Abstract:
An embodiment of a circuit includes first and second branches, an amplifier, a compensation circuit, and a bias unit. The first and second branches are respectively operable to generate first and second currents. The amplifier has a first amplifier input node coupled to the first branch, a second amplifier input node coupled to the second branch, an amplifier output node coupled to the first and second branches, and a first compensation node. The compensation unit is operable to provide a first offset-compensation signal to the first compensation node. And the first bias unit is operable to provide first and second bias signals to the first and second input nodes, respectively, such that the amplifier is operable to cause the first current to approximately equal the second current.

Description:
RELATED APPLICATION DATA 
       [0001]    This application is related to U.S. patent application Ser. No. ______, entitled CIRCUIT FOR GENERATING A REFERENCE VOLTAGE (Attorney Docket No.: 2110-351-03) filed ______, and which is incorporated herein by reference in its entirety. 
     
    
     PRIORITY CLAIM 
       [0002]    The instant application claims priority to Italian Patent Application No. MI2009A001897, filed Oct. 30, 2009, which application is incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0003]    An embodiment relates to a circuit for generating a reference voltage. In particular, an embodiment relates to a circuit for generating a reference voltage of the bandgap type. 
       BACKGROUND 
       [0004]    Circuits for generating a reference voltage, also known simply by the term “voltage reference circuits”, are circuits that may play a vital role in various types of integrated circuits. In particular, a voltage reference circuit may be capable of generating at least one electrical quantity with high accuracy and great stability, which quantity may be used as reference in various types of circuit blocks such as analog to digital converters, voltage regulators, measuring circuits and so on. A voltage reference circuit may, therefore, be provided with specific features such as good thermal stability and good electrical noise rejection, so as to be capable of providing an output voltage whose value is more independent as possible from voltage supply variations and from temperature changes of the circuit wherein it is integrated. 
         [0005]    A class of voltage reference circuits widely known, that is, with the features mentioned above, is the so-called bandgap voltage reference circuits class, or simply bandgap circuits. Briefly, a bandgap circuit exploits the band potential of silicon to generate an accurate reference voltage that is independent of the circuit operating temperature. The operation principle of a bandgap circuit is based on obtaining a bandgap voltage VBG (almost) independent of the circuit operating temperature by means of a bipolar transistor that implements the relation VBG=VBE+nVT, where VBE is the voltage between the base terminal and the emitter terminal of the bipolar transistor, VT is the thermal voltage (equal to kT/q, where k is the Boltzmann constant, T is the absolute temperature, and q the electron charge), and n is a multiplicative parameter calculated to obtain the desired compensation of the temperature variations of the voltage VBE. For a given collector current, the voltage VBE between the base and emitter of a bipolar transistor decreases as the temperature increases—in the jargon, the voltage VBE is a quantity of the CTAT (Complementary To Absolute Temperature) type—while the thermal voltage appears to be proportional to the temperature itself—in the jargon, the thermal voltage VT is a quantity of the PTAT (Proportional To Absolute Temperature) type. 
         [0006]    According to an approach known in the state of the art, the bandgap voltage VBG may be generated by forcing a current Iptat provided by a current generator in a first reference circuit element comprising a transdiode connected bipolar transistor, and mirroring the current Iptat in a second reference circuit element formed by a series of a resistor and a second transdiode connected bipolar transistor having an emitter area different from that of the first bipolar transistor. Coupling the first reference circuit element and the second reference circuit element with respective input terminals of a high gain operational amplifier, and using the output of such operational amplifier to control the generator of the current Iptat, a negative feedback loop is established, which forces the first and second reference circuit elements voltages to a same value. With such a configuration, the current Iptat is found to be: 
         [0000]        Iptat =[ln( L 1 /L 2)*( KT/q )]/Re, 
         [0000]    where L 1  and L 2  are parameters proportional to the emitter areas, respectively, of the first bipolar transistor and of the second bipolar transistor, while Re is the resistance of the resistor comprised in the second reference circuit element; as may be seen from the equation, this current appears to be of the PTAT type, being proportional to the absolute temperature T. The current Iptat is then forced into a third reference circuit element comprising an element characterized by an electrical quantity of the CTAT type for generating the bandgap voltage VGB. 
         [0007]    A major drawback that may afflict a configuration of this type is the extreme variability of the common-mode voltage of the operational amplifier input terminals. Indeed, this voltage being dependent from the base-emitter voltages VBE of the bipolar transistors included in the first and second reference circuit elements, it may vary in a range between 0.3 and 0.8 Volts depending on temperature and tolerances of the manufacturing process. Consequently, the operational amplifier is designed to handle the large input signal excursions without compromising the proper voltage reference circuit operation. However, this may be very difficult if the supply voltage has a reduced value, as happens in the circuits integrated using advanced CMOS (Complementary Metal Oxide Semiconductor) technologies. For example, in the 90 nm CMOS technology the power supply has a nominal value equal to 1.2 Volts; this value may actually decrease until reaching 0.9 Volts when the circuit has been designed to operate during stand-by phases in order to minimize losses due to the leakage currents presence. In these cases, the common-mode voltage excursions due to temperature change may be too large, and the transistors of the operational amplifier input stage may be forced to operate in the triode operation region, and thus the amplifier may not operate correctly. 
         [0008]    In order to solve the above mentioned drawbacks, a solution provides for using an operational amplifier whose input stage consists of n-channel MOS transistors with reduced threshold voltage. However, although this allows the operational amplifier to operate correctly even in the presence of high excursions of the common-mode voltage, forming MOS transistors with reduced threshold voltage may require an additional lithography mask, and this may imply an increase in the whole circuit production costs. 
         [0009]    According to a further solution, the common-mode voltage value is increased by introducing resistors in series with the first and second reference circuit elements and using the voltage drops that are generated as a result of the current Iptat flowing in these resistors. Nevertheless, the problem of the common-mode voltage excursion as a function of temperature may not be resolved; if the amplifier is be supplied with a low-supply voltage value, with this solution the common-mode voltage may, in fact, exceed the supply voltage itself, thus possibly compromising the proper functioning of the amplifier. 
       SUMMARY 
       [0010]    An embodiment overcomes the above-mentioned drawbacks. 
         [0011]    An embodiment relates to a bandgap voltage reference circuit to generate a bandgap reference voltage according to a first current. Said circuit comprises a current generator controlled by a first driving voltage for generating the first current based on the driving voltage. Said circuit further comprises a first reference circuit element adapted to generate a first reference voltage according to the first current and a second reference circuit element adapted to generate a second reference voltage according to the first current. The circuit further comprises an operational amplifier having a first input terminal coupled to the first circuit element for receiving a first reference input voltage according to the first reference voltage, a second input terminal coupled to the second reference circuit element for receiving a second input voltage according to the second reference voltage and an output terminal coupled to the controlled current generator to provide the first driving voltage. The circuit further comprises a control circuit. Said control circuit comprises first capacitive means having a first terminal coupled to the first reference circuit element to receive the first reference voltage and a second terminal coupled to the first input terminal to provide the first input voltage. The control circuit also comprises second capacitive means comprising a first terminal coupled to the second reference circuit element for receiving the second reference voltage and a second terminal coupled to the second input terminal to provide the second input voltage. The control circuit further comprises first biasing means for selectively providing a first common mode voltage to the second terminals of the first and second capacitive means. The operational amplifier is an offset compensated operational amplifier further comprising a first compensation terminal for receiving the first common-mode voltage and a second compensation terminal coupled to an offset management circuit for receiving a first compensation voltage. The offset management circuit comprises an auxiliary operational amplifier having a first input terminal adapted to receive a third input voltage corresponding to the first input voltage, a second input terminal adapted to receive a fourth input voltage corresponding to the second input voltage, and an output terminal adapted to be selectively coupled to the second compensation terminal of the operational amplifier for providing the first compensation voltage. 
