the current supplied by the current sources so that the voltage drop compensates the dV difference between the junction. i.e. so that 
     
       
           IR=kT/q  log  n    
       
     
     Thus a reference current I is obtained. 
     The circuit disclosed in the Nauta article uses two (high impedance) current sources to supply the current I to the two transistors. This is in contrast to more conventional reference circuit designs, which use the (low impedance) input and (high impedance) output of a current mirror to supply the current I to respective ones of the transistors. By the use of two high impedance current sources, the Nauta article achieves high accuracy because it overcomes the detrimental consequences (e.g. supply voltage dependence) of the Early effect on the accuracy of the reference circuit. 
     However, it has been found that the reference circuit disclosed in the Nauta article has a potential instability problem, which can be overcome only by cumbersome additional circuits such as adding a relatively large capacitor between point A and Vnn. This capacitor undoes the elimination of the detrimental consequences of the Early effect at higher frequencies, because it causes an imbalance between the loads of the current sources; moreover the capacitor takes up circuit space. 
     BRIEF SUMMARY OF THE INVENTION 
     Amongst others, it is an object of the invention to provide for a circuit with a voltage and/or current reference circuit that achieves high accuracy and is stable even without a relatively large capacitor. 
     In the Nauta article, the feedback loop adjusts the currents from the current sources to obtain the desired current. This means that a voltage must be sensed on the transistors. This voltage is defined relative to the power supply pole of the transistors and the resistor. The sensed voltage must then be used to generate a control voltage for the current sources. This control voltage is defined relative to power supply pole of the current sources. Thus a shift of voltage reference is needed. It has been found that the circuits needed to shift from the one reference to the other give rise to the instability if no cumbersome measures are taken. 
     The need for this shift of voltage reference is removed by adjusting the current flowing the transistors by deviation of current through a deviation circuit which is connected to the same power supply pole as the transistors and the resistor. Thus stability is improved without a capacitor, at the price of a slightly increased current consumption, whereas the high accuracy may be retained. As a further advantage, the circuit does not need an additional startup circuit, as is the case for conventional PTAT current reference circuits. 
     These and other advantageous aspects of the invention will be described using the attached figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a prior art reference circuit; 
     FIG. 2 shows a first embodiment of the reference circuit according to the invention; 
     FIG. 3 shows a reference circuit with a reference current output; 
     FIG. 4 shows a reference circuit with another PTAT core; 
     FIGS. 5,  5   a  show bandgap reference circuits; 
     FIGS. 6,  6   a  show further bandgap reference circuits; 
     FIG. 7 shows a set of current sources for use in a reference circuit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 shows a reference circuit according to the invention. The circuit contains a PTAT core  20 , which comprises a first NPN transistor  200   a , a second NPN transistor  200   b  and a resistor  202 ; the emitter area of the second transistor  200   b  is a factor n larger than the emitter area of the first transistor  200   a . In addition the circuit contains four current sources  22   a,b ,  24   a,b . The circuit has a positive power supply connection Vpp and a negative power supply connection Vnn. 
     The collector of the first transistor  200   a  is connected to the positive power supply connection Vpp via the first current source  22   a . The emitter of the first transistor  200   a  is connected to the negative power supply connection Vnn. 
     The collector of the second transistor  200   b  is connected to the positive power supply connection Vpp via the second current source  22   b . The emitter of the second transistor  200   b  is connected to the negative power supply connection Vnn via the resistor. 
     The base connections of the first and second transistor  200   a,b  are connected together and to the collector of the first transistor  200   a.    
     The third and fourth current source  24   a,b  are connected between the negative power supply Vnn and the collector of the first and second transistor  200   a,b  respectively. A control input of the third and fourth current source  24   a,b  are connected together and to the collector of the second transistor  200   b.    
     In operation the PTAT core  20  imposes that the base-emitter voltage of the first transistor  200   a  is equal to the sum of the voltage drop across the resistor  202  and the base-emitter voltage  200   b  of the second transistor. As a consequence the natural logarithm of the ratio of the currents I 1 , I 2  through the collector of the first and second transistor  200   a,b  to the negative power supply is 
     
       
         log I 1 /I 2 =I 2 * R*q/kT −log  n    
       
     
     where R is the resistance value of resistor  202  and n is the ratio of the emitter areas of the transistors  200   a,b.    
     The connection between the collector and the base of the first transistor  200   a  ensures that the sum of the currents at the collector of the first transistor is zero. 
     The first and second current source each supply a current I from the positive power supply to the collector of the first and second transistor  200   a,b  respectively. Part I 1 , I 2  of these currents flows through the collector-emitter of the first and second transistor  200   a,b  and through the resistor  202 . A fraction of these currents is deviated from the transistors  200   a,b  by the third and fourth current source  24   a , 24   b.    
     The fraction is controlled by the voltage at the collector of the second transistor  200   b  and reaches a stationary value once the currents I 1 , I 2  through the first and second transistor are equal, that is when 
     
       
         I 2 * R*q/kT =log  n    
       
     
     Thus, a current  12  is realized that depends on absolute temperature T, but not on material properties of the transistors. Both the voltage at the collector of the first transistor  200   a  and that at the collector of the second transistor  200   b  are defined with respect to the same power supply Vnn (through the properties of the first transistor  200   a  and the control input of the fourth current source  24   b  respectively). Because these voltages are defined with respect to the same reference (Vnn), the circuit is hardly susceptible to the effects of a wide frequency range of power supply variations, effects due e.g. to the Early effect in the transistors  200   a,b . No start-up current is needed and no capacitor is needed to make the circuit stable. 
     The voltage at the collector of the first transistor  200   a  may be used as a reference voltage. 
     FIG. 3 shows how reference currents may be obtained. A further transistor  26  is included with properties similar to those of the first transistor  200   a  and having an emitter and base connected to the emitter and base of the first transistor  200   a . From the collector of this further transistor  26  flows a current I 1 . 
     A current from the positive supply connection Vpp is obtained by a first and second output current source  27 ,  28 . An output node  29  is connected to the positive and negative supply connections Vpp, Vnn through the first and the second output current source  27 ,  28  respectively. A control input of the second output current source is connected to the control inputs of the third and fourth current source  24   a,b.    
     In operation, the first output current source supplies the same current I as the first and second current source  22   a,b . The second output current source supplies the same current (I-I 1 ) as the third and fourth current source  24   a,b . As a result the net current at the output node  29  is I 1 . 
     Dependent on the need for reference current sources either further transistor  26  or the combination of output current sources  27 ,  28  or both may be used. 
     Various versions of the PTAT core may be used. For example, one may use transistors  200   a,b  with the same emitter area, provided the current supplied by the first current source  22   a  is a factor n larger than that supplied by the second current source  22   b . In this case, the third and fourth current source  24   a,b  must also be proportioned with a ratio n:  1  so that they deviate the same fractions of the current from the positive power supply Vpp supplied by the first and second current source  22   a,b  respectively. 
     Similarly additional resistors may included, for example in the emitter path of the first transistor  200   a.    
     All kinds of combinations of different currents and emitter areas may be used. What matters is that the junction current densities through the first and second transistor  200   a,b  differs and that the resulting difference in base-emitter voltage is the same as a resistive voltage drop IR, which is proportional to the controlled current. Furthermore third and fourth current source should deviate the same fractions of the currents supplied to the PTAT core. 
     FIG. 4 shows another PTAT core  400  this time with a first and second PNP transistor  400   a,b  and a resistor  402 . The collectors of the PNP transistors  400   a,b  are connected to the negative power supply Vnn. The emitter of the first PNP transistor  400   a  is connected to the positive power supply through the first current source. The emitter of the second PNP transistor  400   b  is connected to the positive power supply Vpp through the resistor, a node  404  and the second current source  22   b . The bases of the transistors  400   a,b  are connected together. The emitter of the first transistor  400   a  and the node  404  are connected as the outputs of the PTAT core  400  in the same way as the collectors of the npn transistors  200   a,b  of FIG.  2 . 
     In addition, the circuit of FIG. 4 contains a base voltage control circuit  42 . The base voltage control circuit  42  has an input connected to the emitter of the first transistor  400   a  and a high impedance output connected to the base of the first transistor  400   a.    