         [0012]    An embodiment relates to a method for operating a bandgap voltage reference circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    One or more embodiments of the invention, as well as features and advantages thereof, will be better understood by reference to the following detailed description, given purely by way of a non-restrictive indication, to be read in conjunction with the attached figures. In particular: 
           [0014]      FIG. 1A  shows a bandgap voltage reference circuit according to an embodiment; 
           [0015]      FIG. 1B  shows a portion of an input stage of an embodiment of an operational amplifier in the circuit of  FIG. 1A ; 
           [0016]      FIG. 2  shows the circuit structure of a biasing block the circuit of  FIG. 1A  according to an embodiment; 
           [0017]      FIG. 3  illustrates the circuit structure of a short-circuit block in the circuit of  FIG. 1A  according to an embodiment; 
           [0018]      FIG. 4  is a timing diagram that shows the evolution over time of some of the signals generated by and received from an embodiment of the circuit of  FIG. 1A . 
           [0019]      FIG. 5  shows a bandgap voltage reference circuit according to a further embodiment; 
           [0020]      FIG. 6  illustrates the circuit structure of a biasing block in the circuit of  FIG. 5  according to an embodiment; 
           [0021]      FIG. 7  illustrates the circuit structure of a short-circuit block in the circuit of  FIG. 5  according to an embodiment; 
           [0022]      FIG. 8  shows a circuit for managing the offset in the circuit of  FIG. 5  according to an embodiment; 
           [0023]      FIG. 9  shows a structure of an operational amplifier in the circuit of  FIG. 5  and of an auxiliary operational amplifier in the offset management circuit of  FIG. 8  according to an embodiment, and 
           [0024]      FIG. 10  is a timing diagram that shows the evolution over time of some of the signals generated and received by an embodiment of the circuit of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    With reference to  FIG. 1 , a bandgap voltage reference circuit  100  is shown according to an embodiment. 
         [0026]    The circuit  100  comprises a first reference circuit element  102  coupled to a first current generator  104  adapted to generate a current identified in the figure with the reference Iptat. The circuit  100  also comprises a second reference circuit element  106  coupled to a second current generator  108 ; the current generator  108  is connected with the current generator  104  in a mirror configuration. 
         [0027]    The reference circuit element  102  comprises a bipolar PNP type transistor  110  having a collector terminal connected to a terminal that provides a ground voltage, a base terminal connected to the collector terminal, and an emitter terminal connected to a terminal of the current generator  104 , which provides the current Iptat. 
         [0028]    The current generator  104  comprises a first p-channel MOS transistor  112  having a drain terminal connected to the transistor  110  emitter terminal for providing the current Iptat, a gate terminal connected to current generator  108 , and a source terminal connected to a drain terminal of a second p-channel MOS transistor  114 . The transistor  114  has a gate terminal connected to the current generator  108  and a source terminal connected to a terminal that provides a supply voltage Vdd. The gate terminal of the transistor  114  is also adapted to receive a hold signal Vphold used to selectively activate/deactivate the current Iptat supply. In particular, when the hold signal Vphold is at a low value, such as the ground voltage, the transistor  114  turns out to be turned on, while when the hold signal Vphold is at a high value, such as the supply voltage Vdd, the transistor  114  turns out to be turned off; in the latter case, the supplying of the current Iptat is be interrupted. 
         [0029]    The reference circuit element  106  comprises a bipolar PNP type transistor  116  having a collector terminal connected to a terminal providing the ground voltage, a base terminal connected to the collector terminal, and an emitter terminal connected to a first terminal of a resistor  118 . The resistor  118  comprises a second terminal connected to a terminal of the current generator  108  that provides a mirrored version of the current Iptat. 
         [0030]    The current generator  108  comprises a first p-channel MOS transistor  120  having a drain terminal connected to the resistor  118  to provide the current Iptat, a gate terminal connected to the gate terminal of the transistor  112  and a source terminal connected to the drain terminal of a second p-channel MOS transistor  122 . The transistor  122  has a gate terminal connected to the gate terminal of the transistor  114  and a source terminal connected to a terminal providing the supply voltage Vdd. 
         [0031]    The circuit  100  also comprises an operational amplifier  124  comprising a non-inverting input terminal (“+”) coupled to the transistor  110  emitter terminal, an inverting input terminal (“−”) coupled to the second terminal of the resistor  118  and an output terminal connected to the current generators  104 ,  108  for providing a driving voltage Vpgate to the gate terminals of the transistors  112  and  120  in order to adjust the value of the current Iptat. According to an embodiment, the operational amplifier  124  has a differential input stage comprising an input pair formed by n-channel MOS transistors, each biased by a bias current corresponding to the current Iptat. The operational amplifier  124  may also be turned on (and off) in a selective manner depending on the value assumed by a control signal Vota provided to a control terminal of the amplifier. In an embodiment, the operational amplifier  124  has a high gain and a high output impedance. 
         [0032]      FIG. 1B  shows the circuit structure of an input stage portion—identified in the figure with the reference  150 —of the operational amplifier  124  according to an embodiment. 
         [0033]    The input stage  150  comprises a pair of n-channel MOS transistors  152 ,  154  connected in differential configuration. In particular, the transistor  152  has a drain terminal connected to a load (not shown in the figure), a gate terminal that represents the non-inverting terminal of the operational amplifier  124 , and a source terminal connected to a drain terminal of an n-channel MOS transistor  156  (circuit node  158 ) adapted to provide the bias current of the transistors  152  and  154 . The transistor  154  has a drain terminal connected to a load (not shown in the figure), a gate terminal which is the inverting terminal of the operational amplifier  124 , and a source terminal connected to the node  158 . 
         [0034]    The transistor  156  comprises a source terminal connected to a terminal that provides the ground voltage and a gate terminal connected to a gate terminal of a further n-channel MOS transistor  160 . The transistor  160  has a source terminal connected to a terminal that provides the ground voltage and a drain terminal connected to the gate terminal. The transistor  160  is configured to conduct a current equal to the current Iptat; such current may, for example, be supplied to the transistor  160  from one of the current generators  104  and  108  of the circuit  100 . 
         [0035]    According to an embodiment, the transistors  152 ,  154  and  160  have the same form factor FF=W/L—where W is the gate region width and L is the gate region length—, while the transistor  156  has a form factor FF′ equal to 2*W/L. In this way, the drain current of the transistor  156  turns out to be twice the drain current of the transistor  160 , i.e. 2*Iptat. Given the structure symmetry, if the pair of transistors  152 ,  154  does not appear to be overly unbalanced (e.g. if the amplifier operates in the so-called amplification region) each transistor of the pair  152 ,  154  is conducting a current value approximately equal to Iptat during steady-state operation. 
         [0036]    Still referring to  FIG. 1A , the presence of the operational amplifier  124  having the inputs coupled to the reference circuit elements  102 ,  106  and the output connected to the current generators  114  and  118  forms a negative feedback loop, through which the voltages of the first and second circuit elements  102 ,  106  are brought to approximately a same value, and the generated current Iptat appears to be proportional to the absolute temperature, i.e. it turns out to be a quantity of the PTAT type. 
         [0037]    The current Iptat thus generated is then mirrored by a third current generator  126  and forced into a third reference circuit element  128  for generating a bandgap voltage Vbg, which represents the circuit  100  output. Without going into details already known, the reference circuit element  128  may be formed by a resistive divider comprising an element characterized by an electrical quantity of the CTAT type—such as the voltage between the base and emitter of a transdiode connected bipolar transistor—which compensates for the PTAT behavior of the current Iptat. 
         [0038]    The current generator  126  comprises, in particular, a first p-channel MOS transistor  130  having a drain terminal connected to the reference circuit element  128  for providing the current Iptat, a gate terminal connected to the gate terminal of the transistor  112 , and a source terminal connected to the drain terminal of a second p-channel MOS transistor  132 . The transistor  132  has a gate terminal connected to the gate terminal of the transistor  114  and a source terminal connected to a terminal providing the supply voltage Vdd. 