     The base voltage control circuit  42  contains a first and second base control current source  420 ,  422  and a current mirror  424 . The current mirror  424  has a supply connection connected to the positive supply connection Vpp. The input and output of the current mirror is connected to the negative supply connection Vnn through the first and second base control current source  420 ,  422  respectively. 
     A control input of the first base control current source  420  is connected to the control inputs of the third and fourth current sources  24   a,b . A control input of the second base control current source  422  is connected to the emitter of the first transistor  400   a.    
     In operation, the function of the base voltage control circuit  42  is to make the emitter voltage of the first transistor  400   a  equal to the voltage at the node  404  between the resistor  402  and the second current source  22   b . To do this, the base voltage control circuit  42  adjusts the base voltage of the transistors  400   a,b  until the net current at the emitter of the first transistor  400   a  is zero. In this respect the base voltage control circuit  42  takes over the function of the connection between the collector and base of the first transistor  200   a  of FIG.  2 . 
     The first base control current source  420  supplies the same current I-I 2  as the third and fourth current source  24   a,b  and the current supplied by the second base control current source  422  is adjusted so that it supplies the same current as the third and fourth current source  24   a,b . This is realized when the voltage at the emitter of the first transistor  400   a  equals the voltage at the node  404 . 
     The current sources can be realized in various conventional ways. One may use for example bipolar transistors with an emitter connected to the supply, optionally via a resistor, a collector coupled to the output of the current source and a base used as control input. Instead of bipolar transistors MOS transistors may be used. Preferably, the MOS transistors are cascoded, at least in the third and fourth current source  24   a,b  and in the first and second base control current sources  420 ,  422 . A control voltage for cascode transistors may be derived for example from the output of the current mirror  424 . 
     In this respect the FIG. 4 is very suitable for MOS implementation, because PNP transistors  400   a,b  can be realized in a CMOS process. Instead of the transistors  400   a,b  or  200   a,b  MOS transistors may be used, but then the reference voltage and current depend on carrier mobility. 
     The reference circuit according to the invention may also be converted to a bandgap reference, by adding a resistive voltage drop to the reference voltage across the base-emitter the transistor  200   a  etc. 
     FIG. 5 shows a bandgap reference circuit according to the invention. Here a further resistor  50  has been included between the negative power supply Vnn on one hand and a connection between the resistor  202  and the emitter of the first transistor  200   a  on the other hand. The components of third and fourth current source  24   a,b  are shown explicitly. Each contains a transistor  52   a,b  and a resistor  54   a,b  connected between the emitter and Vnn. The resistors  54   a,b  serve to raise the collector voltage of the second transistor  200   a,b  so that it does not become too low now that the emitter voltages are raised by the further resistor  400 ; preferably the value of the resistors  54   a,b  is selected so that the collector voltages of the first and second transistor  200   a,b  are substantially equal. (Alternatively, the two resistors  54   a,b  may be merged in a single resistor connecting the emitters of both transistors  52   a,b  to Vnn). 
     The value of the further resistor  400  may be chosen in a known way to ensure a bandgap reference voltage 
     
       
           Vbe/R 400+2*I 1   
       
     
     (approximately 1.2V) at the collector of the first transistor  200   a  relative to Vnn. 
     FIG. 5 a  shows a CMOS version of this bandgap reference circuit. Here, P 1 , P 2  function as a feedback amplifier to steer the deviation currents under control of the difference between the voltages of the emitter of one PNP transistor and the PTAT resistor connected to the emitter of the other PNP transistor. 
     FIG. 6 shows an alternative voltage reference circuit. Here a further resistor  60  is coupled in parallel to the base-emitter junction of the first NPN transistor  200   a . A common resistor  62  couples the connection of the resistor  202 , the emitter of the first NPN transistor  200   a  and the further resistor  60 . A further NPN transistor  64  has its base coupled to the collector of the first NPN transistor  200   a , its emitter coupled to the base of first NPN transistor  200   a  and its collector connected to the positive power supply Vpp. A diode transistor  66  is coupled between the collector of the second NPN transistor  200   b  and the collector of the transistor  52   b  in the fourth current source. 
     In operation the current through both NPN transistors  200   a,b  and the further resistor is collected as a current 
     
       
           IC= 2*I 1 + Vbe /R 60   
       
     
     In the circuit of FIG. 6 the product IC*R 60  takes the place of the bandgap voltage of FIG.  5 : the further resistor R 60  is selected in a similar way as further resistor  400  of FIG.  6 . By means of the common resistor  62 , the current IC can be converted into any desired voltage. 
     The further NPN transistor  64  serves to compensate the current drawn by the further resistor  60 . The voltage at the collector of the first NPN transistor  200   a  will change until the current through the further transistor  60  is substantially equal to the current through the further resistor  60 . The diode transistor  64  introduces a voltage level shift which serves to keep the voltage at the collector of the first and second transistor  200   a,b  substantially equal, so as to minimize the consequence of the Early effect on the reference current. 
     Instead of the further transistor  64  one may also use a compensation resistor in parallel with the collector emitter of the transistor  52   b  in the third current source to compensate the current through the further resistor. This allows the circuit to operate at a lower supply voltage, but it requires resistor matching. In this case, the collector and base of the first NPN transistor  200   a  may be connected to each other and the diode transistor may be replaced by a direct connection. 
     The compensating resistor should have the same value as the further resistor, in order to draw the same current from the collector of the second NPN transistor  200   b  as the further transistor draws from the collector of the first NPN transistor  200   a.    
     Alternatively, the function of transistor  64  may be replaced as shown in the circuit of FIG. 6 a . In this circuit, the function of transistor  64  is replaced by an amplifier circuit Q 11 , Q 12 , Q 13 , Q 14 , R 13 , R 14 . This circuit is suitable for lower supply voltages, because it eliminates the base-emitter voltage drop of transistor  64  in the critical supply path from Vpp through the base emitter junction of first transistor  200   a  to Vnn. Instead, only the collector-emitter voltage drop of Q 13  (plus the drop over R 13 ) occurs in this path. 
     The circuit of FIG. 6 is more accurate than the version with the compensating resistor. In addition, the further transistor  64  provides a buffering of the base voltage of the first and second transistor  200   a,b , so that this voltage may be used as an output voltage. 
     The buffer transistor  64  can also be applied to other versions of the circuit, that is, not only if a further resistor  60  is present in parallel to the base emitter junction of the first transistor  200   a  (as in FIG.  6 ). Generally, the buffering serves to ensure that a current drawn from the base (such as an output current) does not affect the accuracy of the circuit. One may for example use a current bias circuit for the buffer transistor  64  between the base of the first transistor  200   a  and Vpp to drain a quiescent current of the further transistor  64 . Preferably, the bias circuit matches the third and fourth current source, e.g. by using a series arrangement of a resistor and a diode. 
     FIG. 7 shows a circuit which may be used for realizing the first and second current source  22   a,b . This circuit contains a first branch between Vpp and Vnn of successively a resistor  700 , a node  701 , a resistor  702  and the collector-emitter of an NPN transistor  704 , the base of the transistor  704  being coupled to the node  701 . 
     A second branch between Vpp and Vnn contains the channel of a PMOS transistor  720 , the collector emitter of an NPN transistor  722  and a resistor  724 . The collector of the transistor  704  in the first branch is coupled to the base of the NPN transistor  722  in the second branch. This NPN transistor  704  has twice the emitter area of the transistor  704  in the first branch. 
     The drain of the PMOS transistor  720  is coupled to its gate and to the gate of a number of further PMOS transistors  74 ,  76  which serve as first and second current source.