         [0039]    According to an embodiment, the value of the common-mode voltage to the inputs of the operational amplifier  124  is adjusted by a common mode management circuit block—identified in the figure with the reference  134 —so that the transistors of the operational amplifier  124  input stage operate in saturation. As will become clearer later in this description, the circuit block  134  is configured to receive the reference voltages of the reference circuit elements  102 ,  106 , and appropriately shift them using capacitors in series with the operational amplifier  124  inputs. In particular, a first capacitor  136  has a first terminal connected to the emitter terminal of the transistor  110  for receiving a voltage Vplusc and a second terminal connected to the non-inverting input terminal of the operational amplifier  124  to provide a voltage Vplus; a second capacitor  138  has instead a first terminal connected to the second terminal of the resistor  118  and a second terminal connected to the inverting input terminal of the operational amplifier  124  to provide a voltage Vminus. 
         [0040]    The circuit block  134  includes of two main sub-blocks, i.e. a biasing block  140  and a short-circuit block  142 . 
         [0041]    The biasing block has a first terminal for receiving a digital control signal Vswc, a second terminal for receiving the driving voltage Vpgate, a third terminal connected to capacitor  136  second terminal, and a fourth terminal connected to the capacitor  138  second terminal. As will be described below, the biasing block  140  is adapted to generate the common-mode voltage that is actually supplied to the operational amplifier  124  inputs; said voltage is selectively forced to the operational amplifier  124  inputs according to the control signal Vswc short-circuiting the second terminals of the capacitors  136  and  138  connected to the operational amplifier  124  inputs. In this way the voltages Vplus and Vminus are brought to the voltage value determined by the biasing block  140 . 
         [0042]    The short-circuit block  142  has a first terminal for receiving the control signal Vswc, a second terminal for receiving the driving voltage Vpgate, a third terminal connected to the emitter terminal of the transistor  110  for receiving the voltage Vplusc, and a fourth terminal connected to the emitter terminal of the transistor  116  for receiving a voltage Vminusc. Alternatively, the fourth terminal may be connected to the node between the resistor  100  and the capacitor  138 . The short-circuit block  142  is capable of selectively short-circuiting the emitter terminal of the transistor  110  with the emitter terminal of the transistor  116  (or to the opposite node of the resistor  118 ) according to the control signal Vswc. 
         [0043]    In addition, in order to allow the voltages across the capacitors  136  and  138  to be updated with the values generated by the biasing block  140 , and at the same time ensure a proper operation of the system, in an embodiment the circuit  100  has a “sample and hold” type architecture, whose operation comprises an alternating sequence of holding phases (hold signal Vphold high) and regeneration phases (hold signal Vphold low). In particular, the circuit  100  is provided with a first retention capacitor  144  connected between a terminal that provides the supply voltage Vdd and the operational amplifier  124  output terminal for storing the driving voltage Vpgate when the operational amplifier  124  is turned off, and a circuit for sampling and holding the bandgap voltage Vbg. The circuit for sampling and holding the bandgap voltage Vbg comprises a controlled switch  146  having a first conduction terminal connected to the drain terminal of the transistor  130 , a second conduction terminal to provide the bandgap voltage Vbg and a control terminal to receive a sampling signal Vbgref; the circuit for sampling and holding the bandgap voltage Vbg also comprises a second retention capacitor  148  having a first terminal connected to the second conduction terminal of the controlled switch  146  and a second terminal connected to a terminal which receives the ground voltage. When the sampling signal is asserted, for example at the supply voltage Vdd, the controlled switch is closed, and the bandgap voltage Vbg is determined by the voltage drop generated across the reference circuit element  128  crossed by the current Iptat. When the sampling signal is deasserted, for example at the ground voltage, the controlled switch is open, and the bandgap voltage Vbg is determined by the voltage drop across the capacitor  148 . 
         [0044]    The biasing block  140  circuit structure according to an embodiment is illustrated in  FIG. 2 . 
         [0045]    The block  140  includes a bias current generator comprising two p-channel MOS transistors  202 ,  204 . The transistor  202  has a source terminal connected to a terminal providing the supply voltage Vdd, a gate terminal connected to a terminal that provides the ground voltage and a drain terminal connected to a source terminal of the transistor  204 . The transistor  204  has a gate terminal that receives the driving voltage Vpgate, and the drain terminal connected to a circuit node identified in the figure with the reference  206 . The transistors  202 ,  204  are sized in a similar manner (e.g., equal) to the transistors  112 ,  114  of the current generator  104 . Consequently, since the gate terminal of the transistor  204  is driven by the same driving signal Vpgate provided to the gate terminal of the transistor  112 , the current generated by the transistor  204  matches (for example, is approximately equal) to the current Iptat. 
         [0046]    The biasing block  140  also includes a common mode generator comprising two n-channel MOS transistors  208 ,  210  adapted to bias the node  206  with a common-mode voltage Vcm. The transistor  208  has a drain terminal and a gate terminal connected to the node  206 , and a source terminal connected to a drain terminal of the transistor  210  (circuit node  211 ). The transistor  210  has a gate terminal connected to the circuit node  206 , and a source terminal connected to a terminal that provides the ground voltage. 
         [0047]    The biasing block  140  further comprises a first transmission gate adapted to selectively connect the node  206  with the second terminal of the capacitor  136  according to the value assumed by the control signal Vswc, and a second transmission gate adapted to selectively connect the node  206  with the second terminal of the capacitor  138  as a function of control signal Vswc. In particular, the first transmission gate comprises a n-channel MOS transistor  212  having a first conduction terminal connected to the node  206 , a gate terminal driven by the control signal Vswc and a second conduction terminal connected to the second terminal of the capacitor  136  and a p-channel MOS transistor  214  having a first conduction terminal connected to the node  206 , a gate terminal driven by a negated version of the control signal Vswc, identified in the figure with the reference Vnswc, and a second conduction terminal connected the second terminal of the capacitor  136 ; the second transmission gate comprises an n-channel MOS transistor  216  having a first conduction terminal connected to the node  206 , a gate terminal driven by the control signal Vswc and a second conduction terminal connected to the second terminal of the capacitor  138 , and a p-channel MOS transistor  218  having a first conduction terminal connected to the node  206 , a gate terminal driven by the signal Vnswc, and a second conduction terminal connected to the second terminal of the capacitor  138 . 
         [0048]    In this way, when the control signal Vscw is asserted (e.g. to the value of the supply voltage Vdd), both the transmission gates are closed, and the voltages Vplus and Vminus provided to operational amplifier  124  inputs assume the common-mode voltage Vcm value generated by the transistors  208  and  210 . According to an embodiment, the transistors  208  and  210  are sized so that the common-mode voltage Vcm generated by them has a value such that the transistors  152 ,  154  and  156  of the input stage  150  of the operational amplifier  124  operate in the saturation zone. 
         [0049]    In particular, in an embodiment, the transistor  208  has a form factor FF that is equal to the form factor FF of the transistors  152 ,  154  and  160  of the input stage  150 , while the transistor  210  has a form factor FF″=W/(X*L), where X is a scale factor greater than or equal to three. In other words, according to an embodiment, a potentially optimal value of the common-mode voltage Vcm may be obtained by making the transistor  210  more resistive than the transistors  152 ,  154  and  160  of the input stage  150  of the operational amplifier  124 . 
         [0050]    In order to understand why a more resistive transistor  210  may be capable of generating a potentially optimal common-mode voltage Vcm such that the transistors of the input stage  150  of the operational amplifier  124  work in the saturation zone, reference will be now made jointly to  FIGS. 1B and 2 . 
         [0051]    During circuit operation, the transistor  208  of the biasing block  140  operates in the saturation zone, while the transistor  210  operates in the triode zone. Consequently, the current Iptat across these transistors is equal to: 
         [0000]        Iptat =(β/ X )*( Vcm−Vth− ½ *Vx )* Vx   (current of the transistor 210);
 
         [0000]        Iptat =(3)*( Vcm−Vx−Vth )̂2  (current of the transistor 208),
 
         [0000]    where β is the transistor  208  gain, X is the transistor  210  scaling factor, Vth is the threshold voltage of the transistors  208  and  210 , and Vx is the voltage at the node  211  of the biasing block  140 . Equating the above equations yields the following relationship: 
         [0000]        Vx =(√(1 +X )−1)*√(2 *Iptat /β)  (1)
 
         [0000]    where √( ) is the square root operation. 
         [0052]    In order to operate the transistor  156  of the input stage  150  of the operational amplifier  124  in the saturation zone, the following condition is fulfilled: 
         [0000]        Vc &gt;√(2 *Iptat/β ),  (2)
 
         [0000]    where Vc is the voltage at the node  158  of the input stage  150 . 
         [0053]    Since, in the amplification operating region, the transistors  152  and  154  of the input stage  150  of the operational amplifier  124  conduct a current approximately equal to Iptat, applying the common-mode voltage Vcm generated by the biasing block  140  to the gate terminals of the transistors  152  and  154 , the voltage Vc at node  158  of the input stage  150  assumes a value approximately equal to the voltage Vx at the node  211  of the biasing block  140 . In other words, the condition for which the transistor  156  of the input stage  150  works in the saturation zone becomes: 
         [0000]        Vx &gt;√(2 *Iptat/β ).  (3)
 
         [0054]    According to the relation (1), the condition (3) becomes: 
         [0000]      (√(1 +X )−1)*√(2 *Iptat/β )&gt;√(2 *Iptat/β )  (4)
 
         [0000]      i.e.: 
         [0000]      (√(1 +X )−1)&gt;1.  (5)
 
         [0055]    Solving with respect to X, the condition (5) becomes: 
         [0000]      X&gt;3,  (6)
 
         [0000]    i.e. the condition for having a potentially optimum value for the common-mode voltage Vcm to be provided to the operational amplifier  124  involves providing the transistor  210  a form factor FF″=W/(X*L) equal to one third (or less) of the form factor FF of the transistors  152 ,  154  of the operational amplifier  124 . 
         [0056]    The circuit structure of the short-circuit block  142  according to an embodiment is illustrated in  FIG. 3 . 
         [0057]    The short-circuit block  142  comprises two sections, namely a controlled switch  300  adapted to selectively short-circuit the emitter terminals of the transistors  110  and  106  comprised in the reference circuit elements  102  and  106  (or short-circuit the emitter of the transistor  110  to the bottom node of the capacitor  138 ), and a driver circuit  301  capable of driving the controlled switch  300  according to the control signal Vswc. 
         [0058]    The controlled switch  300  is formed by an n-channel MOS transistor, having a first conduction terminal connected to the emitter terminal of the transistor  110  to receive the voltage Vplusc, a second conduction terminal connected to the emitter terminal of the transistor  116  (or to the bottom node of the capacitor  138 ) to receive the voltage Vminusc, and a gate terminal connected to the driver circuit  301  for receiving a driving voltage Vs. Given that in modern integrated circuits the supply voltage Vdd typically has a very small value, in order to correctly drive the controlled switch  300 , it may be necessary that the driving voltage Vs is able to assume values greater than the supply voltage Vdd one. On the other hand, in order to prevent the occurrence of circuit malfunctions, the maximum value the driving voltage Vs may assume is lower than the oxide breakdown voltage of the controlled switch  300 . 
         [0059]    For this reason, according to an embodiment, the driving circuit  301  that generates the driving voltage Vs is a so-called “boost” circuit, and in particular is a circuit of the “clock booster” type. In detail, the driving circuit  301  comprises a current generator comprising two p-channel MOS transistors  302 ,  304 . The transistor  302  has a source terminal connected to a terminal providing the supply voltage Vdd, a gate terminal connected to a terminal that provides the ground voltage, and a drain terminal connected to a source terminal of the transistor  304 . The transistor  304  has a gate terminal that receives the driving voltage Vpgate, and the drain terminal connected to a circuit node identified in the figure with the reference  306 . The transistors  302 ,  304  are sized in a similar manner (e.g., equal) to the transistors  112 ,  114  of the current generator  104 . Consequently, since the gate terminal of the transistor  304  is driven by the same driving signal Vpgate provided to the gate terminal of the transistor  112 , the current generated by transistor  304  corresponds (for example, appears to be equal) to the current Iptat. 
         [0060]    The driving circuit  301  also comprises a bipolar p-channel transistor  308  having an emitter terminal connected to the node  306 , a collector terminal connected to a terminal that provides the ground voltage, and a base terminal connected to the collector terminal. The transistor  308  is sized in a similar way (i.e., approximately equal) to the transistor  110  included in the reference circuit element  102 , so that the voltage of node  306 , identified in the figure with the reference Vb, is as close as possible to the value of the voltages Vplusc, Vminusc of the emitter terminals of the transistors  110  and  106 . 
         [0061]    The node  306  is also connected to a drain terminal of a n-channel MOS transistor  310 , which transistor has a source terminal connected to a first terminal of a capacitor  312  and a gate terminal connected to a source terminal of a further n-channel MOS transistor  314 . The transistor  314  further comprises a drain terminal connected to the node  306  and a gate terminal connected to the source terminal of the transistor  310 ; the source terminal of the transistor  314  is also connected to a first terminal of a capacitor  316  and to the gate terminal of the controlled switch  300  to provide the driving voltage Vs. 
         [0062]    The capacitor  312  also comprises a second terminal connected to an output terminal of a first two-input NOR logic gate, identified in the figure with the reference  318 . The capacitor  316  comprises a second terminal connected to an output terminal of a second two-input NOR logic gate, identified in the figure with the reference  320 . The NOR gate  318  comprises a first input terminal capable of receiving the control signal Vswc and a second input terminal connected to the output terminal of the NOR gate  320 ; the NOR gate  320  comprises a first input terminal connected to the output terminal of the NOR gate  318  and a second input terminal connected to an output terminal of a NOT logic gate  322 . The NOT gate  322  comprises an input terminal capable of receiving the control signal Vswc. 
         [0063]    When the control signal Vswc is deasserted to the ground voltage, the output of the NOR gate  318  is brought to the supply voltage Vdd, while the output of the NOR gate  320  is brought to the ground voltage. In this situation, the gate terminal of the transistor  314  is brought to a voltage value approximately equal to Vb+Vdd; consequently, the transistor  314  is turned on while the transistor  310  is off, and the driving voltage Vs has a value equal to the voltage Vb. Since the driving circuit  301  is sized such that the voltage Vb has a value similar to that of the voltages Vplusc, Vminusc, in this condition the controlled switch  300  is open. 
         [0064]    When the control signal Vswc is instead asserted to the supply voltage, the output of the NOR gate  320  is brought to the power supply voltage Vdd, while the output of the NOR gate  318  is brought to the ground voltage. Consequently, the gate terminal of the transistor  314  is brought to a voltage of about Vb; therefore, the transistor  314  is turned off while the transistor  310  is turned on, and the driving voltage Vs is brought to a value equal to Vb+Vdd. In this condition, the controlled switch  300  is closed. 
         [0065]    The driving circuit  301  also comprises a capacitor  324  connected between the node  306  and a terminal that provides the ground voltage. The function of this capacitor is to provide the electrical charge needed to compensate for the inevitable changes in the voltage Vb of the node  306  that occur due to the switching of the NOR gates  318  and  320 . 
         [0066]    The overall operation of circuit  100  will be now described, referring to the figures previously described in conjunction with the  FIG. 4 ;  FIG. 4  is a timing diagram illustrating the trend in time of some of the signals generated/received by the circuit  100 . 
         [0067]    As described above, the circuit  100  according to an embodiment, has a “sample and hold” architecture, whose operation comprises a sequence of alternating holding phases and regeneration phases scanned by the value assumed by the hold signal—in particular, during the holding phases the hold signal Vphold is at a high voltage level, while during the regeneration phases that signal is at a low voltage level. 
         [0068]    Consequently, at the beginning of each regeneration phase, the hold signal Vphold is brought to a low voltage level (ground voltage). In this situation, the transistors  114 ,  122  and  132  are turned on, and the current generators  104 ,  108 ,  126  are enabled to generate the current Iptat in various branches of the circuit. 
         [0069]    Subsequently, the control signal Vota as well is brought to a low voltage level (ground voltage) in order to disable the operational amplifier  124 , and bring the output thereof in a high impedance state. It is stressed that in this phase the driving voltage Vpgate is equal to the value corresponding to the charge that was stored in the capacitor  144  during the previous holding phase, set in turn by a previous activation of the operational amplifier  124 . 
         [0070]    The control signal Vswc is then brought to a high voltage level (Vdd) to turn on the biasing block  140  and the short-circuit block  142 . In particular, the transmission gates of the biasing block  140  are closed, shorting the second terminal of the capacitor  136  with the second terminal of the capacitor  138 ; simultaneously, the controlled switch  300  of the short-circuit block  142  is closed, too, in order to short circuit the emitter terminal of the transistor  110  (connected to the first terminal of the capacitor  136 ) with the emitter terminal of the transistor  116  (coupled to the first terminal of the capacitor  138  through the resistor  118  or directly with the first terminal of the capacitor  138 ). In this situation, the voltage Vplus at the non-inverting terminal of the operational amplifier  124  and the voltage Vminus at the inverting terminal of the operational amplifier  124  are brought to the common-mode voltage Vcm generated by the transistors  208  and  210  of the biasing block  140 , while a voltage drop dVc equal to: 
         [0000]        dVc =( Vcm−Veb ), 
         [0000]    is set on the capacitors  136  and  138 , where Veb=Vplusc=Vminusc (particularly where the terminal Vminusc of the short-circuit block  142  is connected directly to the first node of the capacitor  138 ). 
         [0071]    Therefore, according to an embodiment, the common-mode voltage at the operational amplifier  124  inputs is set to the value Veb generated by the reference circuit elements  102 ,  106  plus a shift value dVc generated by electronic devices that are subject to conditions (such as biasing and temperature) very similar to those the components of the operational amplifier  124  are subjected to, and that also were manufactured during the same manufacturing process. In other words, according to an embodiment, the common-mode voltage at the operational amplifier  124  inputs is set to an approximately optimal value, which follows the variations in temperature and the polarization the circuit  100  is subject to, and which appears to be calibrated according to the specific parameters of the process by means of which the circuit  100  was manufactured. 
         [0072]    At this point the control signal Vswc is brought to a low voltage level (ground voltage), so that the transmission gates of the biasing block  140  and the controlled switch  300  of the short-circuit block  142  are opened. 
         [0073]    Subsequently, the control signal Vota is brought to a high voltage level (Vdd), so as to enable the operational amplifier  124 , and allow formation of the negative feedback loop between the voltage across the reference circuit elements  102 ,  106  and the driving voltage Vpgate of the current generators  104 ,  108  and  106 . In particular, the driving voltage Vpgate across the capacitor  144  is regenerated by the operational amplifier  124  based on values assumed by the voltages Vplus and Vminus. 
         [0074]    The sampling signal Vbgref is thus brought to a high voltage level (Vdd) to close the controlled switch  146  in order to regenerate the bandgap voltage Vbg across the capacitor  148 . 
         [0075]    Thereafter, both the sampling signal Vbgref and the control signal Vota are brought to a low voltage level (ground voltage) in order to open the controlled switch  146  and disable the operational amplifier  124 . The voltages Vbg and Vpgate are then stored by the capacitors  148  and  144 . 
         [0076]    At this point the hold signal Vphold is brought to a high voltage level (Vdd), and the holding phase starts. During the holding phase, the power consumption is minimized due to the fact that the transistors  114 ,  122  and  132  are disabled, so the generation of the current Iptat by the current generators  104 ,  108 ,  126  is inhibited. The values of the voltages Vpgate and Vbg used by the circuit  100  during the whole holding phase correspond to those stored in the capacitors  148  and  144 . 
         [0077]    The bandgap voltage Vbg generated by the circuit  100  up to now described may, however, assume a different value from the desired one, caused by a possible offset voltage present between the inverting terminal and the non-inverting terminal of the operational amplifier  124 . In particular, considering the presence of this offset voltage, the voltages Vplusc and Vminusc set using the negative feedback loop defined by the operational amplifier  124  may not be equal, thereby altering the value of the current Iptat used for the generation of the bandgap voltage Vbg. 
         [0078]    In accordance with an embodiment, in order to eliminate (or at least minimize as far as possible) the negative effect due to the offset voltage between the inputs of the operational amplifier  124 , the circuit  100  may be opportunely modified as shown in  FIG. 5 . This modified version of the circuit  100  is identified in  FIG. 5  with the reference  100 ′. 
         [0079]    An embodiment of the bandgap voltage reference circuit  100 ′ differs from the circuit  100  since the operational amplifier  124  is replaced with an offset compensated operational amplifier  124 ′, and said operational amplifier  124 ′ is coupled to an offset management circuit—identified in the figure with the reference  600 —whose function is to estimate the offset voltage of the amplifier  124 ′ and to compensate the offset voltage, and thus drive the amplifier in a potentially improved manner. In the same way as for the operational amplifier  124 , the operational amplifier  124 ′ has a non-inverting input terminal (“+”) connected to the second terminal of the capacitor  136  to receive the voltage Vplus, an inverting input terminal (“−”) connected to the second terminal of the capacitor  138  to receive the voltage Vminus, and an output terminal connected to the current generators  104 ,  108  to provide the driving voltage Vpgate to the gate terminals of the transistors  112  and  120  in order to adjust the value of the current Iptat; the operational amplifier  124 ′ is also provided with a reference terminal to receive the common-mode voltage Vcm from the common-mode management circuit block  134 , and a pair of terminals for the compensation of the offset, comprising a non-inverting compensation terminal (“c+”) connected to the offset management circuit  600  for receiving a compensation voltage Vc 1 , and an inverting compensation terminal (“c−”) for receiving the common-mode voltage Vcm from the common-mode management circuit block  134 . As will become clearer in the following, the compensation voltage Vc 1  generated by the offset management circuit  600  has a magnitude such to nullify (or at least minimize as more as possible) the offset voltage between the inverting and the non-inverting terminals when applied to the non-inverting compensation terminal of the operational amplifier  124 ′. The structure and the operation of the operational amplifier  124 ′ and of the offset management circuit  600  according to an embodiment are described in detail below. 
         [0080]    In the circuit  100 ′ the biasing block  140  is replaced by a biasing block  140 ′, an embodiment of whose structure is illustrated in  FIG. 6 . As can be seen in this figure, the biasing block  140 ′ is substantially equal to the biasing block  140 , but has some differences. In the biasing block  140 ′ the common mode voltage Vcm generated by the transistors  208  and  210  is stored in a capacitor  702  connected between the circuit node  206  and a terminal providing the ground voltage, and is periodically regenerated by activating/deactivating the transistors  202  and  208  through a common-mode regeneration signal Vcmr. In particular, the common-mode regeneration signal Vcmr is supplied to the gate terminal of the transistor  208  and to an input terminal of a NOT logic gate  704 ; the NOT logic gate  704  has an output terminal connected to the gate terminal of the transistor  202 . When the common-mode regeneration signal Vcmr is asserted (e.g., at the supply voltage Vdd), the transistors  202  and  208  are turned on, and the biasing block  140 ′ behaves in the same way as the biasing block  140  described in  FIG. 2 ; when instead the common-mode regeneration signal Vcmr is deasserted (e.g., at the ground voltage), the transistors  202  and  208  are turned off, and the common mode voltage Vcm is equal to the value stored in the capacitor  702 . 
         [0081]    Furthermore, in the circuit  100 ′ the short-circuit block  142  is replaced by a short-circuit block  142 ′, an embodiment of whose structure is illustrated in  FIG. 7 . Even in this case, the short-circuit block  142 ′ is substantially similar to the short-circuit block  142 , but has some differences. In particular, in the short-circuit block  142 ′ the controlled switch  300  is controlled by a dedicated control signal Vbsw, different from the control signal Vswc; this makes it possible to control the short-circuit  142 ′ and polarization  140 ′ blocks independently. In addition, in the short-circuit block  142 ′ the voltage Vb of the circuit node  306  may be periodically regenerated by activating/deactivating the transistors  302  and  308  via the common-mode regeneration signal Vcmr; in particular, the common-mode regeneration signal Vcmr is provided to an input terminal of a NOT logic gate  802 , having an output terminal connected to the gate terminal of the transistor  302  and to the base terminal of the transistor  308 . When the common-mode regeneration signal Vcmr is asserted (for example, to the supply voltage Vdd), the transistors  302  and  308  are turned on, and the operation of the short-circuit block  142 ′ is equal to the operation of the short-circuit block  142 ; when instead the common-mode regeneration signal Vcmr is deasserted (for example, at the ground voltage), the transistors  302  and  308  are turned off, and the value of the voltage Vb of the circuit node  306  is equal to the value stored in the capacitor  324 . 
         [0082]    The structure of the offset management circuit  600  according to an embodiment will now be described in greater detail with reference to  FIG. 8 . 
         [0083]    The offset management circuit  600  may have a structure that is typical for the circuit class that operates according to the technique of the continuous zeroing (in the jargon, “Continuous Auto-Zeroing”). The zeroing (reduction) of the offset voltage of the operational amplifier  124 ′ is performed by the offset management circuit  600  via the selective inclusion of an auxiliary operational amplifier  902  in the negative feedback loop of the operational amplifier  124 ′; in more detail, the offset management circuit  600  is configured for measuring the offset voltage of the auxiliary operational amplifier  902 , storing in a capacitor  904  a compensation voltage Vc 2  corresponding to the measured offset voltage, and then insert the auxiliary operational amplifier  902 —compensated by means of the measured compensation voltage Vc 2 —in the feedback of the operational amplifier  124 ′. In this way, the offset voltage of the operational amplifier  124 ′ is nullified by a negative feedback loop comprising an amplifier—the auxiliary amplifier  902 —which ideally has a null voltage offset. 
         [0084]    According to an embodiment of the present invention, the auxiliary operational amplifier  902  is structured in the same way as the operational amplifier  124 ′—i.e., it is approximately the same as the operational amplifier  124 ′. In addition, the inverting and non-inverting inputs of the auxiliary amplifier  902  are supplied by input voltages corresponding to the input voltages supplied to the inverting and non-inverting inputs of the amplifier  124 ′. 
         [0085]    For this reason, the offset management circuit  600  comprises a first capacitor  906  having a first terminal coupled to the emitter terminal of the transistor  116  (or to the first terminal of the capacitor  138 ) comprised in the reference circuit element  106  for receiving the reference voltage Viminusc, and a second capacitor  908  having a first terminal coupled to the emitter terminal of the transistor  110  comprised in the reference circuit element  102  for receiving the voltage Vplusc. The capacitor  906  comprises a second terminal connected to an inverting terminal (“−”) of the auxiliary operational amplifier  902  for providing a voltage Vminus 2  corresponding to the voltage Vminus provided to the inverting terminal of the operational amplifier  124 ′; the capacitor  908  also comprises a second terminal connected to a non-inverting terminal (“+”) of the auxiliary operational amplifier  902  to provide a voltage Vplus 2  corresponding to the voltage Vplus provided to the non-inverting terminal of the operational amplifier  124 ′. 
         [0086]    To ensure that the voltages Vplus 2  and Vminus 2  are as equal as possible to the voltages Vplus and Vminus, the second terminals of the capacitors  906  and  908  are coupled to a biasing block  910  having approximately the same structure of the biasing block  140 ′. The biasing block  910  has a first terminal for receiving the control signal Vswc, a second terminal for receiving the driving voltage Vpgate, a third terminal connected to the second terminal of the capacitor  906 , a fourth terminal connected to the second terminal of the capacitor  908 , and a fifth terminal to receive the common-mode regeneration signal Vcmr. In the same way as for the biasing block  140 ′, the biasing block  910  is adapted to generate the common-mode voltage Vcm′—corresponding to the voltage Vcm generated by the biasing block  140 ′—which is actually supplied to the inputs of the auxiliary operational amplifier  902 ; this voltage is selectively forced to the inputs of the auxiliary operational amplifier  902  according to the control signal Vswc shorting the second terminal of the capacitors  906  and  908  connected to the inputs of the auxiliary operational amplifier  902 . In this way the voltages Vplus 2  and Vminus 2  are brought to the value of the voltage Vcm′ determined by the biasing block  910 . The biasing block  910  also comprises a terminal connected to a reference terminal (“r”) of the auxiliary operational amplifier  902  in order to provide the common-mode voltage Vcm′. 
         [0087]    Similarly to the operational amplifier  124 ′, the auxiliary operational amplifier  902  is provided with a pair of terminals for the offset compensation, comprising a non-inverting compensation terminal (“c+”) connected to the biasing block  910  for receiving the common-mode voltage Vcm′, and an inverting compensation terminal (“c−”) connected to a first terminal of the capacitor  904  for receiving the compensation voltage Vc 2 . The capacitor  904  comprises a second terminal connected to a terminal that provides the ground voltage. The auxiliary operational amplifier  902  also comprises an output terminal connected to a circuit node  912  of a switching circuit  914  to provide an output voltage Vout. The auxiliary operational amplifier  902  may be selectively activated or deactivated by means of a control signal Vota 2 . 
         [0088]    The switching circuit  914  comprises a first switch  916  controlled by a driving signal Vsw 1  having a first conduction terminal connected to the first conduction terminal of the capacitor  904  and a second conduction terminal connected to the circuit node  912 . The switching circuit  914  also comprises a second switch  918  controlled by a driving signal Vswr having a first conduction terminal connected to the circuit node  912  and a second conduction terminal connected to a first terminal of a capacitor  920  for supplying the compensation voltage Vc 1  to the non-inverting compensation terminal of the operational amplifier  124 ′. The capacitor  920  comprises a second terminal connected to a terminal that provides the ground reference voltage. 
         [0089]    Before proceeding with the description of operation of the offset management circuit  600  and of the bandgap voltage reference  100 ′ as a whole, a possible structure of the operational amplifier  124 ′ and of the auxiliary operational amplifier  902  will now be described according to an embodiment shown in  FIG. 9 . 
         [0090]    In particular, given that according to an embodiment the operational amplifier  124 ′ and the auxiliary operational amplifier  902  are equal, we will refer to a single circuit structure, identified with the generic term of “amplifier”. The signals provided/generated to/from terminals of the amplifier are identified in the figure with a double reference, the first of which corresponds to the operational amplifier  124 ′ and the second of which corresponds to the auxiliary operational amplifier  902 . 
         [0091]    The amplifier  124 ′/ 902  is an offset-compensable operational amplifier that comprises two input stage  1002 ,  1004  connected in parallel. 
         [0092]    The stage  1002 , called a gain stage, comprises a pair of n-channel MOS transistors  1006 ,  1008  connected in differential configuration. In particular, the transistor  1006  has a drain terminal connected to a circuit node  1010 , a gate terminal which is the inverting terminal of the amplifier—adapted to receive the voltage Vplus/Vplus 2 —and a source terminal connected to a drain terminal of an n-channel MOS transistor  1012  (circuit node  1014 ) adapted to provide the bias current of the transistors  1006  and  1008 . The transistor  1008  has a drain terminal connected to a circuit node  1016 , a gate terminal which is the inverting terminal of the amplifier—adapted to receive the voltage Vminus/Vminus 2 —and a source terminal connected to the node  1014 . The transistor  1012  has a source terminal connected to a terminal providing the ground voltage and a gate terminal connected to gate terminal of a further n-channel MOS transistor  1013 . The transistor  1013  has a source terminal connected to a terminal that provides the ground voltage and a drain terminal connected to the gate terminal. The transistor  1013  is configured to conduct a current approximately equal to the current Iptat; this current may for example be provided to the transistor  1013  by a current generator mirrored to one of the current generators  104  and  108  of the circuit  100 ′. 
         [0093]    The stage  1004 , called a compensation stage, comprises a further pair of n-channel MOS transistors  1017 ,  1018  connected in a degenerated differential configuration, i.e., with the source terminals coupled together through a degeneration resistor  1020 . In particular, the transistor  1017  has a drain terminal connected to the circuit node  1010 , a gate terminal that represents the non-inverting compensation terminal of the amplifier—adapted to receive the voltage Vc 1 /Vcm′—and a source terminal connected to a drain terminal of a n-channel MOS transistor  1022  (circuit node  1024 ) adapted to provide the bias current of the transistor  1017 . The transistor  1018  has a drain terminal connected to the circuit node  1016 , a gate terminal which is the inverting compensation terminal of the amplifier—adapted to receive the voltage Vcm/Vc 2 —and a source terminal connected to a drain terminal of an n-channel MOS transistor  1026  (circuit node  1028 ) adapted to provide the bias current of the transistor  1018 . The transistors  1022  and  1026  have a source terminal connected to a terminal providing the ground voltage and a gate terminal connected to the gate terminal of transistor  1013 . The degeneration resistor  1020  is formed by a pair of n-channel MOS transistors  1030  and  1032  configured to operate in the triode operating region. In particular, the transistor  1030  has a first conduction terminal connected to the node  1024 , a second conduction terminal connected to the node  1028 , and a gate terminal connected to the gate terminal of the transistor  1017  to receive the voltage Vc 1 /Vcm′. The transistor  1032  has a first conduction terminal connected to the node  1024 , a second conduction terminal connected to the node  1028 , and a gate terminal connected to the gate terminal of the transistor  1018  in order to receive the voltage Vcm/Vc 2 . 
         [0094]    The circuit node  1010  is connected to a source terminal of a n-channel MOS transistor  1034 , while the circuit node  1016  is connected to a source terminal of a n-channel MOS transistor  1036 . The transistor  1034  has a gate terminal connected to a transistor  1036  gate terminal and adapted to receive the control signal Vota/Vota 2 , and a drain terminal connected to a current generator  1038  controlled by the control signal Vota/Vota 2 . The transistor  1036  has its drain terminal connected to the current generator  1038  (circuit node  1040 ). Circuit node  1040  represents the output node of the amplifier, adapted to provide the voltage Vpgate/Vout. 
         [0095]    The operational amplifier above described may be selectively activated/deactivated by turning on/off the transistors  1034 ,  1036  and the current generator  1038  through the control signal Vota/Vota 2 . 
         [0096]    Being equipped with a couple of input stages in parallel, the amplifier may be operated in standard mode, i.e., by activating the gain stage  1002  via input signals Vplus/Vplus 2 , Vminus/Vminus 2  applied differentially and by turning off the compensation stage  1004 , or in a compensated mode, by turning on also the compensations stage  1004  via compensation voltages Vc 1 /Vcm′, Vcm/Vc 2  applied differentially. By applying proper compensation voltages Vc 1 /Vcm′, Vcm/Vc 2 , it may be possible to nullify (or at least reduce) the magnitude of the offset voltage of the amplifier. 
         [0097]    Without descending into technical details that go beyond the scope of the present disclosure, the presence of the degeneration resistor  1020  allows one to decrease the gain of the compensation stage  1004  by a quantity so as to stabilize the amplifier when connected in negative feedback. 
         [0098]    The offset management circuit  600  operation and a bandgap voltage reference circuit  100 ′ as a whole will be now described with reference to the  FIGS. 6-9 , previously described, in conjunction with  FIG. 10 ;  FIG. 10  is a time diagram which shows the trend over time of some of the signals generated/received by the circuit  100 ′. 
         [0099]    As the circuit  100 , the circuit  100 ′ as well has an architecture of the “sample and hold” type, whose operation comprises a sequence of alternating holding phases and regeneration phases timed by the value assumed by the holding signal—in particular, during the holding phases the hold signal Vphold is at a high voltage level, while during the regeneration phases said signal is at a low voltage level. 
         [0100]    During the holding phase (hold signal Vphold at the high voltage level), the current generators  104 ,  108 ,  126  of the circuit  100 ′ are turned off; moreover the control voltages Vota and Vota 2  are deasserted to a low voltage (ground voltage), so that even the operational amplifiers  124 ′ and  902  are turned off. During each holding phase, the value of the bandgap voltage Vbg is equal to that corresponding to the charge that was stored in the capacitor  148  during the previous regeneration phase; similarly, the value of the driving voltage Vpgate is equal to the value corresponding to the charge which was stored in the capacitor  144  during the previously holding phase, set in turn by a previous activation of the operational amplifier  124 ′. 
         [0101]    At the start of the regeneration phase, the hold signal Vphold is reduced to low voltage level, turning on the transistors  114 ,  122  and  132  and thus enabling the current generators  104 ,  108 ,  126  to the generation of the current Iptat in the various branches of the circuit. 
         [0102]    The control signals Vswc and Vbsw are then brought to a high voltage level (Vdd) to turn on the biasing blocks  140 ′,  910  and to activate the short-circuit block  142 ′ so as to regenerate the values of the voltages across the capacitors  136  and  138  connected to the operational amplifier  124 ′, and the voltages across the capacitors  906  and  908  connected to the auxiliary operational amplifier  902 . In particular, the transmission ports of the biasing block  140 ′ are closed, shorting the second terminal of the capacitor  136  with the second terminal of the capacitor  138 ; simultaneously, the controlled switch of the short-circuit block  142 ′ as well is closed, in order to short-circuit the emitter terminal of the transistor  110  (connected to the first terminal of the capacitor  136 ) with the emitter terminal of the transistor  116  (coupled to the capacitor  138  first terminal through the resistor  118  or with the first terminal of the capacitor  138  directly). In this situation, the voltage Vplus at the non-inverting terminal of the operational amplifier  124 ′ and the voltage Vminus at the inverting terminal of the operational amplifier  124 ′ are brought to the common-mode voltage Vcm stored across the capacitor  702  of the biasing block  140 ′, while a voltage drop equal to dVc: 
         [0000]        dVc =( Vcm−Veb ), 
         [0000]    is set on the capacitors  136  and  138 , where Veb=Vplusc=Vminusc. Similarly, the transmission gates of the biasing block  910  are closed, shorting the second terminal of the capacitor  906  with the second terminal of the capacitor  908 . In this situation, the voltage Vplus 2  at the non-inverting terminal of the auxiliary operational amplifier  902  and the voltage Vminus 2  at the inverting terminal of the auxiliary operational amplifier  902  are brought to the common-mode voltage Vcm′ taken from the biasing block  910 , while on the capacitor  906  and  908  it is set a voltage drop dVc′ equal to: 
         [0000]        dVc =( Vcm′−Veb ), 
         [0000]    where Veb=Vplusc=Vminusc. 
         [0103]    The common mode voltages at the inputs of the operational amplifier  124 ′and of the auxiliary operational amplifier  902  are then set to a value, which may be an optimal value, and which follows the possible temperature and biasing variation to which the circuit  100 ′ is subject, and that appears to be calibrated depending on the specific parameters of the process by which the circuit  100 ′ was manufactured. 
         [0104]    Subsequently, the control signal Vswc is brought to a low voltage level (ground voltage), so that the transmission gates of the biasing blocks  140 ′ and  910  are open. 
         [0105]    At this point begins the compensation phase of the offset voltage of the auxiliary operational amplifier  902 . In particular, the control signal Vota 2  is asserted to a high voltage level (Vdd) to turn on the auxiliary operational amplifier  902 , and in the meanwhile the driving signal Vsw 1  is asserted as well to close the switch  916  of the switching circuit  914 . Consequently, the inverting and non-inverting input terminals of the auxiliary operational amplifier  902  are shortened together through the capacitors  906  and  908  (which have the first terminals shortened together because the short-circuit block  142 ′ is still active); moreover, the auxiliary operational amplifier  902  has the output terminal connected in feedback with the inverting compensation terminal, and the non-inverting compensation terminal that receives from the biasing block  910  the common-mode voltage Vcm′. In this condition, the compensation voltage Vc 2  across the capacitor  904  assumes a value adapted to compensate the offset voltage of the auxiliary operational amplifier  902 . Such offset voltage corresponds to a condition of the auxiliary operational amplifier  902  in which the inverting and non-inverting input terminals have a common-mode voltage equal to the common-mode voltage Vcm′ (which in turn corresponds to the common-mode voltage Vcm of the input terminals of the operational amplifier  124 ′). 
         [0106]    Subsequently, the control signal Vsbw is deasserted to a low voltage level (the ground voltage) to turn off the compensation block  142 ′ and interrupt the connection between the first terminal of the capacitor  136  and the first terminal of the capacitor  138 , and between the first terminal of the capacitor  906  and the first terminal of the capacitor  908 . 
         [0107]    In the same time, the control signal Vota is asserted to a high voltage level (Vdd) to turn on the operational amplifier  124 ′ and to enable the negative feedback loop between the voltage across the reference circuit elements  102 ,  106  and the driving voltage Vpgate of the current generators  104 ,  108  and  106 . In particular, the driving voltage Vpgate across the capacitor  144  is regenerated by the operational amplifier  124 ′ according to the values assumed by the voltages Vplus and Vminus. 
         [0108]    Furthermore, the driving signal Vsw 1  is deasserted to open the switch  916 , while the driving signal Vswr is asserted to close the switch  918  of the switching circuit  914 . In this condition, the output terminal of the auxiliary operational amplifier  902  is placed in connection with the non-inverting compensation terminal of the operational amplifier  124 ′ to provide the compensation voltage Vc 1 , which is stored across the capacitor  920 . Consequently, the auxiliary operational amplifier  902 —whose offset voltage appears to be compensated because the compensation voltage Vc 2  stored across the capacitor  904  is provided to the inverting compensation input terminal—is inserted into the feedback loop generated by the operational amplifier  124 ′ between the voltage across the reference circuit elements  102 ,  106  and the driving voltage Vpgate. Providing to the non-inverting terminal of the operational amplifier  124 ′ the compensated voltage Vc 1  generated by the presence of the auxiliary operational amplifier  902  in the feedback loop, the offset voltage may be reduced, since the voltages Vminusc and Vplusc are forced to assume a value substantially equal to each other; therefore the current Iptat may assume a value close to the ideal value (i.e. corresponding to a null voltage offset). In the occurrence that the auxiliary operational amplifier  902  is equal to the operational amplifier  124 ′, and the common-mode voltage Vcm′ generated by the biasing block  910  is equal to the common-mode voltage Vcm generated by the biasing block  140 ′, the offset voltages of the two amplifiers would be equal, and the compensation voltage Vc 2  would be equal to the compensation voltage Vc 1  Consequently, the offset voltage of the operational amplifier  124 ′ is approximately completely nullified, because at the non-inverting compensation terminal of the operational amplifier  124 ′ would be applied a compensation voltage Vc 1  equal to the compensation voltage Vc 2  used to compensate the offset voltage of the auxiliary operational amplifier  902  which generates the compensation voltage Vc 1  itself. Analyzing the topology of the circuit  100 ′ it is noted that to establish a correct negative feedback loop in an embodiment, the compensation voltage Vc 1  is provided to the non-inverting compensation terminal of the operational amplifier  124 ′, and not to the inverting compensation terminal. 
         [0109]    Subsequently, the auxiliary operational amplifier  902  is removed from the negative feedback loop of the operational amplifier  124 ′ by deasserting the driving signal Vswr and then opening the switch  918 . The compensation voltage Vc 1  is still stored across the capacitor  920 . 
         [0110]    The sampling signal Vbgref is thus brought to a high voltage level (Vdd) for closing the controlled switch  146  in order to regenerate the bandgap voltage Vbg across the capacitor  148 . 
         [0111]    Then, both the sampling signal Vbgref and the control signals Vota and Vota 2  are brought to a low voltage level (the ground voltage) in order to open the controlled switch  146  and to disable the operational amplifier  124 ′ and the auxiliary operational amplifier  902 . The voltages Vbg and Vpgate are then stored in the capacitors  148  and  144 . 
         [0112]    Before proceeding to the subsequent holding phase, the common-mode regeneration signal Vcmr is asserted to a high voltage level (Vdd) to activate the transistor  202  of the biasing block  140 ′ and then regenerate the common-mode voltage Vcm across the capacitor  702  (similar considerations can be applied to the biasing block  910  and the common-mode voltage Vcm′), and to activate the transistors  302  and  308  of the short-circuit block  142 ′ and then regenerate the voltage Vb across the capacitor  324 . 
         [0113]    At this point the holding signal Vphold is brought to a high voltage level (Vdd), and the new holding phase starts. 
         [0114]    In summary, an embodiment of the circuit  100 ′ is able to set an optimum common-mode voltage, suitable for applications with low supply voltages and that is not affected by unwanted variations due to the presence of an offset voltage at the input of the operational amplifier; furthermore, thanks to the architecture of the “sample and hold” type used, the current consumption is substantially reduced compared to solutions known in the art. 
         [0115]    Naturally, in order to satisfy local and specific requirements, one may apply to the embodiments described above many modifications. Particularly, although the present disclosure has been described with a certain degree of particularity with reference to embodiments thereof, it should be understood that various omissions, substitutions and changes in the form and details as well as other embodiments are possible; moreover, it is expressly intended that specific elements and/or method steps described in connection with any disclosed embodiment may be incorporated in any other embodiment as a general matter of design choice. 
         [0116]    For example, similar considerations apply if the reference circuit elements have a different but equivalent structure, such as the presence in at least one reference circuit element of more bipolar transistors connected in parallel to each other. 
         [0117]    Moreover, even if the described current generators were sized to mirror the generated current with a mirror ratio equal to 1:1, the concepts of the present disclosure may be applied to cases with different mirroring ratios. 
         [0118]    In addition, the common mode voltage generated by the biasing block may be achieved in other ways, such as using a single transistor sufficiently resistive to allow the transistors of the operational amplifier input stage to operate in the saturation zone. 
         [0119]    Similar considerations may apply to the driver circuit of the short-circuit block, which may have a different circuit structure, such as a boost circuit that does not belong to the category of the clock booster circuits. 
         [0120]    Furthermore, some of the circuitry described above in conjunction with the embodiments of  FIGS. 6-9  may be conceptual for the explanation of concepts; but the actual circuitry may be implemented differently. 
         [0121]    Moreover, the compensation signal Vc 1  may be coupled to one of the signal input terminals of the amplifier  124 ′, thus allowing one to omit the compensation terminals. 
         [0122]    From the foregoing it will be appreciated that, although specific embodiments have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the disclosure. Furthermore, where an alternative is disclosed for a particular embodiment, this alternative may also apply to other embodiments even if not specifically stated.